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Craniopharyngiomas

ABSTRACT

 

Craniopharyngiomas are rare intracranial tumors that mainly arise in the sellar/parasellar (particularly suprasellar) region. They present in both children and adults with a wide range of clinical manifestations. Histologically, they are benign tumors with distinct adamantinomatous and papillary subtypes. Beta-catenin gene mutations have been identified in the adamantinomatous subtype and activating mutations in BRAF (V600E) in the papillary variant, opening further avenues in our understanding of their pathogenesis. Despite their benign classification, management is challenging due to unpredictable growth and the involvement of adjacent critical structures particularly for vision and hypothalamo-pituitary function. Surgery with or without external irradiation currently represents the mainstay of therapy for most patients; however, the optimal protocol for the management of these tumors has not yet been established. Further management options include intracystic irradiation or bleomycin, stereotactic radiosurgery, systemic chemotherapy, or targeted BRAF inhibitors (for the papillary subtype); however, the outcomes of these approaches have not yet been validated with large scale clinical trials. Following treatment, patients face a high burden of morbidity due to visual, endocrine, hypothalamic, and neuropsychological dysfunction, and long-term mortality rates are substantially elevated compared with the general population.

 

EPIDEMIOLOGY

 

Craniopharyngiomas account for 2–5% of all primary intracranial neoplasms and for up to 15% of intracranial tumors in children (1). Their annual incidence is reported as around 0.18 cases per 100,000 people (2), and genetic susceptibility seems unlikely. Craniopharyngiomas may be detected at any age, even in the prenatal and neonatal periods (3) and a bimodal age distribution with peak incidence rates at ages 5–14 and 50–74 years has been proposed (2,4). In population-based studies from the USA and Finland, no gender differences have been found (4,5).

 

PATHOLOGY AND PATHOGENESIS

 

Craniopharyngiomas are epithelial tumors arising along the path of the craniopharyngeal duct (the canal connecting the stomodeal ectoderm with the evaginated Rathke’s pouch). Based on the WHO classification, they are assigned grade I. Rare cases of malignant transformation (possibly triggered by previous irradiation) have been described (1). Two main pathological subtypes have been reported: the adamantinomatous variant and the papillary variant (1,6).

 

The adamantinomatous variant is the most common subtype and may occur at any age (Figure 1). Macroscopically, adamantinomatous craniopharyngiomas have cystic and/or solid components, necrotic debris, fibrous tissue and calcification. The cysts may be multiloculated and contain liquid ranging from “machinery oil” to shimmering cholesterol-laden fluid consisting of desquamated squamous epithelial cells, rich in membrane lipids and cytoskeleton keratin. They tend to have sharp and irregular margins, often merging into a peripheral zone of dense reactive gliosis, with abundant Rosenthal fiber formation (consisting of irregular masses of granular deposits within astrocytic processes) in the surrounding brain tissue and vascular structures. The epithelium of the adamantinomatous type is composed of three layers of cells: a distinct palisaded basal layer of small cells with darkly staining nuclei and little cytoplasm (somewhat resembling the basal cells of the epidermis of the skin); an intermediate layer of variable thickness composed of loose aggregates of stellate cells (termed stellate reticulum), with processes traversing empty intercellular spaces; and a top layer facing into the cyst lumen with abruptly enlarged, flattened and keratinized to flat plate-like squamous cells. The flat squames are desquamated singly or in distinctive stacked clusters and form nodules of “wet” keratin, which are often heavily calcified and appear grossly as white flecks. The keratinous debris may elicit an inflammatory and foreign body giant cell reaction. The presence of the typical adamantinomatous epithelium or of the “wet” keratin alone are diagnostic, whereas features only suggestive of the diagnosis in small or non-representative specimens include fibrohistiocytic reaction, necrotic debris, calcification and cholesterol clefts (1).

Figure 1A. Histology of adamantinomatous craniopharyngioma. Islands of tumor with finger-like protrusions into surrounding brain tissue with central accumulation of keratin nodules; HE x40 magnification.

Figure 1B. Histology of adamantinomatous craniopharyngioma. Well-differentiated epithelium with peripheral palisading, nodular whorls, and pale, microcystic areas termed ‘stellate reticulum’, as well as pale eosinophilic ‘wet keratin’ nodule (right bottom); HE x200 magnification.

Figure 1C. Histology of adamantinomatous craniopharyngioma. Beta-catenin immunohistochemistry of area shown in A, highlighting dark staining nodular whorls; x40 magnification

Figure 1D. Histology of adamantinomatous craniopharyngioma. Basal epithelium demonstrating the aberrant nuclear accumulation of beta-catenin in nodular whorls (arrowheads) due to beta-catenin mutation; Anti-beta-catenin, x400 magnification.

The papillary variant has been almost exclusively described in adult populations (accounting for 14-50% of adult cases but only around 2% of pediatric cases) (Figure 2). It consists of mature squamous epithelium forming pseudopapillae and of an anastomosing fibrovascular stroma without the presence of peripheral palisading of cells or stellate reticulin, and with membranous beta-catenin immunoreactivity only. The differential diagnosis between a papillary craniopharyngioma and a Rathke’s cleft cyst may be difficult, particularly in small biopsy specimens, as the epithelial lining of the Rathke’s cysts may undergo squamous differentiation; however, the lack of a solid component and the presence of extensive ciliation and/or mucin production are suggestive of Rathke’s (1,6).

Figure 2A. Histology of papillary craniopharyngioma. Papillae lined by non-keratinizing squamous epithelium and containing loosely structured connective tissue; HE x20 magnification.

Figure 2B. Histology of papillary craniopharyngioma. Connective tissue harbors a patchy lymphocytic infiltrate (asterix); HE x100 magnification.

Figure 2C. Histology of papillary craniopharyngioma. Non-keratinizing squamous epithelium highlighted by beta-catenin immunostain, x20 magnification.

Figure 2D. Histology of papillary craniopharyngioma. Squamous epithelium showing membranous immunoreactivity of beta-catenin, lacking clusters with aberrant nuclear accumulation, x400 magnification.

Although the pathogenesis of craniopharyngiomas has not been fully elucidated, our understanding in this field has increased significantly in recent years. Beta-catenin gene (CTNNB1) mutations have been identified in the adamantinomatous subtype affecting exon 3 which encodes the degradation targeting box of beta-catenin; the mutant form is resistant to degradation leading to accumulation of nuclear beta-catenin protein (a transcriptional activator of the Wnt signaling pathway) (Figure 1D). Furthermore, strong beta-catenin expression has been shown in the adamantinomatous subtype indicating re-activation of the Wnt signaling pathway and subsequent de-regulation of several downstream pathways (7-10). Molecular analysis also implicates the immune response in the pathogenesis of adamantinomatous craniopharyngiomas. Cells within this subtype show signs of inflammation (in both cystic and solid components), and increased levels of cytokines including Interleukin-6 (IL-6), IL-8 and IL-10 have been identified(10-12). Furthermore, the expression of immune related genes is increased, and the immune check point proteins Programmed Death Ligand 1 (PD-L1), and Programmed Cell Death Protein 1 (PD-1) are expressed in both subtypes of craniopharyngioma(10,13).  For papillary craniopharyngiomas specifically, a number of studies using whole exome sequencing, next-generation panel sequencing, pyrosequencing and Sanger sequencing have shown the presence of activating mutations in the BRAF (V600E) gene;  the prevalence of which varies according to the sequencing method, generally being between 81 and 100% (8). BRAF mutations can lead to activation of the MAPK/ERK (Mitogen Activated Protein Kinase / Extracellular signal Regulated Kinases) pathway, which ultimately results to increased cell growth, proliferation, and cell survival(10). Whilst BRAF mutations are found in numerous cells within papillary craniopharyngiomas, only a small cluster of progenitor cells expressing the SOX2/SOX9 (Sex Region Y Box 2 and 9) transcription factors are believed to be involved in their tumorigenesis(14). It has also been suggested that the two pathological subtypes have different epigenomic and transcriptomic signatures, and that the cell clusters in the adamantinomatous subtype may have a functional role in the promotion of tumor invasion (8).

 

DIAGNOSIS

 

Location/Imaging

 

Most craniopharyngiomas are located in the sellar/parasellar region and the majority (94-95%) have a suprasellar component. Other rare locations include the nasopharynx, the paranasal area, the sphenoid bone, the ethmoid sinus, the intrachiasmatic area, the temporal lobe, the pineal gland, the posterior cranial fossa, the cerebellopontine angle, the midportion of the midbrain or, mainly relating to the papillary variant, within the 3rd ventricle(1). Plain skull X-rays, although seldom used nowadays, may show calcification and an abnormal sella. CT is helpful for evaluation of the bony anatomy, the identification of calcification and the discrimination of the solid and cystic components. They are usually of mixed attenuation; the cyst fluid has low density and the contrast medium enhances any solid portion, including the cyst capsule (1). MRI is particularly useful for the topographic and structural analysis of the tumor. The radiological appearance depends on the proportion of the solid and cystic components, the content of the cyst(s) (cholesterol, keratin, hemorrhage), and the amount of calcification present. A solid lesion appears as iso- or hypointense relative to the brain. On pre-contrast T1-weighted images, it shows enhancement following gadolinium administration, and is usually of mixed hypo- or hyperintensity on T2-weighted images. Large amounts of calcification may be visualized as areas of low signal on both T1- and T2-weighted images. A cystic element is usually hypointense on T1- and hyperintense on T2-weighted sequences, and a thin peripheral contrast-enhancing rim of the cyst can be shown on T1-weighted images. Protein, cholesterol, and methemoglobin may cause high signal on T1-weighted images, while very concentrated protein and various blood products may be associated with low T2-weighted signal (1). Imaging examples from cystic, solid, and mixed solid-cystic craniopharyngiomas are shown in Figure 3.

Figure 3A. MRI images of craniopharyngiomas. Coronal section showing cystic craniopharyngioma on post-contrast T1-weighted MRI. The cyst contents are isointense and the cyst rim enhances following contrast.

Figure 3B. MRI images of craniopharyngiomas. Sagittal section of 3A.

Figure 3C. MRI images of craniopharyngiomas. Sagittal section showing a solid craniopharyngioma on T1-weighted imaging which enhances after contrast.

Figure 3D. MRI images of craniopharyngiomas. Coronal section showing a solid craniopharyngioma on T1-weighted imaging which enhances after contrast.

Figure 3E. MRI images of craniopharyngiomas. Sagittal section showing a craniopharyngioma with mixed solid and cystic components on post-contrast T1-weighted imaging.

Figure 3F. MRI images of craniopharyngiomas. Coronal section showing a mixed solid and cystic craniopharyngioma with mixed signal intensities on T2-weighted imaging.

The consistency of craniopharyngiomas can be purely or predominantly cystic, purely or predominantly solid, and mixed. When present, the calcification patterns vary from solid lumps to popcorn-like foci or, less commonly, to an eggshell pattern lining the cyst wall. Hydrocephalus has been reported in 20-38% and is probably more frequent in childhood-diagnosed disease (41-54%).

 

The differential diagnosis includes a number of sellar or parasellar lesions, including Rathke’s cleft cyst, dermoid cyst, epidermoid cyst, pituitary adenoma, germinoma, hamartoma, suprasellar aneurysm, arachnoid cyst, suprasellar abscess, glioma, meningioma, sarcoidosis, tuberculosis and Langerhans cell histiocytosis. Differentiation from a Rathke’s cleft cyst (typically small, round, purely cystic lesions lacking calcification), or from a pituitary adenoma (in the rare case of a homogeneously enhancing solid craniopharyngioma), may be particularly difficult (1,15).

 

Clinical and Hormonal Manifestations at Presentation

 

Patients with craniopharyngioma may present with a variety of clinical manifestations attributed to pressure effects on vital structures of the brain (visual pathways, brain parenchyma, ventricular system, major blood vessels and hypothalamo-pituitary system) (15-17). Their severity depends on the location, size, and growth potential of the tumor. The duration of the symptoms until diagnosis ranges between 1 week to 372 months(1). The presenting clinical manifestations (neurological, visual, hypothalamo-pituitary) are shown in Table 1. Headaches, nausea/vomiting, visual disturbances, growth failure (in children) and hypogonadism (in adults) are the most frequently reported. Other less common or rare features include motor disorders, such as hemi- or monoparesis, seizures, psychiatric symptoms such as emotional lability, hallucinations and paranoid delusions, autonomic disturbances, precocious puberty, the syndrome of inappropriate secretion of antidiuretic hormone, chemical meningitis due to spontaneous cyst rupture, hearing loss, anosmia, nasal obstruction, epistaxis, photophobia, emaciation, Weber’s syndrome (ipsilateral III cranial nerve palsy with contralateral hemiplegia due to midbrain infarction), and Wallenberg’s syndrome (signs due to occlusion of the posterior inferior cerebellar artery) (1). It has been proposed that in cases of craniopharyngioma diagnosed in childhood, compromised growth rate is already evident in early infancy, whereas an increase in weight tends to present later and is a predictor of obesity (18). 

 

The hypothalamo-pituitary function at presentation may be severely affected; a summary of the results of various studies using different diagnostic tests and criteria shows that GH deficiency is present in 35–100% of the evaluated patients, FSH/LH deficiency in 38–91%, ACTH deficiency in 21–68%, TSH deficiency in 20–42% and diabetes insipidus in 6–38%.

 

Table 1. Presenting Clinical Features in Children and Adults with Craniopharyngioma (10)

 

Children

Adults

 

Total

Headaches

78%

56%

64%

Menstrual disorders

 

57%

 

Visual field defects

46%

60%

55%

Decreased visual acuity

39%

40%

39%

Nausea/vomiting

54%

26%

35%

Growth failure

32%

 

 

Poor energy

22%

32%

29%

Impaired sexual function

 

28%

 

Impaired secondary sexual characteristics

(pts aged ≥13 years)

 

 

24%

Lethargy

17%

26%

23%

Other cranial nerves palsies

27%

  9%

15%

Polyuria/polydipsia

15%

15%

15%

Papilledema

29%

  6%

14%

Cognitive impairment

(memory, concentration)

10%

17%

14%

Anorexia/weight loss

 20%

  8%

12%

Optic atrophy

  5%

14%

10%

Hyperphagia/excessive weight gain

  5%

13%

10%

Psychiatric symptoms/change in behavior

10%

  8%

  8%

Somnolence

  5%

10%

  8%

Galactorrhea

 

  8%

 

Decreased consciousness/coma

10%

  4%

  6%

Cold intolerance

  0%

  8%

  5%

Unsteadiness/ataxia

  7%

  3%

  4%

Hemiparesis

  7%

  1%

  3%

Blindness

  3%

  3%

  3%

Meningitis

   0%

   3%

 2%

 

MANAGEMENT

 

Surgery Combined or Not with External Irradiation

 

Surgery combined or not with adjuvant external beam irradiation is currently one of the most widely used first therapeutic approaches for craniopharyngiomas. These tumors pose challenges mainly due to their sharp, irregular borders and to their tendency to adhere to vital neurovascular structures, making surgical manipulation potentially hazardous to vital brain areas. When large cystic components are present, fluid aspiration provides relief of the obstructive manifestations and facilitates the consecutive removal of the solid portion, which should not be delayed for more than a few weeks due to the significant risk of cyst refilling (15,19). The attempted extent of excision has been a subject of significant debate and depends on the size (achieved in 0% of lesions >4 cm) and location of the tumor, the presence of hydrocephalus (particularly difficult for retrochiasmatic or within the 3rd ventricle), >10% calcification, tumor adherence to the hypothalamus, brain invasion, as well as the experience, individual judgment during the operation, and general treatment policy (aggressive or not) adopted by each neurosurgeon (1,20-22). In recent years, many tertiary centers have adopted a more conservative surgical approach, electing for partial or limited resection with radiotherapy over complete resection, when possible, with aim of hypothalamic sparing and reducing subsequent morbidity (23). In microsurgical series, post-operative mortality ranges between 0 and 5.4% (24), while in a meta- analysis including 2,955 patients early mortality of 2.6% after transsphenoidal and 3.1% after transcranial surgery were reported (25).

 

Interestingly, until 1937, when Carpenter et al. (26) first described the beneficial effects of radiotherapy following aspiration of cyst contents in 4 cases, craniopharyngiomas were considered radioresistant. Historically, the role of irradiation started being established almost two decades later, following the report of the favorable outcome of the combination of minimal surgery and high-dose supervoltage irradiation in a series of 10 patients by Kramer et al. (27). The irradiation of cystic craniopharyngiomas carries the risk of cyst enlargement, arising during or within 6 months after radiotherapy, reported in 10-60% of patients (28-31). Whilst urgent surgical decompression may be needed in some cases, enlargement is transient, and does not represent tumor recurrence (15,30,32).

 

Recurrence Following Surgery

 

Recurrent tumors may arise even from small islets of craniopharyngioma cells in the gliotic brain adjacent to the tumor, which can remain even after gross total removal. The mean interval for diagnosis of recurrence following various primary treatment modalities ranges between 1 and 4.3 years. Remote recurrences as late as 30 years after initial therapy have been reported; possible mechanisms include transplantation during the surgical procedures and dissemination by meningeal seeding or CSF spreading (1). Series with radiological confirmation of the radicality of resection show that recurrence rates following gross total removal range between 0 and 62% at 10 years follow-up. These are significantly lower than those reported after partial or subtotal resection (25-100% at 10 years follow-up). In cases of limited surgery, adjuvant radiotherapy significantly improves the local control rates (recurrence rates 10-63% at 10 years follow-up).  Finally, radiotherapy alone provides 10 years recurrence rates ranging between 0 and 23% (1). These results were based on the use of conventional fractionated external beam radiotherapy; tumor control rates with newer higher precision techniques, such as fractionated stereotactic conformal radiotherapy, have remained optimal with 5-year progression free survival exceeding 90% (33,34). Tumor control rates achieved by proton beam therapy in patients with craniopharyngioma are promising (35), but studies with long-term follow-up are needed. Studies with statistical comparisons of the local control rates achieved by gross total removal or the combination of surgery and radiotherapy have not provided consistent results. The interpretation of data regarding effectiveness of each therapeutic modality must be done with caution, since the published studies are retrospective, non-randomized and often specialty-biased.

 

The growth rate of craniopharyngiomas varies considerably and reliable clinical, radiological, and pathological criteria predicting their behavior are lacking. Thus, apart from significant impact of the treatment modality as mentioned above, attempts to identify other prognostic factors for recurrence (age, group at diagnosis, sex, imaging features, pathological subtypes) have not provided consistent results (1).

 

The management of recurrent tumors remains challenging, as scarring/adhesions from previous operations or irradiation make successful removal difficult. In such cases, total removal is achieved in a substantially lower rate when compared with primary surgery (0-25%). Perioperative mortality is increased following recurrence, occurring in as many as 11-24% (22). The beneficial effect of radiotherapy (proceeded or not by second surgery) in recurrent lesions has been clearly shown(15,36). Recurrent lesions with significant cystic component not amenable to total extirpation may be treated by repetitive aspirations through an indwelling Ommaya reservoir apparatus. In a small series of 11 adult patients with cystic craniopharyngiomas treated with Ommaya reservoirs, local control was achieved in 8 patients (72.7%) without the need for additional treatment over a follow up period of 41.4 months (37).

 

Intracystic Irradiation

 

Intracavitary irradiation (brachytherapy) involves stereotactically guided instillation of beta-emitting isotopes into cystic craniopharyngiomas. It delivers a higher radiation dose to the cyst than external beam radiotherapy, resulting in damage of the secretory epithelial lining, elimination of fluid production, and cyst shrinkage. The efficacy of various beta and gamma-emitting isotopes (mainly 32phosphate, 90yttrium, 186 rhenium, 198gold) has been investigated in a number of studies, but given that none of them has the ideal physical and biological profile, there is no consensus on which is the most suitable therapeutic agent. In a systematic review which included 66 children treated with brachytherapy, a reduction in tumor size was reported in 89% of children with cystic only craniopharyngiomas, and in 58% in those with mixed cystic and solid components (38). In series with mean or median follow-up between 3.1 and 11.9 years providing intracavitary irradiation (mainly with 90yttrium or 32phosphorus) at doses of 200-270 Gy, complete or partial cyst resolution was seen in 71-88%, stabilization in 3-19%, and increases in 5-10% of cases (39-44). New cyst formation or increase in the solid component of the tumor were observed in between 6.5 and 20% of cases. Although beta emitters have short range tissue penetrance, lesions in close proximity to the optic apparatus should be approached with caution (39-44). Deterioration of vision has been reported in 10-58% of cases and has been attributed to failure of cyst collapse, formation of new cysts, increase in the solid tumor, or possibly radiation damage. The reported control rates combined with low surgical morbidity and mortality render brachytherapy an attractive option for predominantly cystic tumors, particularly those that are monocystic. 

 

Intracystic Bleomycin

 

Intracystic installation of the anti-neoplasmatic agent bleomycin has been used in the management of craniopharyngiomas. The drug is administered through an Ommaya reservoir connected to a catheter. In published reports the tumor control rates range between 0 and 100%. However, evidence supporting its efficacy is limited mostly to case reports or non-randomized retrospective studies, and a Cochrane review (45) exploring the effects of bleomycin in children could not recommend its use. Direct leakage of the drug to surrounding tissues during the installation procedure, diffusion though the cyst wall, or high drug doses have been associated with various toxic (hypothalamic damage, blindness, hearing loss, ischemic attacks, peritumoral edema) or even fatal effects (1,46,47).The value of this treatment option in tumor control or even in delaying surgery and/or radiotherapy, as well as the optimal protocol and the clear-cut criteria predicting the long-term outcome, remain to be established in large series with sufficient follow-up.

 

Intracystic Interferon-Alpha

 

Intracystic interferon-alpha is not neurotoxic and is therefore associated with a lower risk of adverse events when compared to other intracystic treatments. Despite encouraging results in a number of studies with short follow up, a large multicenter study demonstrated tumor progression in 75% of patients by a median of just 14 months (48,49).

 

Stereotactic Radiosurgery

 

Stereotactic radiosurgery delivers a single or small number of fraction(s) of high dose ionizing radiation to precisely mapped targets, keeping the exposure of adjacent structures to a minimum. Tumor volume and close attachment to critical structures (e.g., optic apparatus) are limiting factors for its application. Risk of optic neuropathy is <1% when the optic chiasm is constrained to a maximal dose of 10 Gy, 20 Gy, and 25 Gy, for single-fraction SRS, 3-fraction SRS, and 5-fraction SRS respectively (50). SRS achieves tumor control in a substantial number of patients with small volume lesions and reported 5-year progression free survival ranges between 61% and 90.3% (51-55). Rate of tumor control following SRS is negatively associated with tumor volume (56), thus it is particularly useful for well-defined residual disease following surgery or for the treatment of small, solid recurrent tumors situated at least 3-5mm away from the optic chiasm (49). Increasing margin dose and maximum dose >35Gy have been associated with increased risk of neurologic deficit following SRS (57). Studies with long-term follow-up evaluating the optimal marginal dose, its role in the prevention of tumor growth, and its effects on neurocognitive and neuroendocrine function, are needed.

 

Systemic Chemotherapy/Interferon-Alpha 

 

The potential benefit of systemic chemotherapy in craniopharyngiomas has been investigated in a very limited number of patients. Thus, Bremer et al. (58) reported a case of successful management of a recurrent cystic tumor with the combination of vincristine, carmustine (BCNU) and procarbazine. Lippens et al. (59), after administration of five courses of doxorubicin and lomustin in 4 children with multiple or very rapid recurrences, achieved local control in 75% of them after 3-12 years follow-up. Jakacki et al. (60), in a series of 12 patients aged <21 years with progressive or recurrent craniopharyngiomas, showed that after 12 months of treatment with interferon-alpha, tumor reduction of at least 25% was observed in 3 cases. However, during the first weeks of therapy 6 patients experienced an increase in the size of the cystic component, which was finally considered as progressive disease in half of them. Interestingly, 67% of patients that completed one year of therapy without progressive disease had an increase in the size of their tumor at a median period of 11 months after discontinuation of the drug. The cytotoxicity (predominantly hepatic, neurological and cutaneous), requiring temporary discontinuation and/or dose reduction within the first 8 weeks of therapy, was significant (in up to 60% of the cases). In 2012, the same group explored the use of pegylated interferon (a derivative of interferon-alpha with a longer half-life) in five patients; all demonstrated a radiological response to treatment and two of them had a complete response (61). A subsequent phase two multi-center study gave disappointing results. Of 18 adults and children with recurrent craniopharyngiomas who were given systemic pegylated interferon, only one attained a sustained response beyond 3 months (62).

 

Targeted Therapy

 

The finding that most papillary craniopharyngiomas harbor a BRAF (V600E) mutation has opened avenues for use of pharmacological agents specifically targeting and inhibiting mutant BRAF in cases resistant to other treatments. A number of case reports and small case series have demonstrated a significant reduction in tumor size (used alone or in combination with MEK inhibitors), applied neoadjuvantly or after surgery, with or without prior radiotherapy (8,49,63-73). Common side effects associated with BRAF and MEK inhibitors seen from their use in other diseases (such as metastatic melanoma and papillary thyroid cancer) include rash, fever, diarrhea, arthralgia, and liver dysfunction (74). Cases of adamantinomatous craniopharyngiomas responding to MEK inhibitors (75), or controlled with IL-6 inhibitors used alone or in conjunction with VEGR inhibitors (12), have also been reported. The pros and cons of these new treatment modalities, particularly for aggressive tumors, warrant further assessment by trials with large number of patients and adequate follow-up. Two clinical trials (BRAF and MEK inhibitors for papillary craniopharyngiomas, and IL-6 inhibitors for children with adamantinomatous craniopharyngiomas) are currently ongoing (76,77). Initial results from one of these trials – a phase two study which included sixteen patients with papillary craniopharyngiomas harboring the BRAF V600E mutation – have been presented. All patients were treated with oral vemurafenib (BRAF inhibitor) and cobimetinib (MEK inhibitor) in 28-day cycles. Median age and follow up duration were 49.5 years and 1.8 years respectively. In those where volumetric imaging data was available, 14 (93.3%) had a radiological response to treatment, with a median tumor reduction of 83%. Grade 3 (severe) toxicities occurred in 12 patients, whilst grade 4 (potentially life threatening) toxicities occurred in two patients. Three patients stopped treatment due to adverse events (78).

 

MORBIDITY AND MORTALITY

 

Craniopharyngiomas are associated with significant long-term morbidity (mainly involving endocrine, visual, hypothalamic, neurobehavioral, and cognitive sequelae), which is attributed to the damage of critical structures by the primary or recurrent tumor and/or to the adverse effects of the therapeutic interventions. Notably, the severity of the radiation-induced late toxicity is affected by the total and per fraction doses, the volume of the exposed normal tissue, and the young age in childhood populations (1).

 

Endocrine

 

Long-term endocrine morbidity is significant. At last assessment, the rates of individual hormone deficits range between 88-100% for GH, 80-95% for FSH/LH, 55-88% for ACTH, 39-95% for TSH and 25-86% for ADH (1). Restoration of pre-existing hormone deficits following surgical removal is rare, and aggressive surgery leads to more frequent pituitary dysfunction (1,23,79).

 

The phenomenon of «growth without growth hormone» has been reported in some children with craniopharyngioma who show normal or even accelerated linear growth, despite their untreated GH deficiency. The pathophysiological mechanism has not been clarified; the obesity-associated hyperinsulinemia has been proposed as a factor stimulating growth by affecting serum concentrations of IGF-I or by binding directly to the IGF-I receptor (80,81). Review of adult patients with craniopharyngioma and severe GH deficiency but no recent GH treatment (from the KIMS database: Pfizer International Metabolic Database) has shown that those with childhood-onset disease were shorter than those with adult-onset disease, and obesity was more common in the adult-onset patients. Furthermore, quality of life, assessed by Quality of Life-Assessment of Growth Hormone Deficiency in Adults (QoL-AGHDA) and the Nottingham Health Profile, was markedly reduced with no significant differences between those with childhood-onset and those with adult-onset disease (82). A 3-year longitudinal analysis of the changes in height, weight, and body mass index (BMI) SDS in 199 GH-treated pre-pubertal children with post-surgical and/or post-irradiated craniopharyngioma showed that GH therapy induced excellent linear growth compared with children with other forms of organic GH deficiency. Still, the children with craniopharyngioma had a higher BMI; GH had no salutary effect on weight SDS and caused only a mild improvement in BMI SDS (83). A study of 351 patients with adult-onset craniopharyngioma compared with 370 patients with non-functioning pituitary adenomas matched for age and sex (all GH deficient) demonstrated that, after two years of GH replacement, there were significant similar improvements in both groups in free-fat mass, total and low-density lipoprotein and Quality of Life Assessment in the GH-deficient score compared with baseline. Results from a 12-year prospective study showed children with craniopharyngioma treated with GH had improved weight and quality of life outcomes compared to those who were not replaced, or in those who only received GH as adults (84). Observational studies have also shown that growth hormone replacement is not associated with increased risk of tumor recurrence.

 

Diabetes insipidus with an absent or impaired sense of thirst confers a significant risk of serious electrolyte imbalance, and is one of the most difficult complications to manage. In this group of patients, the maintenance of osmotic balance has been shown to be precarious with recurrent episodes of hyper- or hyponatremia contributing to morbidity and mortality. Careful fluid balance with close monitoring of intake/output and daily weights is crucial.

 

Vision

 

Visual outcome is adversely affected by the presence of visual symptoms at diagnosis and by daily irradiation doses >2 Gy (1). Radiation optic neuropathy occurs in 1–2% patients receiving doses to 50 Gy and this is mostly confined to those with pre-radiotherapy visual impairment, with the risk being higher with doses of 55 Gy and above (1,28).

 

Hypothalamic

 

Hypothalamic damage may result in hyperphagia and uncontrollable obesity, disorders of thirst and water/electrolyte balance, behavioral and cognitive impairment, loss of temperature control and disordered sleep pattern. Among these, obesity is the most frequent (reported in 26-61% of the patients treated by surgery combined or not with radiotherapy) and is a consequence of the disruption of the mechanisms controlling satiety, hunger, and energy balance (1,15,85-88). Possible contributing mechanisms include lack of sensitivity to endogenous leptin (89,90) and reduced energy expenditure, and is exaggerated by comorbidities including neurological defects, visual failure, somnolence (91), sleep disturbance, hypopituitarism and psychosocial disorders (92). In a study of 63 survivors of childhood craniopharyngioma, all those with marked obesity after surgery had evidence of significant alterations of the normal hypothalamic anatomy, with their MRI showing either complete deficiency or extensive destruction of the floor of the 3rd ventricle (93). Several image grading systems, used pre- or post-operatively, have been proposed to help predict hypothalamic sequalae and hypothalamic morbidity by defining hypothalamic involvement on imaging and severity of tumor adherence to the hypothalamus (94-98). Furthermore, it has been reported that the basal metabolic rate adjusted to total body weight is significantly lower in adults with craniopharyngioma compared with controls, and that the energy intake/basal metabolic rate ratio is significantly lower in subjects with tumor growth into the 3rd ventricle (99). Children with surgically-treated craniopharyngioma were found to have decreased aerobic capacity during an exercise test, which was most pronounced in those with hypothalamic involvement.  Interestingly, in this study, GH treatment was associated with significant positive effect on aerobic capacity only in the absence of hypothalamic involvement (100). Finally, high levels of the orexigenic gastric hormone ghrelin have not been found in these patients (101). Factors proposed to be associated with significant hypothalamic morbidity are young age at presentation, hypothalamic disturbance at diagnosis, hypothalamic invasion, attempts to remove adherent tumor from the region of hypothalamus, multiple operations for recurrence, and hypothalamic radiation doses >51 Gy (1,102,103). Interestingly, in a retrospective study including 45 adults with craniopharyngioma followed for a median of 26 months, a lower BMI pre-operatively was predictive of greater post-operative weight gain(104). In contrast, a higher pre-operative BMI has been found to be associated with severe post-operative obesity in children(18). Hypothalamic obesity often results in devastating metabolic and psychosocial complications, necessitating provision of dietary and behavioral modifications, encouragement of regular physical activity, psychological counselling, and anti-obesity drugs. Based on a limited number of published cases, gastric bypass surgery results in weight loss; in a systematic review and meta-analysis including 21 cases of bariatric surgery for hypothalamic obesity in patients with craniopharyngioma (6 with adjustable gastric banding, 8 with sleeve gastrectomy, 6 with Roux-en-Y gastric bypass and 1 with biliopancreatic diversion), it was shown that the maximal mean weight loss was achieved in the gastric bypass group after 12 months (105). Furthermore, Weismann et al. (106) in a series of 7 patients with morbid obesity after surgery for craniopharyngioma, who underwent laparoscopic gastric banding or laparoscopic sleeve gastrectomy, reported no significant loss of body weight. A case control study suggested that Roux-en-Y surgery, but not sleeve gastrectomy, yielded equivalent weight loss in craniopharyngioma patients to those with “common” obesity and resulted in significant reductions to BMI after one year (107). The same group subsequently conducted a larger, multi-center case control study, with a median follow up of 5.2 years (108). Obese patients with craniopharyngioma had a mean weight loss of 22% at 5 years after bariatric surgery; irrespective of type of procedure. In contrast to their original findings (107), obese controls lost more weight after Roux-en-Y gastric bypass, whereas sleeve gastrectomy led to similar results in both groups (108). Medical therapies including dextroamphetamine, the combination of diazoxide and metformin (aiming to reduce the hyperinsulinemia), octreotide (aiming to reduce hyperinsulinemia and simultaneously enhance the insulin action), glucagon-like peptide-1 analogues, and a novel methionine aminopeptidase 2 inhibitor, have all been proposed as potential approaches to this significant problem (92). However, outcomes following these therapies are variable and long-term benefits have not yet been established. Studies with large number of patients and longer follow-up are needed to establish the efficacy and safety of these surgical and medical management options.

 

Neuropsychological and Cognitive

 

The compromised neuropsychological and cognitive function in patients with craniopharyngioma after surgery and radiation therapy contributes significantly to poor academic and work performance, disrupted family and social relationships, disrupted body image, and impaired quality of life. Gross total resection, radiotherapy, pre-operative hypothalamic involvement, or intra-operative hypothalamic injury have been associated with a lower quality of life in adults and children with craniopharyngioma (109). It has also been proposed that visual, neurological, and endocrine morbidities negatively impact neuropsychological outcomes (110,111). Areas particularly affected (especially in childhood-onset disease) include memory, attention, executive function, and motivation (110,112-114), with hypothalamic involvement being a risk factor for poorer outcomes (113). In a series of 121 patients followed-up for a mean period of 10 years, Duff et al.(115) found that 40% had poor functional neuropsychiatric outcome. Karavitaki et al. (15), in a series of 121 patients, found cumulative probabilities for permanent motor deficits, epilepsy, psychological disorders necessitating treatment, and complete dependency for basal daily activities at 10-year follow-up of 11%, 12%, 15% and 9%, respectively. There is no consensus on the therapeutic option with the least unfavorable impact on the neurobehavioral outcome, necessitating prospective studies with formal neuropsychological testing and specific behavioral assessment before and after any intervention.  Such data will be particularly important for young children, as there are uncertainties including whether delaying irradiation is a reasonable policy in this age group. 

 

Long-Term Mortality

 

The mortality rates of patients with craniopharyngioma have been described to be 3-6 times higher than that of the general population and reported 10-years survival rates range between 83% and 93% (1,87). Qiao et al.(116) reported a significant fall in the SMR from 6.2 (95% CI 4.1-9.4) to 2.9 (95% CI 2.2-3.8) for studies published before 2010, and after 2010, respectively (116). Apart from the deaths directly attributed to the tumor (pressure effects to critical structures) and to the surgical interventions, the risk of cardio-/cerebrovascular and respiratory mortality is increased (1,117,118). In one study which included 244 patients with childhood onset craniopharyngioma, 11% of patients developed a cerebral infarction. Hydrocephalus and gross total resection were identified as risk factors, and none were attributable to radiotherapy (119). The increased cardiovascular mortality in this population may be driven in part by hypothalamic obesity and related metabolic complications. Long-term follow-up of adult patients with craniopharyngiomas has demonstrated increased prevalence of the metabolic syndrome compared with the general population (120), and hypothalamic involvement has been shown to have negative impact on mortality (121). It has been suggested that in childhood populations the hypoadrenalism and the associated hypoglycemia, as well as the metabolic consequences of ADH deficiency and absent thirst, may also contribute to the excessive mortality. The impact of tumor recurrence on the long-term mortality is widely accepted and the 10-year survival rates in such cases range between 29% and 70%, depending on the subsequent treatment modalities (15).

 

CONCLUSIONS AND FUTURE PERSPECTIVES

 

Craniopharyngiomas present many unique challenges for clinicians.  Whilst controversies regarding the optimal management approach for these rare tumors still exist, the need to prevent hypothalamic morbidities associated with surgical intervention in this area is essential.

Enhanced understanding of the pathogenesis of both adamantinomatous and papillary craniopharyngiomas has led to the concept of targeted medical therapy, an area at the forefront of translational research. Optimal outcomes following the use of BRAF V600 and MEK inhibitors have been described in case reports and are a promising treatment prospect, with hope that their efficacy and safety are supported by the results of large, prospective, randomized studies.

Patients face a high burden of post-treatment morbidity due to endocrine, visual, hypothalamic, and neuropsychological complications, and mortality rates are increased compared with the general population. Obesity is one of the most significant comorbidities with often devastating sequelae; its pathogenesis is multifactorial, and its management is one of the most challenging problems clinicians have to deal with. Given the complexity of these tumors, care for these patients should be provided by an experienced multidisciplinary team.

 

ACKNOWLEDGEMENTS

 

Radiology images provided courtesy of Dr. Swarupsinh Chavda, Consultant Neuroradiologist, Queen Elizabeth Hospital Birmingham, UK.

 

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Disorders of Growth Hormone in Childhood

ABSTRACT

 

Growth is a fundamental process of childhood and growth disorders remain one of the commonest reasons for referral to a pediatric endocrinologist. Growth can be divided into four phases – fetal, infancy, childhood and the pubertal phase with different hormonal components influencing growth at each stage. The GH-IGF1 axis plays a major role in the childhood phase of growth with a significant role alongside sex steroids during puberty while in infancy thyroid hormone and nutrition are vital. Although an uncommon cause of short stature disorders of the GH-IGF1 axis are extremely important due to the effectiveness of recombinant human growth hormone therapy for the child with GH deficiency (GHD). Here we review the diagnosis of growth hormone deficiency through a combination of auxology, biochemistry, imaging, and genetic testing. Particular focus is given to the accuracy of IGF-1/BP3 for diagnosis as well as the known problems with GH stimulation tests and GH assays. Isolated GHD is caused by mutations in GH1, BTK, and RNPC3 while GHD seen as part of multiple pituitary hormone deficiency is known to be caused by mutations in a wide variety of genes. A variety of structural malformations of the brain can be associated with congenital GHD with the commonest being the presence of an ectopic posterior pituitary or Septo-optic dysplasia. Acquired GHD is rarer and caused by tumors, radiotherapy, hypophysitis, and traumatic brain injury.  Treatment with recombinant human GH is highly efficacious in improving height in children with GH deficiency and extremely safe. Short stature disorders are, rarely, also caused by a variety of other disorders of the GH-IGF1 axis. Resistance to growth hormone is seen in Laron syndrome and in mutations in IGF1 and IGF1R while decreased bioavailability of IGF1 is seen in ALS deficiency and PAPPA2 deficiency. Treatment with recombinant human IGF1 (rhIGF1) is available for those with IGF-I deficiency caused by either Laron syndrome or IGF1 mutations. rhIGF1 is effective in improving height but treatment is less effective than the use of GH to treat GH deficiency.  The role of IGF1 in pre-natal growth is highlighted by the phenotype of patients with IGF1R or IGF1 mutations where pre-natal growth is commonly impaired and children born small for gestational age. GH excess is much rarer than GH deficiency in childhood and can be caused by pituitary adenomas, optic nerve gliomas (seen predominantly with NF1), McCune Albright syndrome, or Carney complex. Treatment is with surgery, somatostatin analogs, or GH receptor antagonists.

 

CASE STUDY

 

 A 5-year-old girl was referred to her local community pediatrician by her health visitor with concerns about growth and poor calorie intake. Height at presentation was 91.5 cm (-4.1 SD) with weight 12.5 kg (-3.4 SD) and head circumference 48.8 cm (-2.5 SD).  Her teeth were affected by multiple caries which made chewing hard foods painful and she therefore ate only soft foods. Development was reported to be normal and she was performing well in school. Her parents had noticed loud snoring and tonsils were enlarged on examination.

 

She was born at term by vaginal delivery with a birth weight of 3.5kg and was the youngest of 6 children. The parents were consanguineous (first cousins) and there was a family history of short stature in distant cousins. Mother was 147 cm tall (-2.7 SD) and father 165.1 cm (-1.5 SD). There was a history of diabetes mellitus type 2, diabetic nephropathy and thalassemia in mother and the father had a history of recurrent kidney stones. 

 

On review in the endocrinology clinic prominent forehead, depressed nasal bridge and a high-pitched voice were noted.  General investigations (detailed below) were normal; however, IGF-I and IGFBP-3 concentrations were low with high basal GH and peak GH concentrations (the latter >40µg/L). The combination of low IGF-I with raised GH concentrations suggested a diagnosis of GH insensitivity. In view of the history of snoring the patient was referred to an ENT surgeon who noted large prolapsing tonsils with mild apneic episodes on sleep study. Due to the propensity of IGF-I therapy to induce tonsillar hypertrophy, she underwent tonsillectomy.

 

Treatment with recombinant human IGF-I was started at the age of 6 years and 1 month initially with 0.6 mg (38 mcg/kg/) BD, increasing after 1 week to 1.1 mg (70 mcg/kg) BD and then to 1.7 mg (108 mcg/kg) BD. There were no problems with hypoglycemia. Height velocity increased from 3.6 cm/year to 10.3 cm/year over the first year of treatment. Sequencing of the GH receptor identified a known intronic point (A>G) mutation between exons 6 and 7 in which leads to inclusion of a pseudoexon and an additional 36 amino acids in the extracellular domain of the GHR.

 

At the age of 9 years and 3 months she was noted to be at breast stage 3 and in order to preserve height potential she has been treated with GnRH analogue (Zoladex LA). The IGF-I dose has been increased to maintain dose in the range 100 – 120 mcg/kg/BD and at 10 years 3 months height is 125.8 cm (-2.1 SD) with weight 32 kg (-0.2 SD). There has been some lipophypertrophy around the injection sites and she required an adenoidectomy due to a recurrence of her snoring (with daytime somnolescence) caused by a large obstructing adenoidal pad.

 

Baseline Investigations

Serum electrolytes, urea, creatinine, liver function tests, calcium, phosphate, hemoglobin – all normal

Karyotype 46 XX

TSH 2.2 mU/L (0.3 -5.0) free T4 17 pmol/L (11 - 24)

Prolactin 174 mU/l (85 – 250)

IGF-I <25 ng/mL (55 – 280)

IGFBP-3 0.7 mg/L (1.5 – 3.4)

ALS 3.2 mg/L (2.3 – 11)

Fasting glucose 4.0 mmol/L Insulin 2.1 mIU/L (2.3 - 26)

Skeletal survey – no evidence of skeletal dysplasia

Bone Age delayed by 18 months

 

Arginine stimulation Test

Time (min)       Growth Hormone (µg/L)

-15                   19.3

0                      4.0

15                    4.8

30                    14

60                    >40

90                    >40

120                  15.6

 

Standard Synacthen Test

Time (min)       Cortisol (nmol/L)

0 min               213

30 min             624

60 min             742

 

GnRH Test at age 5 years

Time                LH (IU/L)         FSH (IU/L)

0                      <0.1                 1.7

30                    2.7                   14

60                    3.3                   18

 

INTRODUCTION

 

Growth is a fundamental process of childhood. It can be divided into four phases – fetal, infancy, childhood, and pubertal growth. Although growth occurs as a continuum, the endocrine control of each phase is distinct. The fetal phase includes the fastest period of growth with a crown-rump velocity of 62cm/year during the second trimester. Growth during this phase is dependent upon placental function and maternal nutrition in addition to hormonal factors especially IGF-I, IGF-II and insulin (1,2).  Although size at birth (and hence fetal growth) is profoundly affected by IGF-I deficiency during fetal life (3), the effects of congenital GH deficiency are much less marked with a mild reduction in birth size (4). 

 

Fetal Phase

 

During the first year of life, growth declines from an initial velocity of around 25cm/year to around 10cm/year. Previously it has been thought that during this period growth hormone did not have a significant influence on growth however it is now clear that children with growth hormone deficiency display reduced height velocity from birth (5). In addition to growth hormone, thyroid hormone and adequate nutrition are vital for normal growth during infancy.

 

Infancy Phase

 

During the first two years of life there is a significant period of catch-up or catch-down growth so while size at birth is not well correlated with parental height, by two years of age the correlation between parental and child heights significantly improves (6). It has been hypothesized that this catch up growth is the result of a central mechanism which detects the difference between the actual and expected size and acts to increase growth velocity (7). No experimental evidence exists for this hypothesis. The second hypothesis on the origin of this catch up/down growth is that it arises from alterations in growth plate senescence. Catch down growth is associated with a reduction in the number of stem cell divisions within the growth plate while catch up growth would be due to a compensatory increase in the number of stem cell divisions within the growth plate (8).

 

Childhood Phase

 

There is a gradual transition from the infancy phase into the childhood phase of growth from 6 months to 3 years of age. Prepubertal growth velocity is relatively constant between 4-7 cm/year with the lowest growth velocity of life occurring immediately before the onset of puberty. During childhood growth is mainly controlled by the influence of the GH-IGF-I axis along with thyroid hormone.

 

Pubertal Growth

 

The final phase of growth is puberty – the period of transition from the pre-pubertal state to the full development of secondary sexual characteristics and achievement of final height. Puberty begins with the onset of activity within the hypothalamic-pituitary-gonadal axis leading to the production of androgens (in males) and estrogen (in females). In males the first sign of pubertal development is enlargement of the testes while in females it is development of breast buds. The production of androgens and estrogen is associated with an increase in activity within the GH-IGF-I axis. Administration of testosterone to boys increased both GH and IGF-I concentrations (9) but this effect is dependent upon aromatization as co-administration of an estrogen receptor antagonist (10) or administration of dihydrotestosterone (11) (the active form of testosterone that cannot be aromatized) does not lead to an increase in GH or IGF-I concentrations. In girls there is also an increase in IGF-I levels and GH secretion during puberty but the mechanisms underlying this are less clear. Administration of oral or transdermal estrogen induces a decline in serum IGF-I concentrations and a consequent increase in GH secretion (12).  

 

Fusion of the epiphyseal growth plates is induced by the activity of estrogen on ERα as patients with mutations in the genes encoding Erα (13) or aromatase enzyme (14) result in failure of fusion of the epiphyses and tall stature.

 

This chapter will firstly discuss the physiology of the GH-IGF-I axis along with signal transduction of GH and IGF-I and then consider the diagnosis and treatment of growth hormone deficiency before discussing individual pathological conditions associated with both GH deficiency and GH excess. Disorders leading to GH deficiency have been divided into congenital and acquired. 

 

GH-IGF-I AXIS

 

Physiology of the GH-IGF-I Axis

 

Release of Growth Hormone Releasing Hormone (GHRH) from the hypothalamus regulates the secretion of GH from the anterior pituitary both by increasing GH1 gene transcription and by promoting the secretion of stored GH. GHRH release is pulsatile and influenced by somatostatin and Ghrelin. Ghrelin is a 28 amino acid peptide produced in the stomach (15) and acts via the GH secretagogue receptor (GHSR). The active hormone is the octanoylated form produced by Ghrelin O-acetyltransferase(16) and is cleaved from the 117 amino acid preprohormone. In addition to the role in GH secretion Ghrelin also acts as an appetite stimulant (17) and stimulates the secretion of insulin (18), ACTH (19), and prolactin (19). In vivo the action of Ghrelin requires an intact GHRH system to influence GH secretion (20) but in vitro is capable of directly stimulating GH (15). Somatostatin is a peptide derived from pre-pro-somatostatin within neurons of the anterior periventricular nucleus which project to the median eminence. There are two main forms of somatostatin – 14 and 28 amino acid variants.  It acts via the somatostatin receptors of which there are 5 subtypes (SSTR1-5). The anterior pituitary expresses SSTR1, 2, 3 and 5 (21). Somatostatin acts to decrease the secretion of GH by inhibiting GHRH secretion, directly inhibiting GH secretion in the anterior pituitary (22), antagonizing the activity of Ghrelin (20) as well as inhibiting its secretion (23). Somatostatin tone determines trough levels of GH and reductions in somatostatin tone are a major factor in determining the time of a pulse of GH.  GH secretion is also stimulated by hypoglycemia and exercise. A summary of the factors influencing GH secretion is given in Figure 1.

 

GH is released from the somatotrophs of the anterior pituitary in a pulsatile manner with the pulses predominantly overnight, increasing in amplitude with age (24). The pulse amplitude is maximal in the pubertal years consistent with the raised IGF-I levels and growth velocity at this time (25).  In males there is greater diurnal variation in peak amplitude, with higher peaks overnight and a lower baseline GH level compared to females. Overall GH production is higher in females. GH peak amplitude is linked to IGF-I concentrations while nadir GH is linked to waist-hip ratio (26).  

 

Growth Hormone and GH signal Transduction

 

Growth Hormone (GH) is encoded  by the GH1 gene located at chromosome 17q23.3 and is a 191 amino acid single chain polypeptide (27). There are 20 and 22kDa isoforms of GH generated by alternative splicing (the smaller isoform lacks amino acids 32-46) with the 20kDa accounting for around 10-20% of circulating GH (28).  While GH1 is expressed within the anterior pituitary a 20kDa variant of GH is encoded by the GH2 gene but this is expressed in placenta and not in the pituitary (29).

Figure 1. Physiology of the GH-IGF-I Axis. Release of GHRH from the hypothalamus is under the control of somatostatin (inhibitory) and Ghrelin (stimulatory). Alterations in GHRH tone led to pulsatile release of GH from the anterior pituitary. GH has widespread effects on muscle, fat and in the growth plate. IGF-I is produced in liver and in local tissues in response to GH stimulation. Red lines indicate feedback loops. Figure reproduced and adapted from Butcher I Molecular and Metabolomic Mechanisms Affecting Growth in Children Born Small for Gestational Age PhD thesis University of Manchester 2013.

In the circulation GH is bound to Growth Hormone Binding Protein (GHBP). GHBP is generated either by proteolysis cleavage of the extracellular domain of the growth hormone receptor (GHR) by metzincin metalloproteinase tumor necrosis factor-α converting enzyme (30) or by alternative splicing of the GHR (31). The 22kDa isoform of GH has the highest affinity for GHBP with the 20kDa and placental GH having a lower affinity (32).  GHBP has a molecular mass of 60kDa and acts to prolong the half-life of GH with an increase from 11 minutes to 80 minutes (33). GHBP also acts to maintain the circulating pool of GH within the vasculature (34), reducing the ability of the circulating pool of GH to bind to peripheral GHRs.

 

The actions of GH are mediated via the GHR, a 620 amino acid protein containing a 246-residue extracellular domain, a single24 amino acid transmembrane helix and a 350 amino acid intracellular domain. The GHR gene is located on chromosome 5p13 and contains 10 exons. The GHR exists in a pre-dimerized form on the cell surface. In contrast to previous models, it is now recognized that dimerization per se is insufficient to initiate signaling (35).  GH binds to the GHR via two binding sites – initial binding is via the high affinity site 1 followed by binding to the low affinity binding site 2 (36). GH binding induces a conformational change in the dimerized GHR including rotation of one of the GHR subunits (see Figure 2).  This results in locking together of the extracellular receptor-receptor interaction domain and repositioning of the box 1 motifs in the intracellular domain increasing the distance between them. In turn this leads to repositioning of tyrosine kinases, including JAK2 (37). This repositioning is crucial to JAK2 activation. In the inactive state two JAK2 molecules (each attached to one of two dimerized GHRs) are positioned so that the kinase domain of one JAK2 molecule interacts with the inhibitory pseudokinase domain of the other JAK2 molecule. After repositioning, due to the conformational change induced by GH binding, the inhibitory kinase-pseudokinase interaction is lost and the kinase domains of each JAK2 molecule interact with each other leading to JAK2 activation (38).

 

Activation of the GHR results in JAK2 mediated phosphorylation of the signal transducers and activator of transcription proteins (STAT), including STAT1, STAT3, STAT5A and STAT5B. STAT5A and 5B are recruited to the phosphorylated GHR where their Src homology 2 (SH2) domain is phosphorylated by JAK2. STAT5A/B then homo- or heterodimers and translocate to the nucleus (37,39) (see Figure 3). Activation of STAT1 and STAT3 is also via phosphorylation by JAK2 but this does not require recruitment to the GHR.  JAK2 also phosphorylates the Src homology domain of SHC (leading to activation of the mitogen activated protein kinase pathway) and the insulin receptor substrates (IRS-1, IRS-2 and IRS-3), which, in turn activate phosphatidylinositol-3 kinase and induces translocation of GLUT4 to the membrane. In addition to activation of JAK2, activation of the GHR also leads to direct activation of the Src family kinases, which are capable of activating the mitogen activated protein kinase pathway (40), and activation of protein kinase C via phospholipase C. Activation of protein kinase C stimulates lipogenesis, c-fos expression and increases intracellular calcium levels by activating type 1 calcium channels.

Figure 2. Growth hormone binding to the extracellular domain of the growth hormone receptor reorients the pre-existing homodimer so that one growth hormone receptor subunit rotates relative to the other. This structural reorientation is transmitted through the transmembrane domain resulting in a repositioning of tyrosine kinases bound to the cytoplasmic domain of the receptors. The distance between the box 1 motifs increases between inactive and active states and this movement is fundamental to activation of JAK2. Phosphorylation of JAK2 in turn leads to phosphorylation of STAT molecules, activation of the MAPK cascade and activation of IRS-1. STAT5a and STAT5b homo/heterodimerize and translocate to the nucleus. Figure kindly supplied by Dr Andrew Brooks, Institute for Molecular Bioscience, The University of Queensland.

GH signal transduction is regulated via several mechanisms: JAK2 is autoinhibitory with the pseudokinase domain inhibiting the catalytic domain (41), SHP1 binds to and dephosphorylates JAK2 in response to GH and GH also phosphorylates the transmembrane signal regulatory glycoprotein SIRPα1 which dephosphorylates JAK2 and the GHR.

 

The net result of GH signal transduction is the transcription of a set of GH dependent genes and the production of IGF-I the combination of which mediates the actions of GH including effects on cell proliferation, bone density, glucose homeostasis and serum lipids.

 

Insulin Like Growth Factors, Their Binding Proteins and Signal Transduction

 

INSULIN LIKE GROWTH FACTORS

 

The two insulin-like growth factors, IGF-I and IGF-II, are single chain polypeptide hormones sharing 50% homology with insulin. IGF-I is a 70 amino acid 7.5 kDa protein with four domains – A, B, C and D. The prohormone also contains a c-terminal peptide that is cleaved in the Golgi apparatus before secretion. IGF-II is a 67 amino acid peptide also with a molecular weight of 7.5 kDa. The mitogenic and, in part, the metabolic effects of GH are mediated via IGF-I rather than IGF-II.  The IGFs circulate bound to the IGF binding proteins (IGFBPs), of which there are six classical high affinity IGFBPs. The IGFs form a ternary complex with an IGFBP and the Acid Labile Subunit (ALS), an 85kDa protein secreted by the liver.  99% of serum IGF-I is bound to a ternary complex which acts to prolong the half-life of IGF-I (42). IGF-I is produced in both the liver and in peripheral tissues and thus can act in an autocrine and paracrine manner.

 

IGF RECEPTORS

 

The IGF-1R is a transmembrane heterotetramer consisting of consisting of two extracellular α chains and two membrane-spanning β chains linked by several disulphide bonds (43). Ligand binding sites are present in the α subunits while the β subunits contain the juxtamembrane domain, tyrosine kinase domain and a carboxy terminal domain (44). Ligand binding to the α subunit activates the intrinsic tyrosine kinase activity of the β subunit which leads to autophosphorylation of tyrosine kinases in the juxtamembrane, tyrosine kinase and carboxy terminal domains. This autophosphorylation provides docking sites for substrates including the insulin receptor substrates (IRS-1, -2, -3, -4) and Shc. IRS-1 and Shc recruit the growth factor receptor bound protein 2 that associates with son of sevenless to activate the MAPK pathway. IRS-1 also activates PI3K via its regulatory subunit, p85, leading to activation of AKT which phosphorylates BAD and activates mTOR leading to inhibition of apoptosis and stimulation of proliferation. A summary of IGF-I signal transduction is given in Figure 3.

 

Mouse studies have delineated the relative contribution to growth of the GH-IGF system – deletion of Igf1 or Igf2 results in a 40% reduction in birth weight with a reduction of 55% where Igf1r is deleted (45). Deletion of Igf1 with Igf1r or Igf2 leads to a 70% reduction in birth weight and death from respiratory distress at birth (45) whereas the Igf2r appears to negatively regulate growth as deletion of this gene results in an increase in size to 130% of wild type. IGF-I is produced in both the liver and in peripheral tissues and thus can act in an autocrine and paracrine manner. It appears that autocrine/paracrine IGF-I is more important for growth than liver derived IGF-I as a hepatic specific deletion of Igf1 in mouse resulted in no impairment of growth despite a 75% reduction in serum IGF-I concentrations (46) while a triple liver specific deletion of Igf1/Igfals/Igfbp3 resulted in a 97.5% reduction in circulating IGF-I concentrations with a 6% decrease in body length (47).

Figure 3. IGF-I Signal Transduction. Binding of IGF-I leads to phosphorylation and activation of IRS-1 which, in turn, activates the PI3K and MAPK pathways.

DIAGNOSIS OF GROWTH HORMONE DEFICIENCY IN CHILDHOOD

 

The diagnosis of growth hormone deficiency in childhood is multifactorial process and includes 1) auxological assessment 2) biochemical tests of the GH-IGF-I axis and 3) radiological evaluation of the hypothalamus and pituitary (normally with MR imaging). Prior to evaluation of the GH-IGF-I axis in a short child other diagnosis such as familial short stature, hypothyroidism, Turner syndrome, chronic illness such as Crohn’s disease and skeletal dysplasias should be considered and evaluated appropriately. Patients to be evaluated for growth hormone deficiency include (48,49):

 

  1. Severe short stature (defined height >3 SD below mean)
  2. Height more than 1.5 SD below mid parental height
  3. Height >2 SD below mean with height velocity over 1 year >1 SD below the mean for chronological age or a decrease of more than 0.5 SD in height over 1 year in children aged >2 years
  4. In the absence of short stature – a height velocity more than 2 SD below mean over 1 year or >1.5 SD below mean sustained over 2 years
  5. Signs indicative of an intracranial lesion or history of brain tumor, cranial irradiation, or other organic pituitary abnormality.
  6. Radiological evidence of a pituitary abnormality
  7. Signs and/or symptoms of neonatal GHD

 

Etiology

 

Disorders of GH can be divided into those that cause growth hormone deficiency or growth hormone excess. In childhood growth hormone deficiency is rare with an incidence of 1 in 4000 while the incidence of childhood GH excess is not known but only around 200 cases have been reported in the literature (50).  Causes of GH deficiency are listed in Table 1.

 

Table 1. Causes of Growth Hormone Deficiency

Cause

Examples

Idiopathic

 

 

Genetic

GHRHR mutations

GH1 mutations

Structural brain malformations

Pituitary stalk interruption syndrome

Rathke’s cyst

Agenesis of corpus callosum

Septo-optic dysplasia

Holoprosencephaly

Midline Tumors

Craniopharyngioma

Optic nerve Glioma

Germinoma

Pituitary adenoma

Cranial Irradiation

 

 

Traumatic Brain Injury

Road Traffic Accident

 

CNS infections

 

 

Inflammation and Auto-immunity

Sarcoidosis

Langerhans Cell Histiocytosis

Hypophysitis

Psychosocial deprivation

 

 

Clinical Presentation of GH Deficiency

 

GH deficiency can present either in isolation (isolated GHD - IGHD) or in combination with other pituitary hormone insufficiencies (multiple pituitary hormone deficiency - MPHD). In the neonatal period MPHD typically presents with reduced penile size, episodes of hypoglycemia, and prolonged unconjugated hyperbilirubinemia. MPHD is associated with breech delivery, adverse incidents in pregnancy, and admission to the newborn intensive care unit (51).  Children with severe growth hormone deficiency often appear young for their age and have midface hypoplasia and increased truncal adiposity (see Figure 4). The major clinical feature of GH deficiency is growth failure; typically, this occurs after the first year of life but may be apparent earlier in severe GHD. The earliest manifestations are a reduction in height velocity followed by a reduction in height standard deviation score (SDS) adjusted for mean parental height SDS. The child’s height SDS will ultimately fall below -2SD with the time taken to achieve this depending on the severity and duration of GHD.

Figure 4. Child with Laron syndrome. Short stature with typical facial appearance of GH insensitivity with midface hypoplasia, this finding is common to GH deficiency as well.

Biochemical Assessment of the GH-IGF-I Axis

 

Multiple assays have been developed to measure GH in serum. A consensus statement of the GH-IGF-I research society in 2000 recommended that assays used should use monoclonal antibodies to measure the 22kDa variant of human GH and that the reference preparation should be the WHO standard 88/624 (a recombinant human 22kDa GH at 3 IU = 1mg) (48,52).

 

Growth Hormone Stimulation Tests and GH Profiles

 

A number of growth hormone stimulation tests have been developed and can be divided into screening tests or definitive tests. Screening tests include exercise, fasting, levodopa, and clonidine and are characterized by low toxicity, ease of administration but low specificity. Definitive tests include the insulin tolerance test, glucagon, and arginine stimulation tests. Using the auxological criteria above a peak GH concentration below 10µg/L has traditionally been used to support the diagnosis of GHD. GHD is not a dichotomous state but exists as a continuum from severe GHD to normality and there is known to be an overlap in peak GH concentrations between normal children and those with GHD.  For this reason, and due to the advent of more sensitive monoclonal antibodies based on the recombinant human GH reference standard, some units will use a more stringent cut-off for the diagnosis of GHD e.g., 7µg/L. Where the diagnosis is isolated idiopathic GHD two pharmacological tests are required. Only one provocative test of GH secretion is required in children with one or more of the following criteria:

 

  1. Central nervous system pathology affecting the pituitary or hypothalamus
  2. A history of cranial irradiation
  3. An identified pathological genetic variant known to be associated with GHD
  4. Multiple pituitary hormone deficiency

 

INSULIN TOLERANCE TEST

 

The gold standard test is considered to be the Insulin Tolerance Test.  This test relies upon an intravenous dose of insulin to induce hypoglycemia with a subsequent rise in GH expected as part of the counter regulatory response to hypoglycemia (53). Cortisol secretion also rises in response to hypoglycemia and thus this test also assesses the hypothalomo- pituitary-adrenal axis. The patient is required to fast overnight and, in the morning, a reliable intravenous line is inserted following which an insulin dose of 0.1units/kg is administered. The dose is reduced to 0.05 units/kg in children under 4 and where there is known or likely multiple pituitary hormone deficiency. This test is generally not recommended for infants and in this group the dose of insulin would be reduced further to 0.01units/kg. After administration of insulin there is careful bedside monitoring of blood glucose concentration and once the blood glucose has reached <2.6 mmol/L (47 mg/dL) the patient eats a high carbohydrate meal. Administration of 10% glucose at 2ml/kg may be required in order to restore adequate blood glucose concentrations. This should be prepared in advance of the start of the test along with an appropriate dose of IV hydrocortisone (this should be given after hypoglycemia where there is known adrenal insufficiency or where hypoglycemia is more severe or prolonged than expected). 50% dextrose is recommended by some for the correction of hypoglycemia during the test but administration of such hyperosmolar solutions has been associated with adverse outcome (54) including cerebral edema. Due to the risks associated with this test it should only ever be performed in a center with appropriate experience.

 

GLUCAGON TEST

 

The glucagon test is one of a number of safer alternative GH provocation tests. Intramuscular administration of glucagon leads to an increase in GH due to a rise in insulin levels compensating for the increase in serum glucose (55). Maximum GH peak occurs 2-3 hours after injection of glucagon. Although less common than with the insulin tolerance test hypoglycemia can occur with the glucagon stimulation test where there is an excessive insulin response. There should therefore be blood glucose monitoring throughout the test and a meal consumed at the end of the test. Nausea and vomiting are other common side effects.

 

ARGININE STIMULATION TEST

 

Arginine administration stimulates the release of GH by inhibiting somatostatin release. Following an overnight fast arginine is administered intravenously at 0.5g/kg (maximum dose 30g) over 30 minutes. Unlike glucagon or insulin, arginine does not directly cause hypoglycemia and thus the arginine stimulation test may be safer, particularly for those patients with predisposition to hypoglycemia. Examples of patients where an arginine test would be suitable where the insulin or glucagon-based tests would not be suitable include patients with diabetes and a history of seizures or children with disorders of cerebral glucose uptake (GLUT2 deficiency) where the patient should be continuously ketotic. Arginine can be combined with L-dopa or GHRH. For combined tests, particularly the arginine-GHRH test it is important to have a test specific cut off for the diagnosis of GHD as with a powerful stimulus of GH secretion a higher cut off is required (a normal peak GH response for arginine-GHRH has been defined at 19-120 µg/L(56)).  GHRH can be used on its own as a provocative agent but is greatly affected by variations in somatostatin tone leading to a highly variable response. In addition, false negative tests may occur in children with hypothalamic damage.

 

Oral agents used in GH stimulation tests include clonidine and L-Dopa. Both clonidine and L-Dopa act by increasing adrenergic tone to increase GHRH and decrease somatostatin levels. A fast of 6 hours is required prior to the test. Since clonidine is a drug used to lower blood pressure hypotension is a potential side effect. Drowsiness is also a frequent occurrence during this test.

 

INTERPRETATION

 

Significant problems exist with GH stimulation tests – peak GH varies according to the stimulus used (57), false positive results in normal pre-pubertal children are frequent (56), the tests have poor reproducibility and there is also variability in GH level with GH assay used (58). Peak GH is also reduced in obesity and for adults BMI specific cut-offs for the diagnosis of GHD have been developed (59).

 

Low GH levels to provocation tests frequently occur in the immediate peripubertal period. Given the known action of the sex steroids to augment endogenous GH secretion this has led some pediatric endocrinologists to prime children of peripubertal age but without clinical signs of puberty undergoing GH stimulation testing with exogenous sex steroids (diethylstilbestrol, ethinylestradiol and testosterone can be used). Around 50% of pediatric endocrinologists routinely use priming for GH stimulation tests(60). Some endocrinologists will prime boys >9 years and girls >8 years others will prime only those with a delayed puberty >13-14 years in boys and > 11 or 12 years in girls. In one study by Marin et al(61) where 61% of healthy prepubertal children failed to demonstrate a peak GH >7µg/L to three GH provocative tests (exercise, insulin and arginine) but after administration of estrogen 95% of these children demonstrated a peak GH >7 µg/L. Multiple other studies have confirmed this result in healthy peripubertal children with growth impairment (62).  Thus, the argument in favor of priming is that it prevents false positive diagnoses of GHD in this group. The concerns about priming are that it only briefly augments the GH response which then returns to suboptimal levels which may be insufficient for normal growth. Thus priming may result in failure to treat children with transient peripubertal GH deficiency who would have benefitted from treatment (62).

 

24 hour or overnight 12-hour GH profiles with measurement of serum GH every 20 minutes have been proposed as an alternative assessment of GH secretion. The obvious disadvantages are the large number of samples required and costs, particularly of the overnight hospital admission. While a 24 hour GH profile has a high reproducibility there is also a large degree of inter individual variability limiting the usefulness of the procedure as a diagnostic test (63).

 

A diagnosis of GH neurosecretory dysfunction can be made where the patient presents with signs/symptoms of GHD with low IGF-I concentration, a normal peak GH level to pharmacological stimulation but absence of spontaneous GH peaks on 24 hour serum GH profile (64). This diagnosis has not been identified in adults and given the interindividual variability in 24-hour GH profiles caution should be made before coming to GH neurosecretory dysfunction as a diagnosis, particularly where there is no history of cranial irradiation.

 

Measurement of IGF-I and/or IGFBP-3

 

IGF-I and IGFBP-3 are, unlike GH, present at relatively constant concentrations in serum throughout the day and can therefore be measured by a simple blood test without the need for pharmacological stimulation. IGF-I is suppressed in states of poor nutrition and both IGF-I and IGFBP-3 concentrations vary with age and pubertal stage, thus normative ranges taking into account age, Tanner stage, and BMI have been recommended (52). The majority of IGF-I exists bound in the ternary IGF-I/IGFBP-3/ALS complex (thus free IGF-I is very low and difficult to measure) and assays therefore require a step to remove the IGF binding proteins before measurement of total IGF-I. Incomplete removal of IGF-I can potentially lead to false low IGF-I concentrations. Both IGF-I and IGFBP-3 have a low sensitivity (~50%) with a high specificity (97%) (65,66) and thus are of limited value in isolation. They do, however, form a vital component of the assessment of a child for GHD combined with auxological, other biochemical and radiological data.

 

Neuroimaging

 

Identifying abnormalities of the hypothalamo-pituitary axis provides powerful evidence for the diagnosis of GH deficiency in the short child. The most common abnormality identified in congenital GHD is the so-called pituitary stalk interruption syndrome consisting of a variable combination of anterior pituitary hypoplasia, ectopic posterior pituitary, and thinning or interruption of the pituitary stalk (67). Loss of the vascular pituitary stalk increases the risk of MPHD 27-fold but required gadolinium-DTPA administration to reliably distinguish presence/absence of vascular stalk (68). Other potential findings in congenital GHD include

 

  1. Septo-optic dysplasia – combination of absence of septum pellucidium, optic nerve hypoplasia and hypopituitarism. May be associated with an ectopic posterior pituitary and anterior pituitary hypoplasia.
  2. Abnormalities of the corpus callosum – agenesis, corpus callosum cysts
  3. Holoprosencephaly
  4. Eye abnormalities – microphthalmia or anophthalmia (GLI2 or OTX2 mutations)
  5. Absent olfactory bulbs (FGFR1, FRF8 and PROKR2 mutations)
  6. Pituitary hyperplasia (seen in patients with PROP1 mutations)
  7. Hypothalamic hamartoma (Pallister-Hall syndrome)
  8. Empty sella
  9. Absence of the internal carotid artery
  10. Arnold-Chiari malformations
  11. Arachnoid cysts
  12. Syringomyelia

 

In acquired GHD tumors affecting the hypothalamo-pituitary axis will frequently be identified – craniopharyngiomas, adenomas, and germinomas. Thickening of the pituitary stalk may be identified in Langerhans cell histiocytosis.

 

As well as a role in the diagnosis of GH deficiency MR imaging can also help predict which patients will require re-testing of growth hormone status at the end of growth. Young adults with MRI abnormalities have an increased risk of persisting GHD into adulthood (69).

 

GH Therapy

 

All children diagnosed with GH deficiency should be treated with recombinant human growth hormone as soon as possible after the diagnosis is made. The aim of treatment is to normalize height – both to within the normal range for the population and to achieve a height within the child’s target range. GH is administered as a once daily subcutaneous injection in the evening. Starting dose is usually in the range of 25-35µg/kg/day with maximum dose being 50µg/kg/day. In children with more severe GHD (evidence by a lower peak GH level, more severe presentation, MRI abnormality) the response to GH is better and often height can be normalized with lower doses of GH e.g., 17-35 µg/kg/day (70). Prediction models (discussed below) are available and in GHD have been shown to reduce variability in response but do not improve height gain (71). Children receiving GH therapy should be seen every 3-6 months and the GH dose titrated to height velocity and height gain. Monitoring of IGF-I concentrations is recommended to avoid prolonged periods of supraphysiological IGF-I levels. In general, IGF-I should be measured at least annually but can be measured more frequently particularly where there has been a recent increase in dose. A reduction in dose would normally be considered were two consecutive IGF-I levels were above +2 SD. As a guide to dose adjustment a 20% alteration in dose leads, on average, to a 1 SD change in IGF-I concentration (72).  Treatment is continued until the child is post-pubertal and growth is either completely ceased or is <2cm per year.  A growth chart from a child with congenital GHD treated with recombinant human GH therapy is shown in Figure 5.

 

Currently there is no single accepted definition of poor response to GH treatment with suggestions including change in height SDS <0.3 or 0.5 during the first year of treatment, change in height velocity <+3cm/year during 1st year of treatment, change in height velocity <+1SD or a height velocity <-1 SD during the first year of therapy.  Depending on the definition used 20-35% of patients display a poor response (73). It is important to discuss the possibility of a poor response with the family prior to staring therapy.

Figure 5. Growth Chart from child with GH deficiency. GH therapy is started at age 4 with height SDS -3.7 SD. There is a sustained improvement in height velocity leading to a final height of +1.5 SD.

Multiple long-acting preparations of growth hormone are at various stages of development (74). A phase three trial in adults with GHD have been completed and has demonstrated similar efficacy with a once weekly injection of a long-acting GH compared to conventional daily GH (75). Trials in children are currently ongoing.

 

Prediction of Response to GH Therapy and the d3-Growth Hormone Receptor Polymorphism

 

Initial work predicting the response to GH therapy was based on auxological and biochemical data, particularly from the Kabi International Growth Study (KIGS), a large surveillance study of over 62,000 patients treated with GH in childhood. Prediction models developed included models for idiopathic isolated GH deficiency (76) and early onset isolated GH deficiency (77).  For the idiopathic isolated GH deficiency prediction model the model explained 61% of the variability on GH response. Factors included in the prediction model were peak GH during stimulation test, age at start of GH therapy, height SDS minus mean parental height SDS, growth hormone dose and weight SDS. Other prediction models derived from alternative datasets have also been produced for GHD (78,79).

 

Around 50% of the European population are homo- or heterozygous for a polymorphism of the GHR that leads to deletion of exon 3 and 22 amino acid residues near the N-terminal. In 2004 it was reported that GH signaling via the GHR with the d3 was increased and that children treated with GH under the SGA license or with idiopathic short stature showed an increased first year growth velocity where they were homo- or heterozygous for the d3 polymorphism (80). Since this original report there have been many studies assessing the effect on the d3 polymorphism on response to GH therapy in GH deficiency, Turner syndrome, SGA children and in children with idiopathic short stature. A meta-analysis of these studies in 2011 indicated that, compared to children homozygous for the full-length allele, children homozygous for the d3 polymorphism have an increase in 1st year height velocity SDS of 0.14 SD and children heterozygous for the d3 polymorphism has an increase of 0.09 SD (81).  Thus, it appears that the d3 polymorphism has a modest effect mediating the response to GH therapy.

 

The PREDICT study was a large international observational study which assessed the contribution of single nucleotide polymorphisms in over 100 candidate genes to GH response in a cohort of children with GH deficiency or Turner syndrome (82,83). GH response was assessed by change in IGF-I concentrations over 1 month and by height velocity change over the first year of treatment. Carriage of 10 polymorphisms within 7 different genes, related in particular to cell signaling, were identified to be associated with change in IGF-I over the first month of GH treatment and height velocity over the first year of treatment. In addition to assessing association between genotype and response to GH therapy the PREDICT study also assessed the use of basal gene expression in peripheral blood mononuclear cells to predict GH response. There were 1188 genes where the expression level was associated with low response and 865 genes where expression level was associated with a high response to GH therapy (83). Network analysis of the human interactome associated with these genes indicated that glucocorticoid, estrogen, and insulin receptor signaling, and protein ubiquitination pathways were most represented by the genes where association was linked to high or low response to GH therapy.

 

A recent genome wide association study examining GH responsiveness did not identify any significant SNPs in their primary analysis (the primary analysis utilized all diagnostic groups for GH treatment together) (84).  They did identify 4 SNPs in a secondary analysis stratifying by diagnosis and limiting to European ancestry – the closest associate genes are UBE4B, LAPTM4B, COL1A1/NT5DC1 and CLEC7A/OLR1(84).

 

INHERITED DISORDERS OF THE GH-IGF-I AXIS

 

Genetic Disorders Causing Isolated Growth Hormone Deficiency

 

Initial reports suggested that only around 12% of cases of isolated growth hormone deficiency were associated with abnormalities of the hypothalamus or pituitary on MR imaging (85). More recent studies have indicated that up to 26% of cases of isolated GHD are associated with MR abnormalities (86), particularly anterior pituitary hypoplasia and ectopic posterior pituitary.  Within the remaining cohort of patients with IGHD an increasing number of genetic causes have been identified.

 

IGHD TYPE 1

 

IGHD type 1a is inherited in an autosomal recessive manner and is due to homozygous deletions and nonsense mutations in the GH1 gene leading to a complete absence of the GH protein from serum. The clinical presentation is with severe growth hormone deficiency and growth failure from 6 months of life with height SDS >4.5 SD below mean. Typically patients respond well to initial therapy with GH but then develop anti-GH antibodies leading to a loss of efficacy (87). Treatment with IGF-I is an option for such patients.

 

IGHD type 1b is also autosomal recessive and caused by mutations in the GH1 gene – either mis-sense, splice site or nonsense or by mutations within the GHRHR (the gene encoding the GHRH receptor). The clinical phenotype in IGHD type 1b is milder than that of IGHD 1a with the presence of low but detectable levels of GH to stimulation tests. These patients show a good response to treatment with GH without the development of anti-GH antibodies.

 

The GHRHR is a 423 amino acid G-coupled protein receptor. It contains seven transmembrane domains encoded for by a 13-exon gene on chromosome 7p15. While human mutations leading to isolated GH deficiency have been found in the GHRHR gene, to date no such mutations have been identified in the gene encoding the ligand, GHRH. The initial link between a GHRHR mutation and impaired growth was in the little mouse, where Lin et al identified an amino acid substitution in codon 60 of the mouse GHRHR (88). The substitution of glycine for aspartic acid (D60G) prevented the binding of GHRH to the mutant receptor. Subsequent to the identification of the mutation in mouse a nonsense mutation (p.E72X) was identified in two patients in a consanguineous family of Indian ethnic origin (89). Since this initial report multiple families have been reported and splice site mutations, missense mutations, nonsense mutations, microdeletions and one mutation in the promoter (90). The clinical phenotype of an individual with a GHRHR mutation is that of autosomal recessive inheritance of IGHD, anterior pituitary hypoplasia (defined as pituitary height more than 2 SD below mean), GH concentrations are either undetectable or very low in response to provocation tests and IGF-I/IGFBP-3 levels are low. In contrast to patients with GH1 mutations midface hypoplasia, neonatal hypoglycemia and microphallus are less common. Intelligence is normal and affected individuals are fertile.

 

Expression of GHRHR is upregulated by the pituitary transcription factor POU1F1 and this results in somatotroph hypertrophy. Because of this effect on somatotrophs anterior pituitary hypoplasia is commonly seen on MR imaging but there have been reports of GHRHR mutations with normal pituitary morphology (91).

 

IGHD TYPE 2

 

IGHD Type 2 is an autosomal dominant disorder caused by mutations in the GH1 gene.  The severity of GH deficiency is highly variable. While the name of the condition suggests only GH is affected, in practice loss of other pituitary hormones has been reported and patient must be followed up to identify these additional hormone deficiencies. Loss of TSH, ACTH, prolactin and gonadotrophins have all been reported (92).

 

IGHD type 2 is most commonly caused by mutations that affect splicing of GH1, particularly splicing of exon 3 (93).  The most frequent mutations are within the first six bp of the exon 3 donor splice site (93) but mutations in the exon 3 splice enhancers and intron splice enhancers have also been reported (90). The exon 3 splice mutations lead to the exclusion of exon 3 and the production of a 17.5kDa isoform of GH lacking amino acids 31-71, responsible for connecting helix 1 and helix 2 of the mature GH molecule. This abnormal 17.5 kDa variant GH is retained within the endoplasmic reticulum, disrupts the Golgi apparatus and reduces the stability of the 22kDa GH isoform (94). In addition to GH trafficking of other hormones including ACTH is disrupted. A mouse model overexpressing the 17.5kDa isoform demonstrated anterior pituitary hypoplasia with invasion by activated macrophages. The loss of additional pituitary hormones is likely to result from the disrupted hormone trafficking as well as the pituitary inflammation and destruction. Children with IGHD type II may display anterior pituitary hypoplasia on MR imaging. Currently there is no specific treatment in man to ameliorate the effects of the 17.5kDa isoform. A small interfering RNA based therapy has been successful in the mouse model of IGHD type 2 (95) but the delivery system used involved inserting the short hairpin RNA as a transgene. Successful implementation of such a therapy in humans will require an alternative mode of delivery capable of crossing the blood-brain barrier. As well as the classical exon 3 splice site mutations IGHD type 2 is also caused by missense mutations. These have been reported to lead to impaired GH release (96) or to alter folding of GH (97).  

 

IGHD TYPE 3

 

IGHD Type 3 is of x-linked recessive inheritance and the males described were both immunoglobulin and GH deficient. A single patient has been reported with a mutation in the BTK gene (resulting in exon skipping) with x-linked agammaglobulinemia and GH deficiency (98).

 

One family has been reported with isolated GHD caused by mutations in RNPC3 (99). The three affected sisters had compound heterozygous mutations in RNPC3 (p.P474T and p.R502X) and presented with classical severe isolated GHD with anterior pituitary hypoplasia on MR imaging. RNPC3 encodes a component of the minor spliceosome responsible for splicing of a small subset (<0.5%) of introns which are present in ~3% of human genes. Given that splicing is an essential basic process present in all tissues it is interesting that the phenotype seen is pituitary specific.  The patients displayed relatively minor perturbations in splicing which is hypothesized to be tolerated in most tissues, but not in the developing pituitary. Response to GH treatment is reported to be excellent (100).

 

Genetic Disorders Leading to Abnormal Pituitary Development and Multiple Pituitary Hormone Deficiency

 

Mutations in an increasing number of genes lead to loss of multiple pituitary hormones including growth hormone (summarized in Table 2).  A brief summary of each is given below – for an extensive review of pituitary development and it’s genetic control see Bancalari et al (101).

 

HESX1

 

The paired homeobox domain protein HESX1 is one of the earliest specific markers of the pituitary primordium and it acts as a transcriptional repressor. Mutations in HESX1 are associated with septo-optic dysplasia (102) and MPHD (103,104) which can be inherited in an autosomal recessive or autosomal dominant pattern. In addition to the MRI appearances associated with septo-optic dysplasia patients with HESX1 mutations can have an ectopic posterior pituitary (104).

 

OTX2

 

The OTX2 homeobox gene is a homologue of the Drosophila orthodenticle protein. It is expressed early in gastrulation and is involved in development of the central nervous system and eye. In humans OTX2 mutations have been identified in patients with anophthalmia or microphthalmia with isolated GHD or MPHD (105). On MR imaging an ectopic posterior pituitary and small anterior pituitary have been associated with OTX2 mutations. 

 

SOX3

 

SOX3 is a single exon gene located on the X chromosome, is expressed widely throughout the ventral diencephalon and is involved in the development of Rathke’s pouch (106). In humans SOX3 duplications (107) or polyalanine expansion (108,109) have been associated with X-linked hypopituitarism with or without mental retardation. The pituitary phenotype is variable from isolated GHD to MPHD. MRI findings may include anterior pituitary hypoplasia, ectopic posterior pituitary, and corpus callosum abnormalities.

 

PITX2

 

PITX2 is a homeodomain transcription factor expressed in the rostral brain and oral ectoderm during development and throughout the anterior pituitary in adult life. Axenfeld-Riegler syndrome is an autosomal dominant disorder characterized by ocular, dental and craniofacial abnormalities in addition to pituitary abnormalities. Mutations in PITX2 have been found in patients with Axenfeld-Riegler syndrome and GH deficiency (110).

 

LHX3 and LHX4

 

LHX3 and LHX4 encode LIM domain proteins expressed in Rathke’s pouch involved in transcriptional regulation. Homozygous loss of function mutations in LHX3 have been associated with hypopituitarism, sensorineural deafness and cervical abnormalities (rigid cervical spine and cervical spina bifida occulta) (111,112). The MRI appearance may be of a small or enlarged pituitary or a hypointense lesion compatible with a microadenoma.  Mutations in LHX4 lead to a range of pituitary dysfunction from GHD to MPHD (113) with a pituitary phenotype including anterior pituitary hypoplasia, ectopic posterior pituitary and in one family there was pointed cerebellar tonsils suggestive of an Arnold Chiari Malformation (114).

 

GLI2

 

GLI2 is a mediator of Sonic Hedgehog signal transduction and is expressed in the oral ectoderm and ventral diencephalon. Heterozygous mutations in GLI2 lead to a variable combination of holoprosencephaly and hypopituitarism (115,116). Other clinical findings may include a cleft lip/palate, postaxial polydactyly and anophthalmia.

 

FGFR1, FGF8 and PROKR2

 

FGFR1, FGF8 and PROKR2 were previously known to be involved in the pathogenesis of Kallmann syndrome (hypogonadotropic hypogonadism with anosmia). Screening of a cohort of 103 patients with hypopituitarism identified mutations in these Kallmann syndrome genes in eight patients (FGFR1 n=3, FGF8 n=1, PROKR2 n=4) (117).  An EPP was identified in one patient with an FGFR1 mutation and a hypoplastic anterior pituitary in one patient with a PROKR2 mutation.

 

PROP1

 

Prophet of Pit-1 (PROP1) is a homeodomain transcription factor with expression limited to the anterior pituitary. It acts as a transcriptional repressor downregulating HESX1 and as an activator of POU1F1. PROP1 mutations are associated with GH, prolactin, TSH and LH/FSH deficiency with rare cases of ACTH deficiency. PROP1 mutations are the commonest genetic cause of hereditary MPHD accounting for ~50% of familial cases (117).  MRI findings include both small and large anterior pituitary glands and even extension of the pituitary to form a large suprasellar mass which waxes and wanes before involuting (118).  Gonadotrophin deficiency in patients with PROP1 mutations is highly variable and can present with micropenis and cryptorchidism to delayed pubertal onset potentially indicating a role of PROP1 in maintenance of gonadotrophin function.

 

POU1F1

 

The first genetic cause of multiple pituitary hormone deficiency, identified in 1992, was mutations in the POU1F1transcription factor (119).  It is essential for the development of somatotrophs, lactotrophs, and thyrotrophs, consequently mutations in POU1F1 lead to deficiency of GH, TSH and prolactin. Anterior pituitary size is most often small but can be normal with normal stalk and normally sited posterior pituitary. The hormone deficiencies can present at any time from birth to adolescence.

 

IGSF1

 

Mutations in IGSF1 (immunoglobulin superfamily member 1) were identified initially as a cause of central hypothyroidism and macro-orchidism (120). IGSF1 is a membrane glycoprotein expressed in Rathke’s pouch. The identified mutations lead to aberrant protein trafficking and protein mislocalisation.  In a small number of subjects mild or transient GHD has been identified (121,122). It is clear that the immunoglobulin superfamily of proteins may have a wider role in controlling pituitary hormone secretion with mutations in immunoglobulin superfamily member 10 associated with constitutional delay in growth and puberty (123).

 

ARNT2

 

A single family with a homozygous frameshift loss of function mutation in ARNT2 has been described. The affected individuals demonstrated multiple pituitary hormone deficiency including diabetes insipidus along with post-natal microcephaly, frontal and temporal lobe hypoplasia, seizures, developmental delay, visual impairment and congenital abnormalities of the urinary tract (124). ARNT2 is a HLH transcription factor which is known to dimerize with SIM1, a known regulator of neuronal differentiation.

 

TCF7L1

 

Transcription factor 7-like 1 is a regulator of WNT/β-catenin signaling and is expressed in the developing forebrain and pituitary. Two patients with heterozygous missense variants have been reported – one diagnosed with GHD and one with low IGF-I concentrations (124). MRI findings are listed in Table 2. In both families there were unaffected family members also carrying the variant. Given functional studies confirmed the deleterious nature of the variant this is likely to represent autosomal dominant inheritance with variable penetrance.

 

RAX

 

RAX encodes a transcription factor involved in eye and forebrain development. A child with a homozygous frameshift truncating mutation in RAX has been identified with a phenotype including anophthalmia, bilateral cleft lip and palate with congenital hypopituitarism (125).

 

LAMB2

 

Laminin b2 is a basement membrane protein with autosomal recessive mutations associated with congenital nephrotic syndrome, ocular abnormalities and developmental delay. One patient has been reported with isolated growth hormone deficiency, optic nerve hypoplasia, and a small anterior pituitary in association with focal segmental glomerulosclerosis with a compound heterozygous missense mutation in LAMB2 (126).

 

TBC1D32

 

TBC1 Domain Family member 32 is thought to be a ciliary protein and a cause of oral facial digital syndrome type IX (127). Two families with biallelic mutations in TBC1D32 and hypopituitarism have been reported (128). For the first family there were two affected siblings and they had panhypopituitarism with an absent anterior pituitary, ectopic posterior pituitary and retinal dystrophy while in a third family the affected proband had anterior pituitary hypoplasia, growth hormone deficiency and developmental delay (128). Facial dysmorphism was present with prominent forehead, low set posteriorly rotated ears, hypertelorism and a flat nasal bridge.  Autosomal recessive mutations in another ciliopathy related gene IFT172 have been reported to cause GHD with an ectopic posterior pituitary (129).

 

MAGEL2 and L1CAM

 

MAGEL2 and L1CAM mutations have been identified in patients with a combination of hypopituitarism and arthrogryposis (130). MAGEL2 mutations cause Schaaf-Yang syndrome which is similar to Prader-Willi Syndrome with hypotonic, obesity, developmental delay, contractures and dysmorphism.  GHD, diabetes insipidus and ACTH deficiency have been reported in 4 patients. In one patient with L1 syndrome due to a L1CAM mutation arthrogryposis was present with GHD.

 

EIF2S3

 

EIF2S3 encodes a protein involved in the initiation of protein synthesis with mutations associated with developmental delay and microcephaly. In three patients’ mutations in EIF2S3 have been associated with GHD and central hypothyroidism (131). Inheritance is X-linked.

 

FOXA2

 

FOXA2 is a transcription factor involved in pituitary and pancreatic B-cell development and de novo heterozygous mutations cause a phenotype of congenital hypopituitarism with congenital hyperinsulinism (132).

 

OTHER MUTATIONS

 

In addition to the above mutations in CDON (133) (nonsense heterozygous), GPR161(134) (homozygous missense) and ROBO1(135) (heterozygous frameshift, nonsense and missense) have been associated with pituitary stalk interruption syndrome.

 

Table 2. Genetic Defects of Pituitary Development and their Phenotype

Gene

Pituitary Deficiencies

MRI phenotype

Inheritance

Other phenotypic features

ARNT2

 

GH, TSH, ACTH, LH, FSH, ADH

Absent PP, ectopic PP, thin stalk, thin corpus callosum, delayed myelination

AR

Hip dysplasia, hydronephrosis, vesico-ureteric reflux, neuropathic bladder, microcephaly, prominent forehead, deep set eyes, retrognathia

CDON

GH, TSH, ACTH

Small anterior pituitary, ectopic posterior pituitary, absent stalk

AD

 

EIF2S3

GH, TSH

Small anterior pituitary, white matter loss,

X-linked recessive

Developmental delay and microcephaly, glucose dysregulation (hyperinsulinemia hypoglycemia and post-prandial hyperglycemia)

GPR161

GH, TSH, ADH

Small anterior pituitary, ectopic posterior pituitary

AR

Congenital ptosis, alopecia, syndactyly, nail hypoplasia

FGFR1

GH, TSH, LH, FSH and ACTH

Normal or small anterior pituitary, corpus callosum agenesis

AD

ASD and VSD, brachydactyly, brachycephaly, preauricular skin tags, ocular abnormalities, seizures

FGF8

GH, TSH, ACTH, ADH

Absent corpus callosum, optic nerve hypoplasia

AD or AR

Holoprosencephaly, Moebius syndrome, craniofacial defects, high arched palate, maxillary hypoplasia, microcephaly, spastic diplegia

FOXA2

GH, TSH, ACTH

Small shallow sella turcica, anterior pituitary hypoplasia, absent stalk

AR

Congenital hyperinsulinism

GLI2

GH, TSH and ACTH with variable gonadotrophin deficiency

Anterior pituitary hypoplasia

AD

Holoprosencephaly, cleft lip and palate, anophthalmia, postaxial polydactyly, imperforate anus, laryngeal cleft, renal agenesis

GLI3

GH, TSH, LH, FSH, ACTH

Anterior pituitary hypoplasia

AD

Pallister-Hall syndrome Postaxial polydactyly, hamartoblastoma

HESX1

Isolated GHD through to panhypopituitarism with TSH, LH, FSH, ACTH, prolactin and ADH deficiency

Optic nerve hypoplasia, absence of the septum pellucidum, ectopic posterior pituitary, anterior pituitary hypoplasia

AR and AD

Developmental delay

IFT172

GHD

Ectopic posterior pituitary, anterior pituitary hypoplasia

AR

Retinopathy, metaphyseal dysplasia, and hypertension with renal failure

IGSF1

 

GH (transient/partial), TSH, prolactin

Normal in the majority of cases.  Frontoparietal hygroma, hypoplasia of the corpus callosum, and small stalk lesion reported.

X-linked recessive

Macro-orchidism, delay in puberty

L1CAM

GHD

Generalized white matter loss and thin corpus callosum

X-linked recessive

Arthrogryposis, hydrocephalus, VSD, developmental delay, scoliosis, astigmatism

LAMB2

GHD

Small anterior pituitary, optic nerve hypoplasia

AR

Congenital nephrotic syndrome, focal segmental glomerulosclerosis, developmental delay

LHX3

GH, TSH, LH, FSH, prolactin

Small, normal or enlarged anterior pituitary

AR

Short neck with limited rotation

LHX4

GH, TSH and ACTH deficiency

Small anterior pituitary, ectopic posterior pituitary, cerebellar abnormalities, corpus callosum hypoplasia

AD

 

MAGEL2

GHD, ACTH, ADH

Small posterior pituitary, thin corpus callosum and optic nerve hypoplasia

Heterozygous mutations on paternal allele

hypotonia, obesity, developmental delay, contractures and dysmorphism

OTX2

GH, TSH, LH, FSH and ACTH

Normal or small AP, pituitary stalk agenesis, ectopic posterior pituitary, Chiari I malformation

AR or AD

Microcephaly, bilateral anophthalmia, developmental delay, cleft palate

POU1F1

GH, TSH, prolactin

Small or normally sized anterior pituitary

AR and AD

 

PROKR2

GH, TSH, ACTH

Hypoplastic corpus callosum, normal or small anterior pituitary

AD

Club foot, syrinx spinal cord, microcephaly, epilepsy

PROP1

GH, TSH, LH, FSH, prolactin, evolving ACTH deficiency

Small, normal or enlarged anterior pituitary – may evolve over time

AR

 

RAX

GH, TSH, LH, FSH, ACTH, ADH

Absent sella turcica and pituitary

AR

Anophthalmia, bilateral cleft lip and palate

ROBO1

GH, TSH

Small or absent anterior pituitary, ectopic or absent posterior pituitary, interrupted or absent stalk

AD

Strabismus, ptosis

SOX3

GH, TSH, LH, FSH, ACTH.  Most commonly isolated GHD

Anterior pituitary and infundibular hypoplasia, ectopic posterior pituitary, corpus callosum abnormalities including cysts

X-linked recessive

Learning difficulties

SOX2

LH, FSH variable GH deficiency

Anterior pituitary hypoplasia, optic nerve hypoplasia, septo-optic dysplasia, hypothalamic hamartoma

AR

Microphthalmia, anophthalmia, micropenis, sensorineural deafness, gastro-intestinal tract defects.

TBC1D32

Isolated GHD to panhypopituitarism

Absent or hypoplastic anterior pituitary, ectopic posterior pituitary

AR

Retinal dystrophy, developmental delay, facial dysmorphism (prominent forehead, low set posteriorly rotated ears, hypertelorism and a flat nasal bridge). 

TCFL7

 

GH

Absent posterior pituitary, anterior pituitary hypoplasia, optic nerve hypoplasia, parital agenesis of corpus callosum, thin anterior commissure

AD

 

 

Bioinactive GH

 

Short stature associated with normal to high levels of growth hormone with low serum IGF-I concentrations “bioinactive GH” was first described in 1978 (136). This disorder is associated with a good clinical response to GH therapy and multiple subsequent cases have been reported in the literature (90). These multiple case reports contained no information on the genetic cause of the disorder.  The first demonstration of the mechanism responsible for bioinactive GH came in 1997 (137) when Takashi and co-workers described a heterozygous glycine to aspartic acid substitution at amino acid 112 of the GH molecule resulting in impaired binding of the mutant GH to GHR. Reported mutations such as the R77C mutation (138,139) have also been found in normally statured relatives and functional work has failed to identify any difference between wild type and R77C GH on GHR binding, activation of the JAK/STAT pathway, secretion studies or ability to induce cell proliferation (140,141). The clinical scenario of normal to high GH concentrations with low IGF-I levels is not uncommon and a diagnosis of bioinactive GH should not be made unless a mutation is identified where there is a demonstration that the function of the variant GH is impaired.

 

A homozygous missense mutation (C53S) in the GH1 gene was reported in a Serbian patient with height SDS of -3.6 at 9 years of age (142). Altered affinity for the GH receptor was demonstrated in functional studies, presumably due to alteration of the disulphide bond between Cys-53 and Cys-65 in the GH molecule.

 

Laron Syndrome

 

Laron syndrome, caused by loss of function mutations in the GHR gene(143), was first described in 1966 (144). Since then more than 250 patients have been described in the literature with over 70 missense, nonsense, indels and splice mutations within the GHR gene (145). The majority of mutations describe are inherited in an autosomal recessive manner but autosomal dominant inheritance has been described in a small number of cases (146).  Patients present with severe short stature having been born with normal birth size. The facial phenotype is similar to severe GH deficiency with frontal bossing and midface hypoplasia. Intellect, development and head circumference are normal. IGF-I, IGFBP-3 and ALS concentrations are low in serum with normal to raised baseline GH levels with raised peak stimulated GH level. Typical adult height is around -5 SD. Measurement of GHBP in serum is useful as, when markedly low, indicates absence of the extracellular component of the GHR. Since mutations can occur in the transmembrane or intracellular domains, the presence of GHBP in serum does not exclude a diagnosis of Laron syndrome.  The standard diagnostic test is an IGF-I generation test. Specificity of this test is around 77-91% and when applied to a population with low prevalence of GH insensitivity the positive predictive value of the test is likely to be low (147). In addition, there is a limited normative data for the IGF-I generation test. Buckway at al reported the results of IGF-I generation tests in normal subjects and subjects with GH deficiency, Laron syndrome and idiopathic short stature (148). Sensitivity of the IGF-I generation test in this population (who all had the same E180 splice mutation in the GHR, was 77% (the cut off for a normal result on this test was an increase in IGF-I to >15ng/mL post-GH stimulation (149)). Diagnosis of Laron syndrome therefore relies upon integration of clinical and biochemical findings and selecting patients for further genetic studies.

 

Recombinant human IGF-I therapy provides limited benefit in improving height. In an observational study containing 28 patients with Laron syndrome the results of treatment with 120 mg/kg/day IGF-I for a mean duration of 5 years increased height SDS from -6.1 SD to -5.1 SD (150). In the first year of treatment there was a marked increase in height velocity from 2.8 to 8.7 cm but height velocity markedly decreased after the first year of treatment. In a separate report of 21 individuals with GH insensitivity – 5 of whom had Laron syndrome there was an increase in height SDS from baseline of +1.9 SD with treatment of 120 mcg/kg/day IGF-I for a mean of 10.5 years (151). The treatment effect is markedly lower than that of GH in children with severe congenital GH deficiency (an example of a growth chart of a child with Laron syndrome treated with IGF-I is given in Figure 6). While GH therapy stimulates both hepatic and local IGF-I production, subcutaneous injections of IGF-I do not simulate this local IGF-I production. In addition, GH therapy normalizes not only IGF-I levels but levels of IGFBP-3 and ALS whereas in GH insensitive subjects treated with IGF-I there is no increase in IGFBP-3 or ALS concentrations. Thus, it would be expected that the injected IGF-I would have a much lower half life than endogenous IGF-I. A combined therapy of IGF-I with IGFBP-3 disappointingly was less effective in improving height (152).

Figure 6. Growth chart of girl with Laron syndrome treated with recombinant human IGF-I (Increlex) from age of 5.8 years when height SDS was -4.2 SD. There is an increase in height velocity over the first year of treatment which is reduced in subsequent years of therapy. Height SDS improves to -2.1 SD by 10.25 years but this has been associated with the onset of puberty at 9 years (treatment with the GnRH analogue Zoladex was introduced at 9.8 years). Current height lies within parental target range. M denotes maternal height and F denotes adjusted paternal height.

STAT5b Mutations

 

The signal transducers and activators of transcription (STAT) family contains seven proteins (STAT1, -2, -3, -4, -5a, -5b and -6). Mutations in STAT1(153) and STAT3 are associated with immune deficiency and a mutation in STAT5b was described in a patient with growth hormone insensitivity and immune deficiency (154).  The initial report was of a homozygous missense mutation in exon 15, encoding the critical SH2 domain leading to aberrant folding and aggregation of the protein. Six other mutations have been described including a nonsense mutation in exon 5 (155), two distinct nucleotide insertions (156,157) in exons 9 and 10 containing the DNA binding domain, a missense variant within the SH2 domain (158), a four nucleotide deletion in exon 5 (159) and a single nucleotide deletion in the Linker domain (160).

 

Until recently all the mutations identified were homozygous and the disorder is predominantly inherited in an autosomal recessive manner but dominant negative mutations have now been reported (161).  There is some evidence of a mild effect of the heterozygous state as height SDS in parents of affected children is consistently below mean height for the population with range from -0.3 SD to -2.8 SD. Birth weight appears to be within normal limits but postnatal height is severely impaired with height SDS range of -3 to -9.9 (158). Growth is comparable to children with Laron syndrome. Bone age and puberty is commonly delayed perhaps reflecting in part the chronic state of ill health. A prominent forehead, depressed nasal bridge and high-pitched voice are seen in some patients. The biochemical findings are compatible with growth hormone insensitivity with normal to high basal growth hormone concentrations and a raised stimulated peak GH level. Of note, 1 subject had a low stimulated peak GH concentration of 6.6 mcg/. Serum IGF-I, IGFBP-3 and ALS concentrations were consistently low in all subjects, remaining low at end of an IGF-I stimulation test.

 

Clinical differentiation of patients with STAT5b mutations form those with Laron syndrome can be made with the immunodeficiency. All but one of the reported cases has presented with chronic pulmonary disease, particularly lymphoid interstitial pneumonia, with the other child having severe hemorrhagic varicella. Two patients have died from their lung disease and a further patient has required lung transplantation. Patients with STAT5b mutations also have raised serum prolactin levels which can also be helpful with diagnosis.

 

Acid Labile Subunit Deficiency

 

The human IGFALS gene is located on chromosome 16p13.3 and ALS deficiency is inherited in an autosomal recessive pattern with homozygous and compound heterozygous mutations identified including missense, nonsense, deletions, duplications and insertions. The mutations are spread throughout the IGFALS gene which contains 2 exons and encodes a protein of 605 amino acids (162). The majority of the mutations are located in the 20 central leucine rich domains.  The clinical phenotype, first described in 2004 (163), is of very low serum concentrations of IGF-I, IGFBP-3 and ALS with a moderate degree of short stature (-2 to -3SD). 

 

Limited data is available on size at birth but weight appears to be within the lower half of the normal range (-0.2 to -1.9 SD) with only one individual reported to be SGA with a birth weight of -2.2 SD. The data on birth length is even more limited but all individuals measured were within normal range at -1.5 to +1.0 SD. Data on height during childhood is more abundant and hemorrhagic it is clear that postnatal growth is affected in the majority of individuals carrying ALS mutations.  Mean prepubertal height in 17 patients was reported as -2.61 SD (range -3.9 to -1.06 SD) with final adult height of -2.15 SD (range -0.5 to -4.2 SD). There is a preponderance of males in the literature (88% reported cases) which may represent the increased likelihood of males with short stature to present to health care providers. In male’s pubertal onset is commonly delayed (6/11 with onset puberty >14 years and 3/11 onset >15 years).  Serum IGF-I and IGFBP-3 standard deviation scores are very low (-3.3 to -11.2 SD for IGF-I and -3.6 to -18.5 for IGFBP-3), with undetectable ALS concentrations in all but one case (164). Levels of GH are increased with a mean peak GH of 46µg/L.

 

The relatively modest growth impairment in ALS deficiency is likely to be due to the preservation of the local production and action of IGF-I with deficiency of hepatic derived IGF-I. The diagnosis should be suggested by the presence of very low concentrations of IGF-I and, in particular, IGFBP-3 in the presence of moderate growth impairment. Although measurement of ALS is not routinely available this would also be a useful diagnostic tool.

 

Response to treatment with GH therapy has been poor and one child treated with recombinant human IGF-I did not improve height after 1 year of treatment.

 

IGF-I Gene Mutations

 

Deletions and mutations within the IGF1 gene are an extremely rare cause of GH insensitivity. The first patient was reported in 1996 (3) and there have been four subsequent affected families reported (165-168). The first patient described had a homozygous deletion of exons 3 and 5 of the IGF1 gene leading to frameshift and generation of a premature termination codon. He had undetectable levels of serum IGF-I with normal concentrations of IGFBP-3 and ALS with raised baseline and spontaneous GH peak levels. He was born small for gestational age at 1.4 kg at term and displayed profound post-natal growth impairment with sensorineural hearing loss, microcephaly and developmental delay. 

 

One subsequent report identified a similar phenotype of growth impairment, developmental delay, microcephaly and hearing impairment with a homozygous missense variant in exon 6 of IGF-1(167). The patient also had low IGF-I concentrations and high GH levels. Subsequent studies have identified this variant in individuals with normal height and there may be an alternative cause for this child’s growth impairment.

 

There have been two cases reported with similar phenotype of growth impairment, microcephaly and hearing impairment in individuals associated with homozygous mutations within the IGF1 gene (166,168). These mutations (V44M and R36Q) reduce the binding affinity of IGF-I for IGF1R. A large family with short stature and a heterozygous IGF1 mutation (c.402+1G>C) inducing splicing out of exon 4 with subsequent frameshift and truncated peptide (165)has also been reported.  This family included 5 short individuals with the heterozygous IGF1 mutation and an additional 5 individuals who are short but do not have the IGF1 variant. The phenotype of the proband was less severe than other IGF1 mutation patients with normal birth size (3.0kg) but significant post-natal growth impairment (presenting height -4.0 SD), normal hearing, normal development except for attention deficit hyperactivity disorder and mildly reduced serum concentrations of IGF-I (-2.2 SD) with normal IGFBP-3 serum levels (-1.25 SD).

 

For all patients reported to date, treatment with GH has been ineffective. Treatment with recombinant human IGF-1 may be more effective but may be complicated by the development of antibodies in those patients with IGF1deletions. It should however be effective for patients with bioinactive IGF-I.

 

Chromosome 15 Abnormalities and Mutations Affecting the IGF-I Receptor

 

The phenotype of patients with mutations in the IGF1R gene is similar, if slightly milder, to patients with IGF1 gene defects. They are born SGA and continue to grow poorly with microcephaly and variable developmental delay. Reported birth weights are from -1.5 to -3.5 SD with head circumference of -2.0 to -3.2 SD. Birth length SDS is highly variable at -1.0 to -5.0 SD while childhood height ranges from -2.1 to -4.8 SD (169). The initial patient described had a compound heterozygous mutation (170) within IGF1R while all other patients to date have heterozygous mutations. These mutations are dispersed throughout the gene (169). Missense (171,172), nonsense (170), small deletions (173)and duplications (174) have already been identified leading to a variety of deleterious effects on the IGF1R including loss via nonsense mediated decay (174), production of a truncated protein (170), altered trafficking(171), reduced ligand binding (175) and altered tyrosine kinase activity (172). Serum IGF-I concentrations can be normal or raised but are generally > +1 SD.

 

Response to treatment with GH therapy is variable – of 5 patients reported no response was seen in two patients, an equivocal response seen in another two patients and only one patient responded well to therapy (169). GH dose ranged from 0.025 to 0.07 mg/kg/day with the best responder treated with the lowest dose of GH. The rationale behind GH therapy is that it increases hepatic and local production of IGF-I to improve growth. Where there is resistance to IGF-I it is not surprising that GH therapy is less effective. For most disorders clinicians the aim of GH therapy is to improve growth without generating IGF-I concentrations above the normal rage. For IGF1R mutations, given the IGF-I resistance, it may not be possible to achieve adequate growth without using high dose GH therapy with subsequent IGF-I concentrations above the normal reference range.  The long-term effects of such therapy in this patient group are unknown and before embarking on such a strategy a careful discussion about the risks and benefits should be undertaken with the child/parents

 

Prior to the identification of mutations within the IGF1R gene there were reports of patients with abnormalities of chromosome 15 including monosomy, ring chromosome and unbalanced translocations. Allelic loss of chromosome 15 was described to result in growth impairment (176) while trisomy of chromosome 15 results in overgrowth (177), given the location of IGF1R at chromosome 15q26 it was hypothesized that the growth alterations were due to a dosage effect on IGF1R. The clinical phenotype is highly variable depending on the chromosomal aberration e.g., 15q26 deletion is associated with congenital diaphragmatic hernia as well as growth impairment (178). Response to GH therapy appears better for patients with chromosome 15 abnormalities with a first-year increase in height SDS of 0.8-1.5 (179).  

 

Pregnancy-Associated Plasma Protein A2 Deficiency

 

Pregnancy-associated plasma protein A2 is a metalloproteinase responsible for the cleavage of IGFBP-3 and IGFBP-5, an essential step in releasing IGF-I from the ternary complex and allowing it to bind to the IGF1R. Two families have been reported with loss of function mutations in PAPPA2 leading to growth impairment with increased concentrations of IGF-I, ALS, IGFBP-3 and IGFBP-5 and a resultant reduction in free IGF-I (180). GH concentrations are raised due to the reduced free IGF-I. Birth size is moderately reduced in some subjects and the degree of postnatal growth impairment is highly variable ranging from -3.8 SD to -1.0 SD. Other clinical features include mild microcephaly, small chins and long thin fingers. Treatment with rhIGF-I in one family demonstrated an increase in height SDS of +0.4 SD over 1 year of treatment (181) while in the second family treatment was discontinued due to headache in one of two siblings (182).

 

ACQUIRED GH DEFICIENCY

 

Tumors of the Hypothalamus or Pituitary

 

CRANIOPHARYNGIOMA

 

Craniopharyngiomas are non-glial intracranial tumors derived from malformed embryonal tissue thought to originate from ectodermal remnants of Rathke’s pouch or residual embryonal epithelium of the anterior pituitary (183). More than 70% of adamantinomatous craniopharyngiomas contain a mutation of the β-catenin gene (184). Although rare, craniopharyngiomas are the commonest childhood tumor affecting the hypothalamo-pituitary axis accounting for 55-90% of sellar and parasellar lesions in childhood (185). The incidence is 0.5-2 per million per year (186) with 30-50% of cases diagnosed in childhood. In contrast to adulthood where the commonest histological type of craniopharyngioma is papillary, in excess of 70% of childhood craniopharyngiomas are adamantinomatous and associated with cyst formation. Survival rates with craniopharyngiomas are excellent exceeding 90% at 10 years (187)after diagnosis but morbidity with visual defects, hypothalamic obesity, and pituitary hormone deficiency is high.

 

Presentation is with a combination of symptoms of raised intracranial pressure, visual impairment, and endocrine deficits. Up to 87% of cases present with a least one pituitary hormone deficiency – the commonest being GH deficiency present in up to 75% of cases at diagnosis (188). The prevalence of GH deficiency rises after treatment to >90% of patients – with both surgical intervention and radiotherapy implicated in this increase in GH deficiency.  Additional pituitary hormone deficits are common including diabetes insipidus which is present in 92% of cases (189).

 

Therapy for craniopharyngiomas can include a combination of surgery, radiotherapy and intra-lesional chemotherapy. Surgery can be via the transcranial or transsphenoidal route.  Where it is possible to remove the entire tumor without causing damage to the hypothalamus or optic nerves this is the treatment of choice. For larger tumors involving these structures controversy exists on whether the benefits of a complete resection, namely a reduction in the risk of recurrence/progression, are outweighed by the surgical morbidity particularly hypothalamic obesity, visual impairment, and adipsic diabetes insipidus (190,191).  The alternative strategy is a limited surgical resection followed by adjuvant treatment with either conventional radiotherapy or proton beam therapy.  Recurrence rates for complete resection are 15-46% (192), 70-90% for patients treated with surgical partial resection alone and 21% for patients treated with surgical resection and radiotherapy (193).

 

There is good evidence to suggest that replacement GH therapy does not increase the risk of recurrence in craniopharyngioma (194,195) and that the gain in height is similar to that seen in congenital isolated GH deficiency. In one report the mean time between diagnosis and initiation of GH therapy was 2.3 years (194). A period of time after diagnosis, prior to the introduction of GH therapy, allows the completion of surgery and radiotherapy and a period of observation. Despite the reports on the overall safety of GH in craniopharyngioma rapid regrowth of the tumor after the initiation of GH therapy has been reported (196).

 

PITUITARY ADENOMAS

 

Pituitary adenomas are rare in childhood comprising only 3% of supratentorial tumors of childhood (197). Functioning adenomas are more common than non-functioning adenomas with the commonest being prolactinomas, followed by ACTH secreting adenomas then GH secreting adenomas. In one series of 41 patients with childhood onset adenomas, 29 (70%) were prolactinomas, 5 (12%) were ACTH secreting adenomas, one patient (2%) presented with a GH secreting adenoma and the remaining 6 patients (15%) presented with non-functioning adenomas (198).  GH deficiency was present in four out of the 41 patients during childhood and 13 patients during follow up into adulthood. All patients who developed GH deficiency had a macroadenoma. In approximately 5% of cases pituitary adenomas are familial and this is known to be caused by mutations in the genes encoding MENIN (199) and Aryl Hydrocarbon Receptor Interacting Protein (200).

 

OPTIC PATHWAY GLIOMA

 

Optic pathway gliomas are tumors of the pre-cortical visual pathway which may also involve the hypothalamus. In around 1 in 3 cases they are associated with neurofibromatosis type 1 (201). They commonly present with ophthalmological signs and symptoms with the main endocrine presentation being precocious puberty. In the majority of cases there is limited or no progression of the tumor and only monitoring is required. Surgery not recommended for most cases due to the possibility of post-operative visual impairment. Where required initial treatment is with chemotherapy with radiotherapy reserved for teenagers and younger children who have not responded to chemotherapy. Although effective with a 90% 10-year progression free survival radiotherapy is associated with an increased risk of worsening visual impairment, endocrine deficits, cerebrovascular disease, and neurocognitive deficits.

 

GH deficiency in optic pathway gliomas can be present prior to radiotherapy but is much more common post radiotherapy. In one study of 68 children with optic pathway gliomas 19 developed GH deficiency, 15 of whom had received radiotherapy (202). In another study of 21 patients with optic pathway gliomas treated with radiotherapy only one patient had GH deficiency pre-radiotherapy while all patients had GH deficiency post radiotherapy (203).  GH therapy is highly effective and restores adult height to within normal range (204). Optic pathway gliomas can be associated with GH excess, especially in NF1 syndrome related cases.

 

LANGERHANS CELL HISTIOCYTOSIS

 

Langerhans cell histiocytosis (LCH) is a rare disorder with a prevalence of ~4 per million children (205). It is a condition in which there is proliferation and accumulation of clonal dendritic cells (LCH cells) bearing an immunophenotype very close to that of the normal epidermal Langerhans cells of the skin (205). LCH cells can spread to nearly any site in the body, proliferate and lead to local inflammation and tissue destruction. The commonest pituitary hormone deficit in Langerhans cell histiocytosis is diabetes insipidus which develops in around 25% of childhood patients with LCH while GH deficiency is the second commonest endocrinopathy present in 9-12% of childhood LCH patients.

 

Radiation

 

Neuroendocrine abnormalities of the hypothalamo-pituitary axis evolve with time after radiation induced damage. The first, and sometimes only, hormone deficiency following radiation exposure of the HPA axis is growth hormone deficiency. The risk of GH deficiency is related to the total radiation dose, fraction size and time between fractions. Almost all children exposed to >30 Gy cranial irradiation will develop GH deficiency around 65% of those receiving <30 Gy develop GH deficiency by 5 years post radiotherapy (206). Isolated GH deficiency has also been reported in children exposed to 18-24 Gy as used prophylactically in acute lymphoblastic leukemia (207) and in children exposed to as little as 10 Gy as part of total body irradiation (208).  

 

The hypothalamus is thought to be the site of radiation induced damage to the HPA as when exposed to radiation <50 Gy hormone deficiencies remain common ~90% after 10 years (209)  but in contrast delivery of radiation doses 500-1500 Gy to the pituitary alone result in lower rates of endocrinopathy – 40% 14 years after exposure (210).  Additional evidence of the susceptibility of the hypothalamus to radiation induced damage comes from the high prevalence rates of hypothalamic dysfunction on dynamic endocrine tests observed after radiation exposure (211)  and the presence of impaired GH secretion to stimuli acting through the hypothalamus with normal GH secretion to stimuli acting directly on the pituitary (212). Within the hypothalamic-pituitary axis there is differential sensitivity to radiation induced damage with the somatotrophic axis being the most vulnerable to damage, followed by GnRH-FSH/LH and then the CRH-ACTH and TRH-TSH axes which are the least sensitive to radiation induced damage (213).  This sequence of loss of pituitary hormones in radiation induced damage is seen in both animal models (213,214) and in humans where lower doses of radiation (e.g. 18-30 Gy used in treatment of childhood leukemia and brain tumors) leads to isolated GHD(215) whereas higher doses of radiation >60 Gy, used in the treatment of skull base tumors and nasopharyngeal carcinomas, leads to multiple pituitary hormone deficiency (216,217). Risk of pituitary hormone deficiency increases with time elapsed after radiation exposure in addition to the radiation dose – in one study around 50% of children treated with 27-32 Gy for a brain tumor were GHD after one year of treatment, with 85% GHD by 5 years post treatment and almost all GHD by 9 years post treatment (206).

 

GH NEUROSECRETORY DYSFUNCTION

 

One form of GHD particularly well described following radiation injury to the hypothalamic-pituitary axis is GH neurosecretory dysfunction (218-220). Neurosecretory dysfunction is characterized by normal responses to pharmacological stimuli of GH secretion but reduced spontaneous physiological GH secretion.  GH neurosecretory function in seen most frequently with lower radiation doses of <24 Gy (220) and it appears that doses >27 Gy both spontaneous and pharmacologically stimulated GH responses are reduced (221).  The possibility of GH neurosecretory dysfunction makes the diagnosis of GHD in children exposed to cranial irradiation challenging. The presence of normal IGF-I and IGFBP-3 concentrations in many children with radiation induced GH deficiency (222-225) (proven with multiple pharmacological stimulation tests) compounds these difficulties.

 

For children with brain tumors that can exfoliate cells into the cerebrospinal fluid (e.g., ependymoma or medulloblastoma) radiotherapy is delivered to the spine in addition to cranial irradiation. Spinal irradiation has a profound effect on growth and leads to reduced height and disproportionate growth with decreased upper to lower segment ratio (226).   Brauner et al compared children treated with craniospinal irradiation to those receiving cranial irradiation alone with height SDS being significantly lower in the craniospinal group at 1.46 SD compared to the cranial irradiation only group with a height SDS of -0.15 (221). Final height in adults who received craniospinal irradiation is also significantly lower than adults receiving cranial irradiation alone (-2.37 v -1.14 SD) (227). Lower age at radiation exposure is associated with a lower adult height SDS (227) with height loss from spinal irradiation estimated at 9 cm when exposed at 1 year, 7cm when exposed at 5 years and 5.5 cm when exposed at 10 years.

 

Response to treatment with GH therapy is poorer in children with radiation induced GH deficiency than in children with congenital GHD. For most patients with congenital GHD, GH therapy will lead to significant catch-up growth but in patients with radiation induced GH deficiency catch up growth is rare (228,229). However, while GH treatment does not appear to induce catch up growth it prevents a further decline in height SDS (228,230).  The cause of the poorer response seen in patients with radiation induced GH deficiency are likely to be multifactorial including early puberty, delayed GH therapy, use of lower doses of GH and the direct effect of spinal irradiation. GH therapy in children who have previously received craniospinal radiotherapy does prevent further height loss but does so at the expense of further exaggerating the skeletal disproportion seen in these patients.

 

There is extensive evidence linking the GH-IGF-I system to risk of cancer via several sources:

 

  1. Up-regulating the activity of the GH-IGF-I axis in leads to increased development of tumors in animal models (for review see Yaker et al (231)).
  2. In vitro evidence of expression of GH, GHR and IGF-I/II by tumors and the ability GH and IGF-I to induce cancer cell proliferation and metastases (for review see Clayton et al (232))
  3. Epidemiological evidence has linked higher serum IGF-I concentrations to cancer risk (233-236)
  4. Increased risk colorectal and thyroid cancers in patients with acromegaly (a condition of chronic GH excess) (237-239)

 

This evidence did lead to concerns about the risk of administration of GH therapy to patients with GH deficiency and a history of cancer. The majority of studies examining risk of recurrence in children with cancers treated with GH indicate that there is no increased risk of recurrence (240-244).  One notable exception is the Childhood Cancer Survivor Study based in centers in North America where the standardized incidence ratio of second malignancy was elevated (2.1 at average follow up of 18 years) in the 361 individuals treated with GH (245).  The majority of brain tumor recurrences occur during the first two years after completion of primary treatment and this has led to the recommendation that treatment with GH should be considered after this time point. This strategy prevents the association between early tumor recurrence and GH therapy by families but potentially denies children with tumor or radiation induced GH deficiency treatment for a considerable length of time. Children with brain tumors require monitoring of growth and consideration should be given to testing for GH deficiency in children with growth failure who have completed primary treatment. GH therapy should be carefully discussed with the family and oncologist where it is considered before 2 years post primary treatment.

 

Trauma

 

Traumatic brain injury is relatively common in childhood with ~180 children per 100,000 population sustaining a closed head injury each year. The proposed mechanism for traumatic brain injury induced hypopituitarism is that injury to the hypophyseal vessels which transverse the stalk leads to anterior pituitary ischemia and infarction. Postmortem studies of fatal closed head injuries identified hypothalamic lesions suggestive of infarction and ischemia in 43% of cases and pituitary lesions in 28% of cases (246). Although there have been multiple published case reports of anterior pituitary dysfunction in traumatic brain injury for many years (247) there has been a large increase in the number of systematic studies of pituitary function in survivors of traumatic brain injury since 2000. Several moderately sized studies of adult traumatic brain injury survivors have demonstrated risk of post-injury hypopituitarism. Deficiency of GH and gonadotrophins was more common than TSH or ACTH deficiency with 10-28% of patients being GH deficient and 8-30% of patients being gonadotrophin deficient (248-253). In the majority of these studies there has been no relationship between time post injury or injury severity and risk of pituitary dysfunction.

 

Until 2006 the literature on childhood traumatic brain injury and hypopituitarism was limited to case reports (for review of the case reports see Acerini et al (254)). The first report of pituitary function in children with traumatic brain injury studies a cohort of 55 patients (22 studied retrospectively and 30 studied prospectively) and identified 2 patients with low peak GH concentrations (255). Khadr et al reported a 39% rate of abnormalities of pituitary function tests in 33 childhood traumatic brain injury patients (256). None of these were felt to be clinically significant.  In this study 7 patients had a low peak GH concentration but 6 out of the 7 were thought to have peri-pubertal blunting of the GH response with one borderline post-pubertal GHD patient who declined further examination (256).  Poomthavorn et al (257) described a cohort of 54 patients with childhood brain injury 4 of whom had known multiple pituitary hormone deficiency prior to the start of the study, in the 50 patients screened however, there were no patients identified with GH deficiency.

 

The largest study of childhood traumatic brain injury and pituitary function is by Heather et al (258). It examined the pituitary function of 198 survivors of childhood traumatic brain injury. Importantly they used an integrated assessment of GH stimulation tests (including 2nd test with priming where required), auxology and IGF-I concentrations in order to reach a diagnosis of GHD. While a low peak GH concentration (<5µg/L, used as the cut off for diagnosis of GHD in New Zealand at the time of the study) was identified in 16 patients, height SDS ranged from -0.9 SD to +3.6 SD and IGF-I concentrations were within normal limits for all subjects. For this study population had the diagnosis of GHD been based solely on a GH stimulation test and a cut off of 10µg/L for the diagnosis of GHD, 33% of patients would have been incorrectly diagnosed as GHD.

 

The risk of hypopituitarism in childhood traumatic brain injury appears to be low and currently routine screening of pituitary function in this group is not justified outside the context of on-going research studies.

 

Hypophysitis

 

Hypophysitis is characterized by cellular infiltration and inflammation and can be classified as lymphocytic, xanthomatous, granulomatous, necrotizing, IgG4-related and mixed forms.  Lymphocytic hypophysitis is the commonest type but overall the disease is extremely rare with an estimated incidence of 1 per 9 million population (259). Presentation is often with visual disturbance, headache and vomiting. MRI may identify a homogeneous enhancing sellar mass. In adults’ deficiency of TSH and ACTH are particularly common and diabetes insipidus is said to be rare (260). The limited pediatric case reports include several children with diabetes insipidus and it may be that the pattern of hormone insufficiency is influenced by age at presentation.  Hypophysitis is more common in pregnant women but can also occur in non-pregnant women, men and in children (261,262).  Definitive diagnosis is with histopathology while treatment includes hormone replacement therapy and surgery where the sellar mass compresses the optic chiasm. The medical treatment of choice is high dose glucocorticoid therapy but alternative reported therapies include azathioprine (263), methotrexate (264), cyclosporin A (265) and stereotactic radiation (266).

 

GROWTH HORMONE EXCESS

 

While short stature and GHD are common reasons to consult a pediatric endocrinologist, tall stature is a far less common reason to present to a pediatric endocrinologist. Within the group of patients presenting with tall stature in childhood the majority will have either familial tall stature or a genetic/syndromic cause for their tall stature (e.g., Beckwith Wiedemann syndrome, Sotos syndrome, Marfan Syndrome, Simpson-Golabi-Behmel syndrome).  GH excess is an extremely rare disorder in pediatric practice. Causes of GH excess include GH secreting pituitary micro or macroadenomas, ectopic GHRH production and genetic abnormalities affecting GH secretion (McCune Albright syndrome and Carney complex).

 

The commonest symptom of GH excess in childhood is rapid growth. In a series of 15 childhood patients (6 female) with GH secreting adenomas reported by Takumi eta al (267) all the patients presented with rapid growth although 3 also had visual signs/symptoms, 3 amenorrhea, 2 headaches, 1 with hypogonadism and 1 with precocious puberty. Microadenomas were present in 4/15 patients. Acromegalic features such as soft tissue growth of the hands and feet, mandibular overgrowth with prognathism, forehead protrusion and deepening of voice can also occur. The presence of acromegalic features in likely to be linked to the timing of onset (more common with onset in adolescence) and the presence of hypogonadism.  Additional clinical features include excessive sweating, carpal tunnel syndrome, lethargy, arthropathy, impaired glucose tolerance and hypertension. Although rare in childhood, hypertension and glucose intolerance are seen in approximately 15% of adolescents presenting with GH excess (268).

 

The diagnosis of GH excess is based on the clinical features and auxology in combination with biochemical evidence. Measurement of IGF-I concentration is useful but the reference range used must be specific for the gender, age and pubertal stage of the child. As IGF-I concentrations rise during puberty a precocious puberty will lead to a raised growth velocity with a serum IGF-I concentration which may be raised for age and gender will not be raised for pubertal stage. Due to the variability of GH levels throughout the day assessment of growth hormone levels is either via an oral glucose tolerance test for GH suppression or a GH day curve. The oral glucose tolerance test for GH suppression is essentially identical to a standard oral glucose tolerance test but with measurement of glucose, insulin and GH at 0, 60, 90, 120 and 150 minutes. A normal response is suppression of GH levels to < 0.4 mcg/L (269). Some centers will undertake a GH day curve – measurement of at least 5 separate GH levels over 12 hours, however, given that adolescence is the age at which there is maximal physiological GH secretion and the lack of GH day curve normative data in adolescence interpretation of this test can be challenging.

 

Benign GH secreting adenomas are the most common cause of GH excess. Mutations in the genes encoding GPR101 (causing X-linked acrogigantism), MENIN (270), aryl hydrocarbon receptor interacting protein (200) and p27 (271) are known to predispose to the development of pituitary adenomas. Overall, most GH secreting adenomas are sporadic but the proportion with a genetic basis is likely to be higher in childhood.

 

Transsphenoidal surgery is the treatment of choice for patients with microadenomas, macroadenomas without cavernous sinus or bone extension or where the tumor is causing symptoms from compression (272). Surgical removal is expected to lead to a biochemical cure in 75-95% of patients, with lower probability of cure in patients with macroadenomas. There are three classes of medical treatments for GH excess:

 

  1. Dopamine agonists – cabergoline, bromocriptine
  2. Somatostatin analogues – octreotide, pasireotide, lanreotide
  3. GH receptor antagonists - pegvisomant

 

Medical therapy can be used either where there is failure of surgical therapy, where the tumor is not amenable to surgery or prior to surgery/radiotherapy. Dopamine agonists are the only oral therapy available. Of the dopamine agonists available, only cabergoline has shown efficacy (273) in acromegalic patients and as monotherapy achieves a biochemical cure in a minority of patients (274). Cabergoline is most useful either in tumors which co-secrete prolactin as well as GH or in combination with another therapeutic agent. The somatostatin analogues are effective in both reducing GH and IGF-I levels as well as reducing tumor size. Long acting, once monthly preparations of the somatostatin analogues represent the mainstay of therapy. Somatostatin analogues achieve biochemical resolution in up to 70% of patients (275) and tumor shrinkage (mean size reduction of 50%) in 75% of patients (276). Pegvisomant is the only GH receptor antagonist therapy available and is the most effective therapy at achieving a biochemical cure but in a small proportion (~2%) leads to tumor growth.  Radiotherapy is generally reserved as a third line treatment due to the long-time taken to achieve maximum effect (up to 10 years (277)) and risks of hypopituitarism (up to 50% by 5 years post radiotherapy), visual problems and late effects of cerebrovascular disease and second tumors.  Given the rarity of GH secreting tumors in childhood close liaison with an adult endocrinologist experienced in the management of acromegaly is recommended for a pediatric endocrinologist when faces with such a patient.

 

 

McCune Albright Syndrome

 

McCune Albright syndrome is disorder characterized by the clinical triad of polyostotic fibrous dysplasia, café au lait skin hyperpigmentation and gonadotrophin independent precocious puberty. It is caused by postzygotic activating mutations of GNAS which encodes a stimulatory subunit of G protein, Gsα (278).  GHRH receptor is a G protein coupled receptor and thus McCune Albright syndrome can lead to autonomous GH hypersecretion from the pituitary by activating the signal transduction pathway downstream of this receptor. In a cohort of 58 children and adults with McCune Albright syndrome Akintoye et al (279) identified 12 patients (21%) with GH excess including 6 (4 female, 2 male) who were <16 years. IGF-I concentrations in 10/12 were >2.5 SD above mean but in 2 patients surprisingly they were low at -2.5 and -0.2 SD. This may be due to the cyclical nature of the hormone hypersecretion in McCune Albright syndrome. MR imaging identified microadenomas in 4 patients and no tumor visible in the remaining patients. Clinical diagnosis of GH excess remains difficult as the facial changes can be masked or mistaken for the development of fibrous dysplastic changes in bone and the precocious puberty can mask the GH induced growth excess. The presence of a normal final height in a patient with precocious puberty indicates the potential presence of GH excess (279). Co-secretion of prolactin is common and the majority of patients have hyperprolactinemia. Due to bone thickening and fibrous dysplasia surgery is not usually an option for treatment and radiotherapy is contra-indicated because of the potential for sarcomatous change in fibrous dysplasia. Of the 11 patients with MAS associated acromegaly 6 were treated with cabergoline and then octreotide. Although 5/6 responded to cabergoline treatment with a reduction in IGF-I concentrations none normalized their IGF-I concentration and a combination of cabergoline and octreotide normalized IGF-I concentrations in 4/6 patients.  In a crossover trial of somatostatin analogue therapy and Pegvisomant in McCune Albright induced GH excess pegvisomant was effective in normalizing IGF-I concentrations in 4/5 patients while somatostatin therapy was effective in 3/5 patients (280).

 

Carney Complex  

 

The Carney complex is an autosomal dominant disorder characterized by skin pigmentary abnormalities, myxomas, endocrine tumors or overactivity, and schwannomas. It is known to be caused by loss of function mutations in the PRKAR1A gene which encodes the regulatory subunit of protein kinase A (281). Dissociation of the regulatory subunits from the catalytic subunits of protein kinase A leads to activation of signal transduction. Under normal circumstances this dissociation is triggered by cAMP. Carney complex associated mutations lead to loss of the regulatory subunit and increased activity of protein kinase A associated signal transduction.  GH secreting adenomas are reported in 10% of patients with carney complex but these are rare before puberty (282). Mild abnormalities in GH, IGF-I and prolactin levels are present in up to 79% of patients and there probably a long period of sommatomammotroph cell hypertrophy and mild hypersecretion prior to the development of true GH excess (283). Histology of Carney complex associated GH tumors is distinct and includes the presence of multifocal tumors, somatomammotroph hypertrophy and the secretion of multiple hormones from the tumor (284).

 

CONCLUSIONS

 

Growth disorders are one of the most common reasons for referral to a pediatric endocrinologist. GH deficiency can be effectively treated with recombinant human growth hormone but controversy still exists over the diagnosis of GH deficiency in childhood, particularly in relation to priming of GH stimulation tests. Over the past decade there has been a great expansion in our knowledge of the genetic causes underlying the congenital disorders causing hypopituitarism and GH deficiency but this has not yet led to any new therapies. While extremely rare in pediatric practice GH excess is an important diagnosis to consider in the tall child/adolescent and management should be undertaken in conjunction with an adult endocrinologist.

 

Important Concepts

 

  • GH signal transduction is not induced by GHR dimerization but by a conformational change in the predimerized GHR leading to repositioning of the BOX1 motifs
  • The diagnosis of growth hormone deficiency is made by combining information from auxology, biochemistry, and neuroimaging.
  • In addition to GH deficiency and Laron syndrome there are now additional disorders of the GH-IGF-I axis – Stat5b deficiency, ALS deficiency, haploinsufficiency, and mutations in IGF1R and mutations in the IGF-I gene.
  • There is an expanding number of genes where mutations lead to a disturbance of pituitary gland formation and pituitary hormone deficiency, however in the majority of patients with congenital hypopituitarism the genetic etiology remains unknown. Consider genetic screening in patients where there are multiple affected individuals in the family and in children where they have associated eye abnormalities.
  • Response to growth hormone therapy is generally very good in patients with congenital GH deficiency where a final adult height within parental target range should be expected. In contrast, in patients with radiation induced GH deficiency, GH treatment is less effective and acts mainly to prevent further height loss.
  • Recombinant human IGF-I is available for treating children with GH insensitivity. While first year height velocity often improves significantly the long-term effects on height are less effective than in children with congenital GH deficiency treated with growth hormone.
  • GH excess is an extremely rare disorder in childhood. All childhood patients with a GH secreting adenoma should be screened for mutations in AIP and MEN1 and management should be shared with an adult endocrinologist.

 

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Surgical Treatment of Pituitary Adenomas

ABSTRACT

The overwhelming majority of pituitary adenomas are benign and present either with characteristic syndromes of excess hormone secretion or secondary to mass effect by the growing tumor. The common hypersecretory syndromes include Cushing’s disease, acromegaly/gigantism, and hyperprolactinemia. Local mass effects on the pituitary can cause varying degrees of hypopituitarism. As the tumor grows beyond the confines of the sella turcica, the visual pathways are commonly affected and visual field deficits are present. Effective medical therapy is available for prolactin secreting adenomas. With the exception of these tumors, transsphenoidal surgery remains the first-line treatment for most other pituitary adenomas. Medical therapy for growth hormone secreting adenomas and for Cushing’s disease continues to evolve.

CLASSIFICATION

Pituitary adenomas may be classified according to their clinical/radiographic characteristics (Table 1) and, more recently, their cell lineage (Table 2). Those tumors that measure less than 10 mm in diameter are considered microadenomas; macroadenomas are those 10 mm or larger (Fig. 1A, B, C, and D). Macroadenomas may also be sub-categorized as "giant" if their extent reaches far beyond the normal confines of the pituitary region or their greatest diameter exceeds 4cm (Fig 1E, F, and G). Pituitary adenomas may also be categorized based on their functional/secretory status. The hypersecretory adenomas cause distinctive clinical syndromes that include acromegaly/gigantism caused by growth hormone (GH) secreting adenomas, the classic Forbes-Albright syndrome (amenorrhea-galactorrhea) caused by prolactin (PRL) secreting adenomas, TSH-secreting adenomas, the occasional hypersecreting FSH/LH adenoma, and Cushing's disease/Nelson’s syndrome caused by corticotropin (ACTH) secreting adenomas. The non-functioning adenomas (NFAs) are “silent” and only perturb the endocrine system due to mass effects on the normal gland causing hypopituitarism (decreased pituitary hormone production) and generally present either incidentally, because of visual loss, or with secondary subtle hormonal abnormalities. The new histopathological classification considers the majority of tumors to be clinically silent gonadotropin tumors staining for SF-1. The next category is the true null cell adenoma which stains for no pituitary hormones with none of the other transcription factors or hormones being detected.

Table 1. Clinical/Radiographic Classification Schemes of Pituitary Adenomas

 Scheme

 Features

Microadenoma/ Macroadenoma

 £ 10 mmm/ > 10 mm

Non-Functioning adenoma

 

Functioning adenoma

 

 Endocrinologically inactive, patient may present with pituitary deficiency or cranial nerve deficits (CN 2 most commonly)

 

Excess of pituitary hormone secretion:  GH adenoma; PRL adenoma; ACTH adenoma; TSH adenoma; GH -PRL adenoma; FSH/LH adenoma (rare, most are non-functioning)

 

Other plurihormonal hypersecretory adenomas

Abbreviations: CN = cranial nerve, GH = growth hormone, PRL = prolactin, ACTH = adrenocorticotropic hormone, TSH = thyroid stimulating hormone, FSH = follicle stimulating hormone, LH = luteinizing hormone

Figure 1. Tumor Classification based on size.  Microadenoma: Coronal and sagittal T1 weighted MRIs with contrast with arrow indicating the location of the tumor (A and B).  Macroadenoma: Coronal and sagittal T1 weighted MRIs of a typical macroadenoma (C and D).  Giant invasive macroadenoma: Coronal and sagittal T1 MRIs with contrast in a patient in whom the tumor compresses the right temporal lobe and invades the sphenoid sinus (E and F).  In another patient, the sagittal MRI reveals a tumor that has not only invaded the sphenoid sinus but compresses the brainstem; the tumor is highlighted (G and H).

The new cell lineage classification system of pituitary adenomas is a result of recent studies which have uncovered the shared transcription factor profiles present in adenoma cell lines (1). For detailed information on the pathology and pathogenesis of pituitary adenomas, see the corresponding Endotext chapter. The most common transcription factor profile is PIT1, which is shared by somatotroph, lactotroph, and thyrotroph adenomas.  PIT1 mediates differentiation, expansion, and survival of these three cell types (Table 2). In adenomas, evidence supports an HMGA mediated upregulation of PIT1 (2). HMGA genes are usually active during embryogenesis but not in normal adulthood (3). A new paradigm has evolved, which generally begins with transcription factor mediated monoclonal expansion of a single cell line followed by variable differentiation and retention of secretory capability. Patients harboring multiple pituitary adenomas present a unique scenario in which the true pathogenesis and pathogenetic process underlying neoplastic growth could involve distinct multicentric monoclonal expansion (“Multiple-Hit Theory”) or adenoma transdifferentiation across cell lines (“Transdifferentiation Theory”) (4).

Table 2. Cell Lineage Classification of Pituitary Adenomas (1)

Lineage

 Cell type

 

 Immunophenotype

Transcription factor profile

Acidophil

Somatotroph  

GH ± PRL ± a-subunit

PIT1

Lactotroph

PRL

PIT1, ER-a

Thyrotroph

TSH-b, a-subunit

PIT1, GATA2

Corticotroph

Corticotroph

ACTH, LMWCK

TPIT

Gonadotroph

Gonadotroph

FSH-b or LH-b or a-subunit

SF1, GATA2

Unknown

Null cell

None

None

Abbreviations: GH= growth hormone, PRL= prolactin, TSH = thyroid stimulating hormone, ACTH = adrenocorticotropic hormone, LMWCK = low molecular weight cytokeratin

EPIDEMIOLOGY

Pituitary adenomas account for approximately 10 to 15% of surgically-treated primary tumors of the central nervous system (CNS) (5-9). The incidence appears higher in African Americans in whom pituitary adenomas account for over 20% of non-metastatic CNS tumors (10, 11). The incidence rate of pituitary tumors has increased from 2.5 to 3.1 per 100,000 per year (annual percentage change of 4.25%). Although the incidence varies according to age, sex, and ethnic group, between approximately 0.5 and 8.5 per 100,000 in the population are diagnosed annually with a pituitary adenoma (5, 12-14). In a large cohort study between 2004 and 2009, the largest incidence peak was 8.5 for males 75-79 years old (14). Autopsy series indicate that pituitary tumors are quite common, and that nearly 25% of the population may harbor undiagnosed adenomas (15, 16). The majority of these tumors are less than 3-5 mm in diameter and would not require medical or surgical intervention. More recent series using magnetic resonance imaging (MRI) of healthy subjects indicate that approximately 10% of the population harbors pituitary lesions. Some series report a higher rate of diagnosis among women of childbearing age, despite a similar incidence in women and men  (5, 13). Because disruption of the hypothalamo-pituitary-gonadal axis in women is more evident than in men, women with pituitary adenomas may present to clinical attention at a higher rate, and earlier, than men.

 

Among the varying classes of adenomas, prolactinomas and non-functioning adenomas have the highest incidence, and account for nearly two-thirds of all pituitary tumors. Prolactin-secreting adenomas comprise 40 to 60% of functioning adenomas and are the most common subtype of pituitary tumor diagnosed in adolescents (6). The majority of microadenomas occur in women in their second and third decades. Men generally present later, in their fourth and fifth decades, almost always with macroadenomas.

 

GH secreting adenomas represent approximately 20-30% of all functioning tumors. Nearly three quarters of GH secreting adenomas are macroadenomas. Approximately 40 to 60 individuals per million have acromegaly (17-19). Between 3 and 4 new cases per million are diagnosed annually (17-20). Most present in their 3rd to 5th decades after they have been developing symptoms and signs for many years  (18). Acromegaly has been associated with an increased incidence of cardiovascular, respiratory, and cerebrovascular disease, as well as an increased risk of colon cancer. Studies have reported an increased risk of mortality compared to the unaffected population (17, 20). Although some studies report a higher incidence of several cancers, others have only confirmed an increased risk of colon cancer  (21, 22). There is some evidence that mortality risk may be different between the sexes. Etxabe et al. found a higher mortality rate in men than in women  (18). Other reports found similarly increased mortality in both sexes  (23). Still others report increased risks of death in men from cardiovascular, respiratory, cerebrovascular, and malignant disease, but only from cerebrovascular disease in women  (17).

 

ACTH adenomas account for 15 to 25% of all functioning adenomas and are the most common pituitary tumors diagnosed in pre-pubertal children (6). The majority of ACTH adenomas, regardless of age, are microadenomas. Approximately 39 individuals per million have Cushing's disease from an ACTH-secreting adenoma and the annual incidence is estimated at 2.4 per million (24). Cushing's disease is more common in women, most of whom present in their third and fourth decades (24, 25). There is a high incidence of hypertension and diabetes mellitus as well as higher vascular disease-related mortality (24, 26). Nelson’s syndrome can develop after adrenalectomy in patients with Cushing’s disease, as negative feedback is then lost to a previously unrecognized intrasellar ACTH adenoma. These patients may develop hyperpigmentation, and the ACTH-secreting pituitary tumors often become aggressive over time. 

CLINICAL PRESENTATION

Advances in neuroimaging, namely CT, CT angiography and particularly magnetic resonance imaging (MRI) have improved the visualization of the pituitary region. Increasing numbers of adenomas are diagnosed incidentally during the evaluation of sinus disorders (15%), trauma (19%), and stroke (15%), among others. These "incidentalomas" are not necessarily asymptomatic. Visual deficits are present in 5-15% of cases and up to 50% when formal testing is employed (27). Some degree of pituitary dysfunction is found in up to 15-30% (27, 28). More than one third are macroadenomas and, of these, approximately 30% will show significant enlargement over time (28-31). Small asymptomatic incidental microadenomas are less likely to have clinically significant growth and often can be followed over time with repeated MRIs.

Although increasing numbers of tumors are diagnosed incidentally, pituitary adenomas more often present secondary to hypersecretion, hypopituitarism, or mass effect (Table 3).

Table 3. Presenting Features of Pituitary Adenomas

Hypersecretion

GH-secreting adenoma: Acromegaly

ACTH-secreting adenoma: Cushing's disease/Nelson’s syndrome

Prolactin-secreting adenoma: Amenorrhea-galactorrhea

TSH-secreting adenoma: Secondary hyperthyroidism

Pituitary insufficiency

Symptoms: diminished libido, infertility, fatigue, weakness

Gonadal dysfunction, Hypothyroidism, Adrenal Insufficiency, Somatotroph Insufficiency

Mass Effect (symptoms related to compressed adjacent structures)

Optic chiasm: bitemporal visual field deficit and diminished visual acuity

Cavernous sinus: trigeminal nerve, facial pain; cranial nerves III, IV, VI, diplopia, ptosis, mydriasis, anisocoria

Pressure on dura or diaphragma sellae: headache

Hypothalamus: behavior, eating, and vigilance disturbances (somnolence)

Temporal lobe: complex partial seizures, memory and cognitive disturbances

Incidental

Discovered during the evaluation for headaches, trauma, nasal sinus disorders, dizziness

Hypersecretory Syndromes

(For detailed descriptions see corresponding chapters in Endotext)

Acromegaly induces characteristic growth hormone-induced structural changes in physiognomy. There is an insidious coarsening of facial features with an enlarged forehead, enlarged tongue, malocclusion of the teeth, and prognathism (Fig 2). Patients' hands and feet also enlarge. Many patients may develop excessive sweating (hyperhidrosis). The external hypertrophy of tissue is paralleled throughout the body. Enlargement of the tongue and hands is common. Patients may suffer from enlarged organs (visceromegaly) and overgrowth of joints and cartilage, along with high blood pressure, cardiomyopathy, congestive heart failure, sleep apnea, spinal canal narrowing (facet hypertrophy), and carpal tunnel syndrome. Significant numbers of patients with acromegaly also have impaired glucose metabolism and diabetes mellitus.

Figure 2. Acromegaly.  A. Coronal T1 weighted MRI with contrast in a patient with an intrasellar GH secreting adenoma.  Arrows indicate the common finding of “cutis gyrata”.  B. Sagittal T1 weighted MRI in the same patient with arrows indicating the frontal bossing and the enlarged frontal sinus, and * the tumor.

 

Cushing's disease causes changes in body habitus with characteristic increased weight gain, truncal obesity, "buffalo hump", enlargement of supraclavicular fat pads and moon facies. Skin changes are also common and include purple striae, easy bruisability, ruddy complexion, and increased body and facial hair. Patients suffer from fatigue, proximal muscle weakness, osteoporosis, psychological/psychiatric disorders, high blood pressure, and impaired glucose metabolism. They often have headache, menstrual disorders, and cognitive and emotional dysfunction.

 

Women with prolactinomas classically present with amenorrhea or oligomenorrhea and galactorrhea. Most are in their childbearing years, and are more likely to pursue medical attention for infertility and menstrual irregularity. Men, and women beyond their reproductive years, more often have headache, visual symptoms, sexual dysfunction, and signs of decreased pituitary function. Amenorrhea and galactorrhea are not specific to prolactinomas, however. Prolactin secretion is under constant inhibitory control from the hypothalamus. Any lesion that imposes pressure upon the portal venous connection of the pituitary stalk (infundibulum) connecting the hypothalamus and pituitary gland can interrupt these inhibitory dopaminergic signals.  This, in turn, causes an increase in serum prolactin levels, and mimics a prolactinoma, i.e., a 'pseudo-prolactinoma'. In such cases serum prolactin levels are usually only moderately elevated. As a general rule, serum prolactin levels over 200 ng/ml (3600mU/L) are indicative of prolactinomas (32).

Hypopituitarism

Tumor growth impairs the normal secretory function of the anterior pituitary and causes hypopituitarism. Common complaints include diminished sex drive, fatigue, weakness, and hypothyroidism. Pituitary insufficiency generally develops slowly over time.  However, acute pituitary insufficiency may occur in the setting of pituitary apoplexy, a condition in which the tumor infarcts or has internal bleeding (Fig 3). Pituitary tumor apoplexy can be particularly devastating, because it combines acute hypopituitarism and adrenal insufficiency with a rapidly expanding intracranial mass, and often causes visual loss or even sudden blindness.

Figure 3. Pituitary tumor apoplexy.  Sagittal T1 weighted MRI without contrast in a patient presenting with pituitary tumor apoplexy.  Note the fluid-fluid level within the tumor indicative of the apoplectic tumor.

Neurological Dysfunction

Neurologic signs and symptoms develop as adenomas grow beyond the confines of the sella turcica and exert pressure upon adjacent brain structures. As tumors enlarge, they compress the optic nerves and optic chiasm, and patients experience visual deficits and diminished visual acuity. Classically this causes a bitemporal hemianopia, i.e., visual loss in the temporal fields of each eye. Tumor growth may also affect other nerves (such as the 3rd, 4th, 5th, or 6th cranial nerves) and cause facial pain and/or double vision or drooping of the eyelid. Headache, although a non-specific complaint, can occur when a tumor stretches the dural sac that surrounds the pituitary gland. Headache from pituitary lesions is usually frontal or retro-orbital – it may be bitemporal or radiate to the occipito-cervical region.  Many patients will have been previously diagnosed with “migraine”, or “tension-headache” (33).

DIAGNOSIS

A panel of endocrinological tests can often confirm the clinical diagnosis of pituitary adenoma. Serum GH and IGF-1 levels screen for acromegaly. Failure to suppress GH levels after an oral glucose load (oral glucose tolerance test (OGTT)) can further confirm the diagnosis. Although any macroadenoma may cause moderate increases in serum PRL, levels greater than 200 ng/ml (3600 mU/L) are highly suggestive of a prolactin secreting adenoma. Dilution of the samples for assay may be necessary to avoid the “hook effect” related to macroprolactinemia.

Endocrinologic studies that suggest Cushing's disease includes an elevated ACTH and late night salivary or elevated 24-hour urine free cortisol (UFC), loss of the normal diurnal variation in cortisol levels, and suppression of serum cortisol levels after high dose dexamethasone but failure to suppress after low dose dexamethasone. Inferior petrosal vein sampling after corticotropin-releasing hormone (CRH) stimulation (i.e., Inferior Petrosal Sinus Sampling; IPSS) may be required to confirm and localize the pituitary source. At times, prior to diagnosing Cushing's disease, other ectopic sources of excess ACTH, such as bronchogenic or pancreatic carcinoma and pulmonary carcinoid tumors, must be excluded. This can often be accomplished with a CT scan or MRI of the chest and abdomen and with novel nuclear imaging tests (34, 35). Obesity, alcoholism, and depression also elevate serum cortisol levels, and the diagnosis of Cushing's disease should be made with caution in these “pseudo-Cushing’s” settings (36). 

TREATMENT

Although some incidentally-discovered microadenomas that do not cause symptoms may be followed clinically and with repeated MRI, patients with macroadenomas generally need medical or surgical intervention. Therapeutic goals include improved quality of life and survival; elimination of mass effect and reversal of related signs and symptoms, normalization of hormonal hypersecretion; preservation or recovery of normal pituitary function, and prevention of recurrence of the pituitary tumor.

MEDICAL THERAPY  

Medical therapy is available for most hypersecretory tumors (37-40). The majority of prolactin-secreting adenomas are effectively treated with dopamine agonists (bromocriptine and cabergoline). Cabergoline is generally preferred as a result of  a better side-effect profile, and between 80-90% of patients can achieve hormonal control (37). Surgical intervention is ordinarily reserved for those who are intolerant of medical therapy because of multiple side effects (e.g., nausea, headache, impulsive or compulsive behavior), whose prolactin levels remain elevated, or whose tumors continue to grow despite maximal medical treatment.

 

Medical treatment using somatostatin analogues (octreotide, lanreotide, and pasireotide) and dopamine agonists (cabergoline) have varying degrees of efficacy for treating GH adenomas.  The growth hormone receptor antagonist, pegvisamont, can be used in combination with other agents (41-43), and hormonal control can generally be achieved in about 60-90% of patients (37).  Although medical therapy is most often reserved for those patients’ awaiting surgery or those with persistent disease postoperatively, some advocate primary medical therapy, particularly for invasive tumors (44, 45). There is some conflicting evidence that pre-surgical medical therapy may improve surgical outcome (46).

 

Ketoconazole and/or metyrapone therapy can normalize serum cortisol levels in patients with Cushing's disease preoperatively 50-75% of the time. Metyrapone and ketoconazole inhibit enzymes in the adrenal gland required for steroid synthesis. A new and safer formulation, levoketoconazole is now available.  Along with acromegaly, surgery remains the first-line therapy for ACTH secreting tumors and Cushing disease. Clinical trials have also demonstrated some role for medical therapy with cabergoline or pasireotide, and with mifepristone (cortisol receptor blocker) in selected cases (47, 48).A new agent, osilodrostat, is under development.

 

The disadvantage of medical treatment of hypersecretory syndromes is that it is usually suppressive in nature and not fully cytotoxic. Tumors often recur when medications are discontinued, or they become resistant to therapy. Potential new targets are being explored, but have not yet reached clinical practice (49-51). 

RADIATION THERAPY

Radiotherapy is most often employed in conjunction with medical or surgical therapy. Fractionated external beam radiation therapy can reduce excessive hormone production and can reduce the incidence of tumor recurrence (52); however, it can be replaced by  stereotactic radiotherapy with focal conformal fractionated delivery. Gamma knife, Cyberknife, proton beam or linear accelerator stereotactic radiosurgery is increasingly considered as adjunctive therapy for pituitary tumors, and can be effective in normalizing hormonal hypersecretion and preventing recurrence (53-55). Whether by fractionated external beam or radiosurgery, the effects of radiotherapy are delayed. Patients require continued suppressive medical therapy during the period between treatment and effect. There is also a significant incidence of radiation-induced delayed hypopituitarism (52). There is no evidence to date that one of these various modalities is superior to another in efficacy, risks of complications, recurrence rates, or incidence of hypopituitarism. For more information on radiotherapy for pituitary tumors, see the corresponding chapter in Endotext.

SURGERY 

Indications for Surgery

For most pituitary tumors, surgery remains the first-line treatment of symptomatic pituitary adenomas. Large or invasive asymptomatic tumors may also warrant surgical consideration. It is sometime possible to estimate a tumor’s invasiveness on an MRI using the Knosp grading system (56). Asymptomatic tumors with evidence of radiographic invasion or displacement of the optic apparatus may benefit from surgery to prevent neurological deficits and progressive pituitary dysfunction. Surgery is also chosen secondarily when medical treatment fails for the treatment of prolactinoma. Regardless of the tumor type, surgery provides prompt relief from excess hormone secretion and mass effect. There is evidence to suggest that debulking of medically refractory prolactinomas and GH adenomas can return these tumors to a responsive state (57, 58). Rarely is surgery recommended as first line therapy for prolactinomas (59).  Surgery may be indicated in pituitary apoplexy with acute vision loss £ 72 hours as a result of mass effect on the optic chiasm from hematoma formation. Studies have shown that some patients with pituitary apoplexy can be successfully treated without operative intervention, but they are often confounded by selection bias, and the ideal patient has not been conclusively established for operative versus non-operative treatment (60-62).

Peri-Operative Management

A major component of the surgical management of patients with pituitary tumors actually occurs in the peri-operative period. Detailed information on peri-operative management of pituitary tumors can be found elsewhere (63). Briefly, pre-operative planning is very important in order to avoid complications and achieve optimal outcomes. It is obligatory to note any prior nasal surgery, review prior imaging, and obtain adequate pre-operative imaging for integration with neuronavigational systems. Typically, a high resolution T1 post contrast MRI is adequate for neuronavigational registration. The authors advocate additional imaging that includes (1) coronal and sagittal T1-weighted pre and post contrast images with at least 3mm slice thickness through the parasellar region for identification of the tumor, pituitary gland/stalk, cavernous sinus, and vasculature, (2) axial T2-weighted images of the sella to measure intercarotid distance, and (3) a coronal and sagittal strong T2-weighted Constructive Interface in Steady State (i.e., CISS, also known as FIESTA) through the parasellar region to identify midline structures and the optic chiasm. For revision surgery, a CT scan of the sinuses can be helpful to identify abnormal osseous anatomy. The imaging should be reviewed to identify normal gland and pituitary stalk, look for cavernous sinus invasion, identify arachnoid diverticula, and verify anatomical landmarks. Finally, it is critical to assess pre-operative pituitary function and replete necessary hormones (especially cortisol and thyroid hormones) prior to surgery. Remember to replete cortisol before thyroid hormone to avoid precipitating an adrenal crisis. For more information on the evaluation and management of pituitary hormone deficiency, see corresponding chapters in Endotext.

 

Post-operative management varies from routine to very complicated depending on the lesion size and extent of the operation and post-operative pituitary function. Patients with complete removal of intrasellar non-functioning tumors and intraoperative preservation of the normal pituitary gland without a cerebrospinal fluid (CSF) leak can have a relatively benign post-operative course. It is important to monitor closely for diabetes insipidus (DI), check a fasting morning cortisol to rule out secondary adrenal insufficiency, and restrict fluids as appropriate to prevent the syndrome of inappropriate anti-diuretic hormone (SIADH) (64). Patients with larger suprasellar or invasive tumors and/or those with CSF leaks requiring more extensive skull base reconstructions may require ICU care (65, 66). For information on the management of endocrine dysfunction and post-operative care in Cushing’s disease and Acromegaly, please see the corresponding chapters in Endotext.

Surgical Technique

The minimally invasive transsphenoidal approach can be used effectively for 95% of pituitary tumors. Exceptions are those large tumors with significant temporal or anterior cranial fossa extension. In such circumstances, transcranial approaches are often more appropriate. Occasionally, combined transsphenoidal and transcranial approaches are used. Nevertheless, some surgeons extend the basic transsphenoidal exposure in order to remove some of these tumors and avoid a craniotomy (Fig. 4) (67-70).

 

The transsphenoidal approach is a versatile method for treating pituitary tumors (Table 4). Endoscopic approaches may be used in isolation or as an adjunct to the other transsphenoidal approaches (Fig. 4) (71-78). Computer-guided neuronavigational techniques are nearly ubiquitous at major pituitary centers in lieu of traditional fluoroscopic guidance (79, 80). The role of neuronavigation is most pertinent in recurrent adenomas in which the midline anatomy has been distorted by previous transsphenoidal surgery. Intraoperative MRI is increasingly available and appears to be most applicable for large tumors (81).  There are three basic variations of the transsphenoidal approach.

Figure 4. Endoscopic approach. Intra-operative photograph of one surgeon (left) driving the endoscope while the main surgeon (right) resects the tumor.

 

Table 4. Transsphenoidal Surgery for Pituitary Adenomas: Personal Summary of 3744 Cases over a 36 year period

Type of Adenoma

Number of Patients (%)

Functioning adenomas

 

GH adenoma (Acromegaly)

662 (17.7)

PRL adenoma

975 (26.0)

ACTH adenoma (Cushing's disease)

680 (18.2)

TSH adenoma

45 (1.2)

Non-functioning adenomas

1382 (36.9)

SUBMUCOSAL TRANSSEPTAL APPROACH

The patient is placed in a lawn-chair position and a hemi-transfixion incision is made just inside the nostril so that the scar cannot be seen after surgery (Fig. 5). Most often the entire procedure can be accomplished endonasally. Conversion to a sublabial approach may be necessary for large macroadenomas and children in whom the exposure through one nostril is sometimes inadequate. A submucosal plane is developed along the nasal septum back to the level of the sphenoid sinus. Bone of the septum can be harvested for use later in the operation. The bone in front of the pituitary gland is also removed, the dura opened, and tumor is extracted in fragments (Fig. 6). Afterwards the saved bone, cartilage, or artificial material can be used to refashion the normal housing of the pituitary gland. Closure is rapid and consists of several interrupted absorbable sutures in the nasal mucosa and temporary nasal packing to promote healing of the mucosa.

Figure 5. Standard positioning for the endonasal approach (above). Below left, endonasal hemitransfixion incision; below right, direct sphenoidotomy technique.

Figure 6. Left, standard endonasal approach showing the trajectory to sella in sagittal view; Right, sequential steps used in tumor removal and repair of the sellar floor common to all techniques.

SEPTAL PUSHOVER/DIRECT SPHENOIDOTOMY

This approach uses incisions deeper within the nasal cavity (Fig 6, lower right. The incision for the septal pushover technique is made at the junction of the cartilaginous and bony septum. Submucosal tunnels are developed on either side of the bony septum until the sphenoid sinus is reached. Another option to reach the sphenoid sinus is by performing a direct sphenoidotomy. Using this method, no incision is made in the septum. Instead, the posterior part of septum just in front of the sphenoid sinus is deflected laterally and the sphenoid sinus is entered directly. There are several advantages to these techniques. Because there is no submucosal dissection of the cartilaginous septum, the risk of an anterior nasal septal perforation is eliminated. In addition, there is less need for nasal packing postoperatively, a frequent cause of postoperative pain and discomfort. The main drawback of these more direct approaches is that the exposure is not as wide as can be achieved by the standard endonasal transseptal approach in which the cartilaginous septum can be more extensively mobilized.

PURE ENDOSCOPIC APPROACH

The pure endoscopic approach has much appeal and is becoming the procedure of choice at many pituitary centers  (82, 83). Surgery begins at the sphenoid rostrum where a direct anterior sphenoidotomy is performed after identifying the natural sphenoid os within the sphenoidoethmoidal recess. Some surgeons prefer to perform the surgery using a single nostril. A binostril approach, however, provides more maneuverability and two-handed microdissection. To achieve an adequate exposure for the binostril approach, the middle and superior turbinates are lateralized and the bony septum just in front of the sphenoid sinus is removed. The sphenoidotomy is widened from the midline inferior vomer to the ethmoid air cells superiorly and then laterally until the carotid arteries are easily visualized (Fig 7-A). This allows instruments to be used in both nostrils simultaneously. Although a specialized endoscope holder may be used during tumor removal, the “3-hand” technique is advocated by many surgeons. The “3-hand” or “4-hand” technique requires two surgeons; one surgeon maneuvers the endoscope while another has both hands free to remove the tumor using microsurgical techniques. The surgical team is typically a neurosurgeon and otolaryngologist with experience in skull base surgery. Extended approaches are more commonly performed by teams rather than individuals (80, 84). The endoscope provides panoramic magnified views of the sellar anatomy during both the approach to and resection of tumors (Fig 7 – A, B). The option of using angled endoscopes allows surgeons to inspect for residual tumor, particularly along the cavernous sinus walls and the suprasellar region (85) (Fig 7 – C, D). No nasal packing is required as the procedure is performed posterior to the septum. The main disadvantages are the procedure’s learning curve and that the depth of field may problematic for some surgeons. There are 3D endoscopes and continued development of High Defintion (HD) imaging that may help to alleviate this potential problem. A recent international survey showed that about 7% of surgeons report using the 3D endoscope for transsphenoidal surgery. Advances in patient specific anatomical modeling is increasingly available for integration with the neuronavigation in the form of “augmented reality” which helps the surgeon visualize otherwise hidden anatomical structures (86). Finally, given the importance of vision preservation during endonasal surgery, especially with extended approaches, new developments in visual evoked potential monitoring are being studied (87). The clinical benefit of these new technologies is promising but still uncertain.

Figure 7. Endoscopic views.  A.  After the anterior wall of the sphenoid sinus is opened, the endoscope provides a panoramic view of the sella and surrounding anatomy. B. Endoscopic view of the tumor bed after resection.  C. Endoscopic view of the right cavernous sinus wall using the 0 degree endoscope.  D.  Note the dramatically improved view of the right cavernous sinus wall in the same patient using the 45 degree endoscope.  (arrowhead= carotid artery)

Outcome

Surgical outcomes after surgery for pituitary adenomas can be divided into functional outcomes and oncologic outcomes. Functional goals include the relief of symptoms and improvement or preservation of pituitary and visual function, along with improved quality of life (88-90). Visual deficits in patients with non-functioning pituitary adenomas are improved in approximately 80-90%. Some visual deterioration may occur in 0-4%. Most patients with intact pituitary function preoperatively retain their normal function. Those with preoperative pituitary deficiency regain function in 27% of the cases. The remaining patients are managed with hormone replacement therapy. Oncologic outcomes relate to tumor resection, recurrence, and biochemical remission from hormonal excess. Ten-year recurrence rates are approximately 16%, although only 6% require additional treatment (Table 5). On long-term follow-up, 83% of patients are alive and well without evidence of disease.

Table 5. Results of Transsphenoidal Surgery, Personal Summary of 3093 Cases over a 28 year period. Proportions (%) represent cumulative incidence.

Tumor

Remission

10-year Recurrence

Non-functioning adenoma

Not applicable*

16%

GH adenoma

Microadenoma

88%

1.3%

Macroadenoma

65%

PRL adenoma

Microadenoma

87%

13%

Macroadenoma

56%

ACTH adenoma

Microadenoma

91%

12% (Adults)

42% (Pediatric)

Macroadenoma

65%

 *Visual improvement occurs in 87% of those with preoperative visual loss.

 

Currently, using strict criteria for remission and in expert hands, transsphenoidal surgery obtains remission in 85-90% of patients with acromegaly with microadenomas and 65% of those harboring macroadenomas. For functional tumors, remission rates vary by tumor size and tumor type (91). Microadenomas typically have higher biochemical remission rates and remission rates are highest for microprolactinomas (92.3%) and lowest for somatotroph macroadenomas (40%). Currently, acromegalic symptoms are improved in 95% and recurrence is less than 2 percent at ten years. Ninety seven percent of patients have preserved normal pituitary function  (92). Modern criteria for remission include normal IGF-1 levels and either GH suppression to less than 0.4 ng/ml with oral glucose tolerance test or fasting GH less than 1.0 ng/ml. Using these criteria, surgical biochemical remission is over 60% (93). Both repeat surgery and medical therapy are options for those with residual disease and/or biochemical recurrence (37, 94).

 

Patients with prolactinomas who present for surgery are most often those who have failed medical management. Endonasal surgery for prolactinomas is associated with additional risks resulting from tumor fibrosis after dopamine agonist therapy but remission rates are still quite good. Prolactin levels are normalized in about 87% of microadenomas and 56% of macroadenomas (Table 5). The recurrence rate among those patients who are normalized after a transsphenoidal operation is 13% at ten years. Preserved pituitary function occurs in all but 3%.

 

Surgical management of Cushing's disease achieves a 91% remission rate for microadenomas, but falls to 65% for those with macroadenomas. Some 10-20% of adults experience recurrence after ten years. Postoperative stereotactic radiosurgery has achieved remission in approximately 60-70% of patients whose disease either did not remit following surgery or recurred (95).

 

Pituitary surgeons, with all health care professionals, strive for excellence in the care of our patients, it is becoming clear that criteria must be developed in order optimize surgical outcomes. Recently, a consensus statement on Pituitary Tumor Centers of Excellence (PTCOE) was released (96). In brief, PTCOE should be independent non-for-profit organizations, widely recognized by endocrinologist and pituitary surgeons, aimed at the advancement of pituitary science and the highest quality of patient care. They should also be recognized by external societies and act as resident training centers.

Complications of Transsphenoidal Surgery

Complication avoidance is central to transsphenoidal surgery given the close proximity of major neurologic and vascular structures (97, 98). Recently, surgical checklists for endonasal transsphenoidal surgery have been developed in order to optimize surgical outcomes and avoid complications (99). The overall mortality rate for transsphenoidal surgery is less than 0.5% (Table 6). Major morbidity (cerebrospinal fluid leak, meningitis, stroke, intracranial hemorrhage, and visual loss) occurs in between 1 and 3% of cases. Less serious complications (sinus disease, nasal septal perforations, and wound issues) occur in approximately 1-7%. Larger invasive tumors and giant adenomas are associated with a higher morbidity. In the modern era, more aggressive extended approaches to large invasive tumors has led to a higher incidence of CSF leak, but the use of the pedicled nasoseptal flap has been largely successful in preventing recurrent leaks with a success rate of up to 98.6% (100). The nasoseptal flap can also be used again in certain revision cases with good results (101).

 

Table 6. Complications of Transsphenoidal Surgery (1972-2017). Personal historical series and a modern results covering a 45 year period and 4,246 cases.

 Outcome Measure

Cumulative Incidence (%)

 

1972-2000

1992-2017 (102)

 Mortality

<0.5%

<0.3%

Major complication: (CSF leak, meningitis, ischemic stroke, intracranial hemorrhage, vascular injury, visual loss)

1.5%

CSF leak 2.6%

Other 3.2%

Minor complication: (sinus disease, septal perforations, epistaxis, wound infections and hematomas)

6.5%

1.3%

CONCLUSIONS

Pituitary adenomas are a complex set of benign tumors that present with characteristic hypersecretory syndromes and mass effect. Although medical and radiotherapy offer effective treatment for particular functional tumors in specific situations, transsphenoidal surgery continues to provide optimal outcomes for non-prolactin secreting adenomas with a low incidence of major morbidity.

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Atypical Forms of Diabetes

ABSTRACT

 

While most patients with diabetes have Type 1 diabetes (T1D) or Type 2 diabetes (T2D) there are other etiologies of diabetes that occur less frequently. In this chapter we will discuss a number of these less-common causes of diabetes. It is clinically very important to recognize these uncommon causes of diabetes as treatment directed towards the underlying etiology can at times result in the remission of the diabetes (for example Cushing’s Syndrome) or be required to avoid other complications of the underlying disorder (for example hemochromatosis, which in addition to causing diabetes can lead to severe liver disease and congestive heart failure). In this chapter the following disorders that are associated with diabetes are discussed: 1) genetic disorders of insulin action (Type A insulin resistance, Donohue Syndrome/Leprechaunism, Rabson-Mendenhall syndrome); 2) maternally inherited diabetes mellitus and deafness syndrome; 3) disorders of the exocrine pancreas (pancreatitis, trauma/pancreatectomy, neoplasia, cystic fibrosis, hemochromatosis); 4) endocrinopathies (acromegaly, Cushing’s syndrome, glucagonoma, pheochromocytoma, hyperthyroidism, somatostatinoma, primary hyperaldosteronism); 5) drug induced; 6) infections; 7) immune mediated (stiff-man syndrome, anti-insulin receptor antibodies); 8) ketosis prone diabetes (Flatbush diabetes); and 9) genetic disorder sometimes associated with diabetes (Down syndrome, Klinefelter syndrome, Turner syndrome, Wilsons syndrome, Wolfram syndrome, Friedreich ataxia, Huntington chorea, Bardet-Biedl syndrome (Laurence-Moon-Biedl syndrome), myotonic dystrophy, porphyria, Prader-Willi syndrome, Alström syndrome). Gestational diabetes, monogenic diabetes (maturity onset diabetes of the young (MODY) and neonatal diabetes), lipodystrophy, fibrocalculous pancreatic disease, diabetes associated with HIV infection, diabetes due to the autoimmune polyglandular syndromes, and post-transplant diabetes are not discussed in this chapter as they are discussed in other Endotext chapters.

 

INTRODUCTION

 

While most patients with diabetes have Type 1 diabetes (T1D) or Type 2 diabetes (T2D) there are other etiologies of diabetes that occur less frequently. In this chapter we will discuss a number of these less-common causes of diabetes (see table 1). Note that gestational diabetes, monogenic diabetes (maturity onset diabetes of the young (MODY) and neonatal diabetes), lipodystrophy, fibrocalculous pancreatic disease, diabetes associated with HIV infection, diabetes due to the autoimmune polyglandular syndromes, and post-transplant diabetes are discussed in separate Endotext chapters (1-7). It is clinically very important to recognize these uncommon causes of diabetes as treatment directed towards the underlying etiology can at times result in the remission of the diabetes (for example Cushing’s Syndrome) or be required to avoid other complications of the underlying disorder (for example hemochromatosis, which in addition to causing diabetes can lead to severe liver disease and congestive heart failure).

 

Table 1. Non-Type 1 Non-T2D Classification       

Genetic defects of beta-cell development and function

MODY (common causes- GCK, HNF1A, HNF4A, HNF1B) 

Neonatal Diabetes (common causes- KCNJ11, ABCC8, INS, 6q24)

1.     Mitochondrial DNA

Genetic defects in insulin action

1.     Type A insulin resistance

2.     Donohue Syndrome (Leprechaunism)

3.     Rabson-Mendenhall syndrome

4.     Lipoatrophic diabetes

Diseases of the exocrine pancreas

1.     Pancreatitis

2.     Fibrocalculous pancreatic disease

3.     Trauma/pancreatectomy

4.     Neoplasia

5.     Cystic fibrosis

6.     Hemochromatosis (iron overload)

Thalassemia (iron overload)

Endocrinopathies

1.     Acromegaly

2.     Cushing’s syndrome

3.     Glucagonoma

4.     Pheochromocytoma

5.     Hyperthyroidism

6.     Somatostatinoma

7.     Primary hyperaldosteronism

Drug- or chemical-induced hyperglycemia

1.     Vacor

2.     Pentamidine

3.     Nicotinic acid

4.     Glucocorticoids

5.     Diazoxide

6.     Check point inhibitors

7.     Dilantin

8.     Interferon alpha

9.     Immune suppressants

10.  Others (statins, psychotropic drugs, b-Adrenergic agonists, thiazides, etc.)

Infections

1.     Congenital rubella

2.     HCV

3.     HIV

COVID-19

Immune-mediated diabetes

1.     Stiff-man syndrome

2.     Anti-insulin receptor antibodies

3.     Autoimmune polyglandular syndromes

Diabetes of unknown cause

1.     Ketosis-prone diabetes (Flatbush diabetes)

Other genetic syndromes sometimes associated with diabetes

1.     Down syndrome

2.     Klinefelter syndrome

3.     Turner syndrome

4.     Wilsons syndrome

5.     Wolfram syndrome

6.     Friedreich ataxia

7.     Bardet-Biedl syndrome (Laurence-Moon-Biedl syndrome)

8.     Myotonic dystrophy

9.     Prader-Willi syndrome

10.  Alström syndrome

 

MATERNALLY INHERITED DIABETES MELLITUS AND DEAFNESS (MIDD)

 

Maternally inherited diabetes mellitus and deafness is a mitochondrial disorder characterized by diabetes and progressive sensorineuronal hearing loss (8,9). Mitochondrial DNA is only transmitted from the mother as the sperm lacks mitochondrial DNA (8). Therefore, over 50% of affected individuals with MIDD have a mother with diabetes.  A mother with this disorder transmits the mutation to almost all of her offspring (10). However, the proportion of somatic cells with the mutation can vary considerably, a condition called heteroplasmy (9). The higher the number of somatic cells with a mutation the greater is the penetrance of symptoms and disease severity. Additionally, the proportion of somatic cells with a mutation can vary from tissue to tissue and explains the variability in the manifestations of this disorder (9). The prevalence of mitochondrial diabetes in the diabetes population depends on ethnic background and ranges between 0.2% and 2%, with the highest prevalence in Japan (10).

 

MIDD is associated with a point mutation in a transfer ribonucleic acid (tRNA) gene at position 3243 with an A to G transition (8,9). In addition to diabetes and auditory impairment, the m.3243A>G mutation can cause other clinical manifestations including central neurological and psychiatric disorders, eye disease, myopathy, cardiac disorders, renal disease, endocrine disease, and gastrointestinal disease (8,9). Other point mutations in mitochondrial DNA can also result in diabetes and deafness but these mutations are rare in comparison to m.3243A>G (8-10).

 

It is thought that defects in mitochondrial function result in the decreased production of ATP following glucose uptake by beta cells resulting in decreased insulin secretion in response to elevated glucose levels (8,9). Additionally, mitochondrial dysfunction in the highly metabolically active pancreatic islets ultimately results in the loss of B‐cell mass further compromising insulin secretion (9). Insulin sensitivity is usually normal (10). Other tissues that are metabolically active may also be adversely effected by the inability of the mitochondria to produce ATP including the cells in the cochlea (9).

 

The clinical syndrome of MIDD can phenotypically resemble either T1D or T2D (9,10). The age of onset varies between childhood and mid-adulthood. Approximately 20% of patients present acutely with high glucose levels and even ketoacidosis (9). Most patients do not have islet cell antibodies but they are present in a small number of patients (9). This could be due to concomitant T1D or to the development of antibodies secondary to beta cell destruction due to mitochondria dysfunction. Patients tend to be thin rather than obese (9). This disorder can be distinguished from MODY by the presence of multi-organ involvement, particularly sensorineural hearing loss, and maternal rather than autosomal dominant transmission. Initially patients may be treated with diet and/or oral agents but overtime most patients with MIDD progress to requiring insulin therapy (8-10).

 

As the name implies this disorder is recognized by the presence of diabetes and deafness and a family history of these conditions in maternal relatives (9,10). Hearing loss is present in approximately 75% of patients and typically precedes the development of diabetes (9). Hearing loss is more common and severe in males (9). Approximately 10-15% of patients, in addition to having diabetes and deafness, also have the syndrome of mitochondrial encephalomyopathy, lactic acidosis, and stroke‐like episodes (9). The m.3243A>G mutation can cause a wide spectrum of abnormalities that include neurological abnormalities (strokes, dementia, seizures), psychiatric disorders including recurrent major depression, schizophrenia and a variety of phobias, macular retinal dystrophy with pigmentation, proximal myopathy, cardiomyopathy, renal failure, short stature, endocrine dysfunction, and gastrointestinal complaints (9). The finding of classical retinal dystrophy and hyperpigmentation on routine eye exam should suggest the diagnosis of maternally inherited diabetes mellitus and deafness. Once suspected the diagnosis of MIDD should be confirmed by genetic testing for the mitochondrial DNA point mutation at position 3243 (A>G). This is usually initially carried out on blood cells but if negative urinary cells or skeletal muscle can be tested and if necessary one can test for other mutations that cause similar phenotypes (11). Once a diagnosis is confirmed first-degree family members at risk should be screened for the mutation and provided with genetic counseling. For those carrying the mutation without clinical manifestations, screening for diabetes and monitoring of kidney function, hearing, and cardiac function should be carried out.

 

GENETIC DEFECTS IN INSULIN ACTION

 

Overview of Insulin Receptor Defects

 

Mutations in the insulin receptor can cause different degrees of insulin resistance but do not need to be associated with diabetes per se (12). A large number of different mutations have been described and they can be classified as mutations that prevent synthesis of the receptor, inhibit transport of the receptor to the plasma membrane, decrease insulin binding to the receptor, impair transmembrane signaling, or increase receptor degradation (13). Pancreatic beta cell hyperplasia and hyperinsulinemia can compensate for the insulin resistance preventing hyperglycemia. Fasting hypoglycemia and postprandial hyperglycemia may be observed. Overtime the beta cells’ ability to secrete insulin diminishes and frank diabetes usually develops. Treatment of the diabetes may require very high doses of insulin (14). Unfortunately, insulin sensitizers have not been very effective in patients with insulin receptor defects. In contrast to the typical patients with insulin resistance, obesity, dyslipidemia, hypertension, and fatty liver are not usually present (14,15). Acanthosis nigricans, pigmentation in the neck or axillae, is a visible sign of severe insulin resistance (12,14). In females, severe insulin resistance is usually associated with hyperandrogenism, oligomenorrhea or amenorrhea, anovulation, hirsutism, acne, and masculinization (12,14). It is hypothesized that the ovarian dysfunction and acanthosis nigricans is due to high levels of insulin acting via the IGF1 receptors (15). The amount of residual insulin receptor function determines the specific syndrome in patients with insulin receptor mutations (Figure 1).

Figure 1. Insulin Receptor Mutation Syndromes (Modified from reference (12)). Severe disorders are usually homozygous or compound heterozygous mutations while milder forms are often heterozygotes.

Type A Insulin Resistance

 

This autosomal dominant disorder includes patients with severe insulin resistance and acanthosis nigricans (12,14). Patients have normal growth and females show ovarian hyperandrogenism that typically presents in the peripubertal period (14). In females, hyperglycemia develops after ovarian hyperandrogenism and acanthosis nigricans. Males display only acanthosis nigricans and they often remain undiagnosed even after the development of symptomatic diabetes, which may not occur until the patients are adults. These patients have mutations in the insulin receptor gene that decreases the activity of the insulin receptor (13,14). In addition, mutations in transcription factors that stimulate the expression of insulin receptors can lead to a similar phenotype as mutations in the insulin receptor (10). Inherited defects in pathways downstream of the insulin receptor can also lead to clinical abnormalities similar to mutations in the insulin receptor (10,11).

 

Donohue Syndrome (Leprechaunism)

 

Donohue syndrome is a rare congenital, autosomal recessive syndrome characterized by very severe insulin resistance due to mutations in the insulin receptor gene, dysmorphic features such as protuberant and low-set ears, flaring nostrils and thick lips, growth retardation, failure to thrive, and early death (13). The name leprechaunism relates to the elfin features of those affected. Clinical features include in addition to acanthosis nigricans, hypertrichosis, hirsutism, dysmorphic facies, breast enlargement, abdominal distension, and lipoatrophy. Patients have extremely high levels of insulin and can develop impaired glucose tolerance or overt diabetes. The prognosis for infants with this condition is very poor and most will die in the first year of life. When parents, who are heterozygous for mutations in the insulin receptor are studied, many of these individuals are insulin resistant (13).

 

Rabson-Mendenhall Syndrome

 

The Rabson-Mendenhall syndrome represents another disorder of extreme insulin resistance (14). This autosomal recessive syndrome is associated with mutations in the insulin receptor gene (12). Initially fasting hypoglycemia, postprandial hyperglycemia, and marked hyperinsulinemia may be observed (12). When beta-cells decompensate, hyperglycemia may become very difficult to treat. Clinical features include in addition to acanthosis nigricans, phallic enlargement, precocious pseudopuberty, short stature, and abnormal teeth, hair and nails (13,14). Hyperplasia of the pineal gland is an unusual feature (13). Prognosis is poor as diabetes is difficult to control even with high insulin doses (13). Hyperglycemia leads to microvascular disease and/or diabetic ketoacidosis resulting in death in the second and third decades of life (12). Leptin administration has resulted in an improvement in this syndrome (16,17).

 

DISEASES OF THE EXOCRINE PANCREAS

 

Diseases that destroy the pancreas can cause DM even in individuals who do not have risk factors for diabetes (18). In the medical literature this is often referred to as Type 3C diabetes. Acquired causes of damage to the pancreas include pancreatitis, trauma, infection, pancreatic carcinoma, and pancreatectomy. Inherited disorders that affect the endocrine pancreas, such as hemochromatosis, thalassemia, and cystic fibrosis, can also cause insulin deficiency and diabetes. The distribution of causes for diabetes secondary to pancreatic disorders in one study was chronic pancreatitis (79%), pancreatic ductal adenocarcinoma (8%), hemochromatosis (7%), cystic fibrosis (4%), and previous pancreatic surgery (2%) (19). The prevalence of diabetes secondary to pancreatic disease is estimated to range from 1% to 9% and likely will depend on the patient population studied (20). In a population study carried out in New Zealand the prevalence of diabetes secondary to pancreatic disorders was close to that of T1D (21).

 

Pancreatitis

 

Pancreatitis may lead to the destruction of the beta cells due to inflammation and irreversible fibrotic damage  (22). In addition to destroying the beta cells, pancreatitis also leads to the destruction of glucagon secreting alpha-cells and pancreatic polypeptide secreting cells (22). The decrease in insulin secretion is the primary mechanism leading to hyperglycemia. In addition, the decrease in secretion of pancreatic polypeptide leads to a decrease in hepatic insulin sensitivity resulting in increased hepatic glucose production (23). Nutrient malabsorption that occurs secondary to pancreatitis leads to impaired incretin secretion that can result in diminished insulin release by the remaining beta-cells (24). Acute pancreatitis can induce transient hyperglycemia that can last for several weeks or permanent hyperglycemia (20,25,26). Predictors of the development of diabetes include obesity, a family history of diabetes, exocrine pancreatic insufficiency, history of pancreatic surgery, pancreatic calcifications, and long duration of pancreatitis (27).

 

The prevalence of diabetes secondary to pancreatitis varies greatly with studies in North America estimating a prevalence of 0.5%-1.15% whereas in Southeast Asia, where tropical or fibrocalcific pancreatitis is endemic, a prevalence of approximately 15%-20% has been reported (22) (see chapter in Tropical Endocrinology Section of Endotext entitled “Fibrocalculous Pancreatic Diabetes” for an in depth discussion of this entity (7). Recently, data from the UK Royal College of General Practitioners Research and Surveillance Centre found 559 cases of diabetes following pancreatic disease in 31,789 cases of adults newly diagnosed with diabetes (1.8%) (28). Most cases of diabetes following pancreatic disease were classified as T2D (28). In another study approximately 50% of the patients with diabetes secondary to pancreatitis were not recognized and were incorrectly thought to have T2D (5). It is very likely that many cases of diabetes secondary to pancreatitis are not recognized to be due to pancreatic disease.

 

The prevalence of diabetes in patients with diagnosed pancreatitis has ranged between 26-80%, depending on the cohort and duration of follow up (20,22,29). The prevalence of diabetes increases with the duration of pancreatitis and early onset of calcific disease (22). Because of the high risk of diabetes in patients with pancreatitis these patients should be periodically screened for the presence of diabetes with measurement of fasting glucose and/or A1c levels.

 

At times it can be difficult to distinguish diabetes secondary to pancreatitis from T1D or T2D. The following diagnostic criteria have been proposed (Table 2) (22).

 

Table 2. Proposed Diagnostic Criteria for Diabetes Secondary to Pancreatitis

Major Criteria (must be present)

Presence of exocrine pancreatic insufficiency (monoclonal fecal elastase-1 test or direct function tests)    

Pathological pancreatic imaging (endoscopic ultrasound, MRI, CT)    

Absence of T1D associated autoimmune markers

Minor Criteria

Absent pancreatic polypeptide secretion    

Impaired incretin secretion (e.g., GLP-1)    

No excessive insulin resistance (e.g., HOMA-IR)    

Impaired beta cell function (e.g., HOMA-B, C-Peptide/glucose-ratio)    

Low serum levels of lipid soluble vitamins (A, D, E and K)

 

It should be recognized that these proposed criteria have not been rigorously tested nor are all criteria available in routine clinical practice. In addition, there are a number of key considerations. First, long-standing T1D and T2D are associated with exocrine pancreatic failure (30). It has been estimated that 26% to 74% of patients with T1D and 28% to 36% of patients with T2D have evidence of exocrine pancreatic insufficiency (18). Second, patients with diabetes are at a higher risk for developing acute and/or chronic pancreatitis (31). Lastly, patients with previous episodes of pancreatitis may also develop T1D or T2D independently of their exocrine pancreatic disease. When diabetes occurs in patients with a pre-existing diagnosis of chronic pancreatitis it is likely that the pancreatitis is an important contributor to the development of the diabetes.

 

Testing for T1D associated autoimmune markers can be helpful in separating T1D from diabetes secondary to pancreatic disease. The presence of islet cell antibodies supports the diagnosis of T1D. The pancreatic polypeptide response to insulin-induced hypoglycemia, secretin-infusion, or a mixed nutrient ingestion can be helpful in separating T2D from diabetes secondary to pancreatic disease. Patients with diabetes secondary to pancreatitis have an absent or reduced pancreatic polypeptide response while patients with T2D have an elevated pancreatic polypeptide response (22,29). Studies have shown that pancreatic polypeptide regulates hepatic insulin sensitivity and the absence of pancreatic polypeptide leads to hepatic insulin resistance and enhances hepatic glucose production, which could contribute to the abnormal glucose metabolism that occurs with pancreatic disease (20).

 

In patients with diabetes secondary to pancreatitis hyperglycemia can be mild to very severe depending upon the degree of pancreatic destruction leading to impaired insulin production and secretion (18,20,22). Glycemic control may be unstable due to the loss of glucagon secretion in response to hypoglycemia, carbohydrate malabsorption, and inconsistent food intake due to pain and/or nausea secondary to pancreatitis (i.e., “brittle diabetes”) (18,20,22). Whether glycemic control is worse in patients with diabetes secondary to pancreatitis is uncertain as older studies reported worse glycemic control and more recent studies have reported that glycemic control was similar to other patients with diabetes (18). The ability to obtain good glycemic control is likely to be related to the degree of pancreatic insufficiency with patients with a total absence of pancreatic function being more difficult to control.

 

In patients with relatively mild diabetes treatment with metformin is indicated. The GI side effects (nausea, abdominal complaints, diarrhea) of metformin may not be tolerable in some patients with pancreatitis. In observational studies metformin therapy has been associated with a reduction in the development of pancreatic cancer in patients with diabetes (32). Given the increased risk of pancreatic cancer in patients with diabetes and/or pancreatitis a reduction in the development of pancreatic cancer would be a potential added benefit of metformin therapy (33-35). There are conflicting data on whether treatment with DPP4-inhibitors or GLP1-analogues can cause pancreatitis, but until this issue has been unequivocally settled, it is wise to refrain from using these drugs in patients who have had pancreatitis without a clear reversible etiology (for example, gallstone pancreatitis status post cholecystectomy). Thiazolidinediones should probably be avoided as patients with pancreatitis and malabsorption are at increased risk for osteoporosis and thiazolidinediones may potentiate this problem.

 

Chronic pancreatitis is a progressive disease and therefore it is likely that glycemic control will worsen overtime and most patients will eventually require insulin therapy (22). Many patients will have severe insulin deficiency and will need to be treated with insulin therapy using regimens employed in patients with T1D. Because of the absence of glucagon secretion patients with diabetes secondary to pancreatitis are more susceptible to severe hypoglycemia with insulin therapy but diabetic ketoacidosis is not commonly observed due to the absence of glucagon.

 

Patients with diabetes secondary to pancreatitis are at risk for microvascular complications and lower extremity arterial disease and therefore routine testing for eye disease, kidney disease, foot ulcers, and neuropathy should be instituted (36-39).

 

Finally, it should be recognized that patients with diabetes secondary to pancreatitis will almost always have exocrine pancreatic insufficiency (29). Many patients with chronic pancreatitis manifest fat malabsorption without symptoms and therefore a thorough evaluation is required. Oral pancreatic enzyme replacement is beneficial in these patients. Of note, pancreatic enzyme supplementation can improve incretin secretion and thereby may benefit glycemic control (20,24,40). Fat soluble vitamin deficiency commonly occurs (Vitamin A, D, and K) and many patients will require supplementation with fat-soluble vitamins.

 

Pancreatectomy

 

The metabolic abnormalities that occur after pancreatic surgery depend on the amount and area of the pancreas removed and whether the remaining pancreas is normal or diseased (23). The basis for this variability is due to the distribution of β and non-β islet cell types in the pancreas. Islet density is relatively low in the head of the pancreas and gradually increases through the body toward the tail region by greater than 2-fold and thus α- and β-cells predominate in the tail. In contrast, the cells that secrete pancreatic polypeptide are mainly localized in the head of the pancreas. Distal pancreatectomy usually causes little change in the metabolic status unless more than 50% of parenchyma is excised in patients with diffuse disease or more than 80% in patients with normal pancreatic function (23). Approximately 4.8 to 38% of patients develop diabetes after a distal pancreatectomy (23). Resection of the head of the pancreas results in a decrease in pancreatic polypeptide, hepatic insulin resistance, and fasting hyperglycemia. Approximately 20% of patients develop diabetes after resection of the head of the pancreas (23). It should also be recognized that removal of pancreatic tissue can accelerate the development of T2D by decreasing insulin secretion in patients with impaired glucose metabolism (41).

 

Patients who have undergone total surgical pancreatectomy have a deficiency of insulin, glucagon, and pancreatic polypeptide and require insulin treatment. In general, there are several differences from typical T1D, including exocrine deficiency, low insulin requirements, and a higher risk of hypoglycemia due to the decrease in glucagon, which stimulates hepatic glucose production (glycogenolysis and gluconeogenesis). Pancreatectomized patients are prone to hypoglycemia and a delayed recovery from hypoglycemia. In an evaluation of 180 patients post pancreatectomy 42% experienced one or more hypoglycemic events on a monthly basis (42). In addition to treatment with insulin, pancreatic enzyme supplements are always needed. Intraportal islet auto transplantation has been used to prevent the development of diabetes with total pancreatectomy and/or reduce the risk of developing difficult to control diabetes (43-45).

 

Pancreatic Cancer

 

A high percentage of patients with pancreatic carcinoma have diabetes (20,46). In one study 68% of patients with pancreatic cancer also had diabetes (47). The prevalence of diabetes in patients with pancreatic cancer is much higher than in other common malignancies (46,47). In patients with pancreatic cancer who also have diabetes, the diagnosis of diabetes occurred less than 2 years prior to the diagnosis of pancreatic cancer in 74% of patients (48). In a population-based study 0.85% of patients over the age of 50 years with newly diagnosed diabetes were diagnosed with pancreatic cancer within 3 years (49). In a study of 115 patients over 50 years of age who were hospitalized for new-onset diabetes 5.6% were found to have a pancreatic cancer (50). Many patients with pancreatic cancer lose weight and therefore deteriorating glycemic control in conjunction with weight loss and anorexia should raise the possibility of an occult pancreatic cancer (46,51). Other clues to the presence of pancreatic cancer in a patient with new onset diabetes are the lack of a family history of diabetes, a BMI < 25, and the absence of features of the metabolic syndrome such as dyslipidemia and hypertension. Given the high incidence of diabetes relative to the incidence of pancreatic cancer the routine screening of all patients who develop diabetes is not cost effective. However, in selected patients with the features described above screening is appropriate.

 

Conversely, long standing T2D increases the risk of developing pancreatic cancer by approximately 1.5 to 2-fold indicating a bidirectional relationship (23,46,52). This risk may persist even after adjustment for obesity and smoking, risk factors for pancreatic cancer. Diabetes is both a risk factor for the development of pancreatic cancer and a complication of pancreatic cancer.

 

As discussed above diabetes may develop secondary to chronic pancreatitis. Chronic pancreatitis increases the risk of pancreatic cancer. Thus, patients with diabetes secondary to chronic pancreatitis are at a higher risk of developing pancreatic cancer (53).  

 

The strongest evidence linking pancreatic cancer with incident diabetes is the beneficial effects of cancer resection on glycemic control (46). In a small study in 7 patients, Permert and colleagues reported an improvement in diabetes status and glucose metabolism after subtotal pancreatectomy (54). Similarly, Pannala and colleagues in a larger study reported that after pancreaticoduodenectomy, diabetes resolved in 17 of 30 patients (57%) with new-onset diabetes but was unaffected in patients with longstanding diabetes (48). Litwin and colleagues noted similar improvements in glucose metabolism after surgery in patients with pancreatic cancer but a deterioration in patients with chronic pancreatitis (55). Finally, studies have also shown that a good response to chemotherapy in patients with pancreatic cancer can also improve glucose levels (56). Taken together these results demonstrate a benefit from tumor removal and suggest that new-onset diabetes associated with pancreatic cancer may be a paraneoplastic phenomenon.

 

The mechanism accounting for the development of new onset diabetes by pancreatic cancers is unknown (46). In contrast to other pancreatic disorders the etiology of diabetes is not due to destruction of the pancreas as patients with new onset diabetes and pancreatic cancer have hyperinsulinemia rather than low insulin levels and as noted above the diabetes improves after resection (20). Additionally, the pancreatic cancers may be very small and thus unlikely to cause pancreatic insufficiency. Pancreatic cancer is associated with insulin resistance but the factors leading to insulin resistance are unknown (20).

 

In patients with pancreatic cancer the main goal of the treatment of diabetes is to prevent the short- term metabolic complications and facilitate the ability of the patient to tolerate treatment of the pancreatic cancer (surgery and chemotherapy). Given the poor survival of patients with pancreatic cancer prevention of the long-term sequelae of diabetes is not a major focus. Metformin is a preferred hypoglycemic agent because there are observational studies suggesting that metformin may improve survival in patients with pancreatic cancer (57-59). However, randomized trials have failed to demonstrate a benefit of metformin therapy in patients with pancreatic cancer (60,61).

 

Hemochromatosis

 

Hemochromatosis is an autosomal recessive disorder characterized by increased iron absorption by the GI tract and increased total body iron stores (62). The excess iron is sequestered in many different tissues including the liver, skin, heart, and the pancreas. The classic triad of hemochromatosis is diabetes mellitus, hepatomegaly, and increased skin pigmentation (“bronze diabetes”), but clinical features also include gonadal failure, arthropathy, and cardiomyopathy (62).

 

In early studies diabetes was present in over 50% of patients with hemochromatosis (63,64). More recently the prevalence of diabetes in patients with hemochromatosis has decreased to approximately 20% of patients, presumably due to the early diagnosis and treatment of hemochromatosis due to genetic testing (63-66). In patients with hemochromatosis screening for the presence of diabetes should be periodically carried out.

 

Diabetes was typically observed in persons who also had severe iron overload and cirrhosis (64). In recent studies with lower rates of diabetes impaired glucose tolerance was observed in approximately 30% of patients with hemochromatosis (63,66). It should be noted that iron overload from any cause can result in diabetes (67). For example, patients with thalassemia develop iron overload due to the need for frequent transfusions (68,69). The prevalence of diabetes in patients with thalassemia has been declining since the more aggressive and widespread use of iron chelation therapy (69).

 

There are two abnormalities that lead to abnormal glucose metabolism in patients with hemochromatosis and iron overload (63). First, iron overload leads to beta cell damage with decreased insulin production and secretion. Pathologic examination revealed hemosiderin deposition and iron-induced fibrosis of the islets (64). The decrease in insulin secretion is the primary defect leading to the development of diabetes (63,64,66). Of note glucagon secretion does not appear to be deficient in patients with diabetes and hemochromatosis suggesting that the iron overload has a preferential toxicity for beta cells compared to alpha cells (64,70,71). Similarly, basal and stimulated pancreatic polypeptide levels are also not decreased in diabetic patients with hemochromatosis (72). Thus, the hormonal abnormalities differ in patients with iron overload induced diabetes compared to patients with pancreatitis induced diabetes. The second abnormality is insulin resistance that occurs due to iron overload hepatic damage and/or secondary to obesity (63,64). In addition, a genetic predisposition to diabetes potentiates the development of metabolic dysfunction. Many patients with hemochromatosis and diabetes have a relative with diabetes (73).

 

The typical micro and macrovascular complications of diabetes occur in patients with hemochromatosis (64,73). In a study by Griffiths and colleagues, 11 of 49 patients with hemochromatosis and diabetes had diabetic retinopathy (74). Sixty percent of the patients with hemochromatosis who had diabetes for greater than 10 years had retinopathy. The incidence of retinopathy is similar to that observed in the general diabetes population (73,74). Similarly, Becker and Miller observed that 7 of 22 patients with diabetes and hemochromatosis had pathologic evidence of diabetic glomerulopathy (75).

 

The treatment of hemochromatosis by phlebotomy has a variable impact on glucose metabolism (63). In patients who do not yet have complications or organ damage an improvement of insulin secretory capacity and normalization of glucose tolerance has been observed (63,64). Glucose metabolism often improves in patients with impaired glucose tolerance (63,76). In patients with diabetes improvement in glucose metabolism by phlebotomy may occur but is not as common as in “pre-diabetics” (63,76,77). In one study 28% of patients with diabetes and hemochromatosis on insulin or oral agents showed improved glucose control following phlebotomy therapy (78).

 

Cystic Fibrosis

 

Cystic Fibrosis is an autosomal recessive disorder due to a defect in the chloride transport channel (79). Cystic fibrosis related diabetes is rare in children but is present in approximately 20% of adolescents and 40-50% of adults with cystic fibrosis (80,81). As patients with cystic fibrosis live longer it is likely that the number of patients with cystic fibrosis and diabetes will increase. The development of diabetes is associated with more severe cystic fibrosis gene mutations, increasing age, worse pulmonary function, undernutrition, liver dysfunction, pancreatic insufficiency, a family history of diabetes, female gender, and corticosteroid use (80,81).

 

The primary defect in patients with cystic fibrosis related diabetes is decreased insulin production and secretion due to fibrosis and atrophy of the pancreas with a reduction of islet mass (80). In addition, mutations in the cystic fibrosis transmembrane conductance regulator gene may have direct effects on the ability of beta cells to secrete insulin (81,82). Beta cell dysfunction is not complete with residual insulin secretion and thus patients with cystic fibrosis related diabetes do not typically develop ketosis (80). Reduced alpha cell mass also occurs so while fasting glucagon levels are normal, glucagon secretion in response to hypoglycemia is impaired (80). Peripheral and hepatic insulin resistance may also occur secondary to infections and inflammation (81).

 

Some of the clinical features of cystic fibrosis related diabetes are similar to T1D as patients are young, not obese, insulin deficiency is the primary defect, and features of the metabolic syndrome (hyperlipidemia, hypertension, visceral adiposity) are not usually present (80). However, cystic fibrosis related diabetes is not an autoimmune disease (islet cell antibodies are not present) and ketosis is rare because endogenous insulin is still produced (80). Fasting glucose levels are often normal initially with elevated postprandial glucose levels due to a reduced and delayed insulin response to carbohydrates while basal insulin is often adequate to maintain normal glucose levels (83). Patients with cystic fibrosis related diabetes are not at high risk to develop atherosclerosis and heart disease is not a major issue (80-82). This is likely due to malabsorption leading to life-long low plasma cholesterol levels and the shortened length of life (80,81). As life expectancy increases the risk of macrovascular disease may increase. Microvascular complications do occur in cystic fibrosis related diabetes and are related to the duration of diabetes and glycemic control (80,81,83). The American Diabetes Association recommends screening for complications of diabetes beginning 5 years after the diagnosis of cystic fibrosis related diabetes (84)

 

Lung disease is a major cause of morbidity and mortality in patients with cystic fibrosis and both insulin insufficiency and hyperglycemia negatively affect cystic fibrosis lung disease (85). Numerous studies have shown that the occurrence of diabetes in patients with cystic fibrosis is associated with more severe lung disease and increased mortality and this adverse effect disproportionately affects women (80,83,85). In patients with cystic fibrosis lung function is critically dependent on maintaining normal weight and lean body mass. Insulin deficiency leads to a catabolic state with the loss of protein and fat (80). Multiple studies have shown that insulin replacement therapy improves nutritional status and pulmonary function in patients with cystic fibrosis related diabetes (80). In addition, elevated blood glucose levels result in elevated blood glucose levels in the airways, which promotes the growth of pathogenic microorganisms and increase pulmonary infections (80,83). Of note recent studies have shown that the marked increase in mortality in patients with cystic fibrosis related diabetes compared to patients with cystic fibrosis only has diminished (86). It is likely that early diagnosis and aggressive treatment has improved survival in patients with cystic fibrosis related diabetes.

 

Because of the adverse effects of diabetes on lung function in patients with cystic fibrosis routine screening for diabetes is recommended (85). It is recommended that annual screening begin at age 10 (85). While fasting glucose and A1c levels are routine screening tests for diabetes, in patients with cystic fibrosis these tests are not sensitive enough (85). Fasting glucose and A1c testing will fail to diagnose approximately 50% of patients with cystic fibrosis related diabetes (82,85). However, recent studies have suggested that a screening A1c >5.5% would detect more than 90% of patients with diabetes and therefore with further confirming studies measuring A1c levels could become an initial screening approach (84).  As noted above, abnormalities in postprandial glucose characterizes cystic fibrosis related diabetes and it is therefore recommended that an oral glucose tolerance test (OGTT) be utilized for the diagnosis of diabetes in patients with cystic fibrosis (85). Studies have shown that the diagnosis of diabetes by OGTT correlates with clinically important cystic fibrosis outcomes including the rate of lung function decline, the risk of microvascular complications, and the risk of early death (85). Moreover, the OGTT identified patients who benefited from insulin therapy (85). Additional screening recommendations are shown in Table 3 and the interpretation of these tests are shown in Table 4.

 

Table 3. ADA and Cystic Fibrosis Foundation Recommendations for Screening for Cystic Fibrosis Related Diabetes (CFRD) (85)

1.     1) The use of A1C as a screening test for CFRD is not recommended.

2.     2) Screening for CFRD should be performed using a 2-h 75-g OGTT. 

3.     3) Annual screening for CFRD should begin by age 10 years in all CF patients who do not have CFRD.

4.     4) CF patients with acute pulmonary exacerbation requiring intravenous antibiotics and/or systemic glucocorticoids should be screened for CFRD by monitoring fasting and 2-h postprandial plasma glucose levels for the first 48 h. If elevated blood glucose levels are found by SMBG, the results must be confirmed by a certified laboratory.

5.     5) Screening for CFRD by measuring mid- and immediate post-feeding plasma glucose levels is recommended for CF patients on continuous enteral feedings, at the time of gastrostomy feeding initiation and then monthly by SMBG. Elevated glucose levels detected by SMBG must be confirmed by a certified laboratory.

6.     6) Women with CF who are planning a pregnancy or confirmed pregnant should be screened for preexisting CFRD with a 2-h 75-g fasting OGTT if they have not had a normal CFRD screen in the last 6 months. 

7.     7) Screening for gestational diabetes mellitus is recommended at both 12–16 weeks’ and 24–28 weeks’ gestation in pregnant women with CF not known to have CFRD, using a 2-h 75-g OGTT with blood glucose measures at 0, 1, and 2 h. 

8.     8) Screening for CFRD using a 2-h 75-g fasting OGTT is recommended 6–12 weeks after the end of the pregnancy in women with gestational diabetes mellitus (diabetes first diagnosed during pregnancy). 

9.     9) CF patients not known to have diabetes who are undergoing any transplantation procedure should be screened preoperatively by OGTT if they have not had CFRD screening in the last 6 months. Plasma glucose levels should be monitored closely in the perioperative critical care period and until hospital discharge. Screening guidelines for patients who do not meet diagnostic criteria for CFRD at the time of hospital discharge are the same as for other CF patients.

CF= cystic fibrosis; CRFD= cystic fibrosis related diabetes; OGTT= oral glucose tolerance test, SMBG= self-monitored blood glucose

 

Table 4. Criteria for the Diagnosis of Cystic Fibrosis Related Diabetes (85)

1.     1) During a period of stable baseline health the diagnosis of CFRD can be made in CF patients according to standard ADA criteria. Testing should be done on 2 separate days to rule out laboratory error unless there are unequivocal symptoms of hyperglycemia (polyuria and polydipsia); a positive FPG or A1C can be used as a confirmatory test, but if it is normal the OGTT should be performed or repeated. If the diagnosis of diabetes is not confirmed, the patient resumes routine annual testing.

·       2-h OGTT plasma glucose >200 mg/dl (11.1 mmol/l)

·       FPG >126 mg/dl (7.0 mmol/l)

·       A1C > 6.5% (A1C <6.5% does not rule out CFRD because this value is often spuriously low in CF.)

·       Classical symptoms of diabetes (polyuria and polydipsia) in the presence of a casual glucose level >200 mg/dl (11.1 mmol/l)

2.     2) The diagnosis of CFRD can be made in CF patients with acute illness (intravenous antibiotics in the hospital or at home, systemic glucocorticoid therapy) when FPG levels >126 mg/dl (7.0 mmol/l) or 2-h postprandial plasma glucose levels >200 mg/dl (11.1 mmol/ l) persist for more than 48 h.

3.     3) The diagnosis of CFRD can be made in CF patients on enteral continuous drip feedings when mid- or post=feeding plasma glucose levels exceed 200 mg/dl (11.1 mmol/l) on 2 separate days.

4.     4) Diagnosis of gestational diabetes mellitus is diagnosed based on 0-, 1-, and 2-h glucose levels with a 75-g OGTT if any one of the following is present:

·       FPG >92 mg/dl (5.1 mmol/l)

·       1-h plasma glucose >180 mg/dl (10.0 mmol/l)

·       2-h plasma glucose >153 mg/dl (8.5 mmol/l)

5.     CF patients with gestational diabetes mellitus are not considered to have CFRD, but require CFRD screening 6–12 weeks after the end of the pregnancy.

6.     5) The onset of CFRD should be defined as the date a person with CF first meets diagnostic criteria, even if hyperglycemia subsequently abates.

CF= cystic fibrosis; CRFD= cystic fibrosis related diabetes; OGTT= oral glucose tolerance test

 

There is evidence that elevations in glucose below the levels typically used to diagnose diabetes result in adverse effects on the lungs (83). Thus, some experts recommend that treatment should be considered for individuals with abnormal glucose levels which do not meet the criteria for diabetes if there is evidence of declining lung function or weight loss (83).

 

A unique feature in the treatment of patients with cystic fibrosis related diabetes is that insulin is the treatment of choice in all patients (85). Studies have shown that cystic fibrosis patients on insulin therapy who achieve good glycemic control demonstrate improvement in weight, protein anabolism, pulmonary function, and survival (85). No specific insulin treatment regimen is recommended and the regimen should be individualized for the patient. For example, a patient with elevated postprandial glucose levels will benefit from meal time rapid acting insulin. It should be noted that patients with cystic fibrosis induced diabetes still have endogenous insulin production, which allows for the achievement of good glycemic control. Oral diabetes agents are not as effective as insulin in improving nutritional and metabolic outcomes and therefore are not recommended (85). For most patients with cystic fibrosis related diabetes an A1c < 7% is recommended but the A1c goal can be higher or lower for certain patients based on clinical judgement. Also of note is that cystic fibrosis patients require a high-calorie, high-salt, high-fat diet.

 

Ivacaftor, a cystic fibrosis transmembrane conductance regulator modulator, is a relatively new agent to treat cystic fibrosis and has been shown to partially reverse the disease. Interestingly in case reports ivacaftor has been shown to markedly improve glycemic control (81,87). In a recent retrospective observation study approximately 1/3 of patients with CFRD had either a resolution of their diabetes or marked improvement with ivacaftor therapy (88). Additionally, the risk of developing CFRD is decreased in patients treated with ivacaftor (89). This beneficial effect is likely due to an improvement in insulin secretion (81,90).

 

INFECTIONS

 

Viral Infections

 

Viral infections, particularly enterovirus and herpes virus infections, have been postulated to play a role in triggering the autoimmune reaction that leads to the development of T1D (91-93). This phenomenon is discussed in detail in the Endotext chapter on the pathogenesis of T1D and the Endotext chapter on changing the course of the disease in T1D (94,95). In rare instances a viral infection has been associated with the fulminant development of diabetes due to the destruction of beta cells (96).

 

Congenital Rubella

 

Congenital rubella infection has been shown to predispose to the development of T1D that usually presents before five years of age (97). It has been estimated that approximately 1-6% of individuals with the rubella syndrome will develop diabetes in childhood or adolescence (97,98). The mechanism for this association is unknown. In addition, studies have also shown that patients with congenital rubella also develop T2D (98). In one series 22% of individuals with congenital rubella developed diabetes later in life (98). Fortunately, with increased vaccinations, congenital rubella has become a disease of the past in developed countries.

 

Hepatitis C Virus (HCV)

 

Meta-analyses and large data base studies have demonstrated that HCV infection is associated with an increased risk of T2D (99-104). In a meta-analysis of 34 studies the risk of diabetes in patients with HCV infection was increased by approximately 70% (99). Moreover, HCV infection is associated with an increased risk of T2D independent of the severity of the associated liver disease (i.e. occurs in patients without liver disease) but the risk of T2D was higher in HCV patients with cirrhosis (100). As expected, the risk of diabetes is increased in HCV patients if the BMI is increased, there is a family history of diabetes, older age, more severe liver disease, and male sex. Conversely, the prevalence of HCV infection in patients with T2D is higher than in non-diabetic controls (100,104,105). In a meta-analysis of 22 studies with 78,051 individuals it was found that patients with T2D were at a higher risk of HCV infection than non-diabetic patients (OR = 3.50; CI = 2.54-4.82) (105). Finally, diabetes is a significant risk factor for the development of liver cirrhosis and hepatocellular carcinoma in HCV infected patients (104,106-109).

 

Given the increased risk of diabetes in HCV infected patients it seems prudent to routinely screen HCV positive patients for diabetes. Conversely, screening patients with diabetes for HCV infection seems reasonable given the availability of drugs that can effectively treat HCV infections.

 

Patients with diabetes and HCV infection are insulin resistant in the liver and peripheral tissues (104,109,110). Insulin resistance is present in HCV infection in the absence of significant liver dysfunction and prior to the development of diabetes (110). Treatment that reduces viral load decreases insulin resistance and the risk of developing diabetes in HCV (104,110,111). The insulin resistance in individuals with HCV infections may be due to inflammation induced by cytokines such as TNF-alpha or monocyte chemoattractant protein-1, released from HCV-induced liver inflammation (104,109). Additionally, HCV may directly activate the mTOR/S6K1 signaling pathway, inhibiting IRS-1 protein function and thereby down-regulating GLUT-4 and up-regulating the gluconeogenic enzyme phosphoenolpyruvate carboxykinase-2 (104,109).

 

Studies have shown that direct-acting antiviral agents that eradicate HCV infection are associated with improved glycemic control in patients with diabetes indicated by decreased A1c levels and decreased insulin use (109,112). Additionally, in a database study of anti-viral treatment for HCV infection, a decrease in end-stage renal disease, ischemic stroke, and acute coronary syndrome was reported (113). These beneficial results on key outcomes need to be confirmed in randomized trials (this may be impossible as withholding treatment of HCV is not appropriate). Treatment of diabetes with metformin or thiazolidinediones is preferred as studies have suggested that these drugs may lower the risk of hepatocellular carcinoma, liver-related death, or liver transplantation in patients infected with HCV (114,115).

 

COVID-19

 

Studies have shown that COVID-19 infections are associated with hyperglycemia and new onset diabetes (116). In a meta-analysis of 8 studies with a total of 3711 COVID-19

patients 492 cases of newly diagnosed diabetes were found (14.4%) (117). There are a number of possibilities that could explain this association (116):

  • The diabetes could be secondary to acute illness and stress induced hyperglycemia. Stress induced hyperglycemia has been observed after other acute conditions including other infections.
  • Pre-existing diabetes could be first recognized during a COVID-19 infection
  • SARS-CoV-2 virus could directly damage the beta cells leading to decreased insulin secretion and hyperglycemia.
  • SARS-CoV-2 virus could lead to pancreatitis indirectly effecting beta cell function
  • The strong immune response that is seen in COVID-19 infections (cytokine storm) could indirectly lead to beta cell dysfunction.
  • The use of high dose glucocorticoids in patients with severe COVID-19 could lead to hyperglycemia and diabetes.

 

Hopefully, future studies will better characterize the mechanisms leading to new onset diabetes in patients with COVID-19 infections and determine whether there is a unique mechanism for this association.

 

ENDOCRINOPATHIES

 

A number of endocrine disorders are associated with an increased occurrence of diabetes.  Increased levels of GH, glucocorticoids, catecholamines, glucagon cause insulin resistance and increased levels of catecholamines, somatostatin, and aldosterone (by producing hypokalemia) decrease insulin secretion and hence can adversely affect glucose homeostasis. The disturbance in glucose metabolism occurring secondary to endocrine disorders may vary from a moderate degree of glucose intolerance to overt diabetes with symptomatic hyperglycemia. Additionally, the endocrine disorders can worsen glycemic control in patients with pre-existing diabetes.

 

Acromegaly

 

This condition is caused by excessive production of growth hormone (GH) from the pituitary (118). The prevalence of DM in patients with acromegaly is between 10-40%; the prevalence of diabetes and glucose intolerance effects more than 50% of patients (118-120). As expected, there is an increased prevalence of diabetes mellitus with age, elevated BMI, a family history of diabetes, and longer duration of acromegaly (119). Diabetes may be present at the time of the diagnosis of acromegaly (121). Higher plasma IGF-1 concentrations correlate with an increased risk of diabetes suggesting that the biochemical severity of acromegaly influences the risk of developing abnormalities of glucose metabolism (122). Patients with acromegaly should be screened for abnormalities in glucose metabolism (120). The prevalence of acromegaly in patients with diabetes is unknown but is likely to be very low given that acromegaly is an uncommon disorder (60 per million) and diabetes is very common (118).

 

GH is a counter regulatory hormone to insulin and is secreted during hypoglycemia (123,124). In patients with acromegaly insulin resistance is the major abnormality leading to disturbances in glucose metabolism (119,121,125). The insulin resistance is driven primarily through GH induced lipolysis, which results in glucose-fatty acid substrate competition leading to decreased glucose utilization in muscle (119,121,125). Additionally, inhibition of post receptor signaling pathway of the insulin receptor also likely plays a role in the insulin resistance (125). Increased hepatic gluconeogenesis also contributes to the hyperglycemia (119,125). Lipolysis increases the delivery of glycerol and fatty acids to the liver, which serve as a substrate and energy source for gluconeogenesis. In some patients with acromegaly increased insulin secretion compensates for the insulin resistance and glucose metabolism remains normal (119). If insulin secretion cannot increase sufficiently to compensate for the insulin resistance glucose intolerance or diabetes develops (119).

 

Treatment is directed at the cause of the acromegaly (118). Successful surgical removal of the pituitary adenoma improves hyperglycemia and glucose metabolism has been reported to normalize in 23–58% of people with pre-operative diabetes after surgical cure of acromegaly (119-121). Lower IGF-1 and growth hormone levels post operatively correlate with remission of diabetes (126). A meta-analysis of 31 studies with 619 patients treated with somatostatin analogues for acromegaly reported a decrease in insulin levels and glucose levels during a glucose tolerance test but no change in fasting glucose or A1c levels (127). The absence of greater benefit in glucose homeostasis with somatostatin analogues could be secondary to somatostatin analogues inhibiting insulin secretion (118). Of note, while first generation somatostatin analogues appear to have mild or neutral effects on glucose metabolism in patients with acromegaly, treatment with pasireotide, a second-generation somatostatin analogue, aggravated glucose metabolism leading to the development of diabetes in some instances (128,129). The adverse effect of pasireotide is due to inhibiting insulin secretion and decreasing the incretin effect. There are little data on the impact of cabergoline on glucose homeostasis in patients with acromegaly but the available studies suggest that it modestly improves glucose metabolism (120). Studies have shown that bromocriptine can improve glucose homeostasis (119,120,130).  Finally, treatment with the growth hormone receptor antagonist, pegvisomant, has beneficial effects on glucose homeostasis (131,132).

 

The treatment of diabetes in patients with acromegaly is similar to the treatment in other patients with diabetes (119). Patients with acromegaly are often lean with low body fat and therefore dietary recommendations may need to be modified. Additionally, since insulin resistance is the primary defect in patients with acromegaly the use of insulin sensitizers may be especially effective but there are no studies comparing the efficacy of various hypoglycemic agents in patients with acromegaly (119). Data suggest that active acromegaly with elevated GH levels enhances the development of microvascular disease (121). The effect of acromegaly on the development of macrovascular disease is unclear (121). Ketoacidosis is uncommon in patients with diabetes and acromegaly.   

 

Cushing’s Syndrome

 

Cushing’s syndrome is due to elevated glucocorticoids that can be caused by the overproduction of ACTH by pituitary adenomas or ectopic ACTH producing tumors, overproduction of glucocorticoids by the adrenal glands due to adenomas or hyperplasia, or the exogenous administration of glucocorticoids (133). In patients with Cushing’s syndrome diabetes is present in 20-47% of the patients, while impaired glucose tolerance (IGT) is present in 21-64% of cases (134). Risk factors for the development of diabetes in patients with Cushing syndrome include age, obesity, and a family history of diabetes (134). The prevalence of diabetes varies depending on the etiology of Cushing’s syndrome (pituitary 33%, ectopic 74%, adrenal 34%) (135). In patients with endogenous Cushing’s syndrome the relationship of the degree of hypercortisolism and abnormalities in glucose metabolism has been inconsistent with some studies showing a correlation and other studies no relationship (136). For example, in one study, in patients with endogenous Cushing’s syndrome the prevalence of abnormalities in glucose metabolism and diabetes did not differ in patients with slightly elevated (not greater than 2x the upper limit of normal), moderately elevated (2-5X the ULN), and severely elevated (>5x the ULN) levels of urinary free cortisol (137). In patients with exogenous Cushing’s syndrome high doses of glucocorticoids and longer duration of treatment are more likely to cause diabetes (120,136). Elevated glucocorticoids are more likely to cause high glucose levels in the afternoon or evening and in the postprandial state (120). Hyperglycemia resulting from exogenous steroids occurs in concert with the time-action profile of the steroid regimen employed, such that once daily morning administration of an intermediate acting steroid (prednisone or methylprednisone) causes peak hyperglycemia within 12 hours (post-prandial) while long-acting or frequently administered steroids cause both fasting and postprandial hyperglycemia.

 

Patients with Cushing’s syndrome should be screened for the presence of abnormalities in glucose metabolism (136). It should be noted that fasting glucose levels are often normal with abnormalities present during an oral glucose tolerance test (134,136). Screening with A1c levels or with an oral glucose tolerance test are therefore preferred. The abnormalities in glucose metabolism may contribute to the increased risk of atherosclerosis in patients with Cushing’s syndrome.

 

The prevalence of Cushing’s syndrome in patients with diabetes is uncertain with studies reporting very different results ranging from 0 to 9% (136). The selection process used and the criteria used to determine the presence of Cushing’s syndrome likely greatly influences the results with studies that select patients with marked obesity, poor glycemic control, and poorly controlled hypertension finding a higher percentage of patients with diabetes having Cushing’s syndrome. A recent meta-analysis of 14 studies with a total of 2827 patients with T2D reported that 1.4% had Cushing’s syndrome based on biochemical analysis (138). In a multicenter study carried out in Italy between 2006 and 2008, 813 patients with known T2D without clinically overt hypercortisolism were evaluated for Cushing’s syndrome (139). After extensive evaluation 6 patients (0.7%) were diagnosed to have Cushing’s syndrome. Four patients had an adrenal adenoma and their diabetes was markedly improved with the disappearance of diabetes in three patients and discontinuation of insulin therapy in the remaining patient. One patient had bilateral macronodular adrenal hyperplasia and one patient had ACTH dependent Cushing’s syndrome with a normal pituitary MRI. In approximately 15-35% of patients with an incidental adrenal nodule mild autonomous cortisol secretion with T2D is present (140,141). After surgical removal of the adenoma in patients with autonomous cortisol secretion diabetes normalized or improved in 62.5% of patients (5 of 8) (142). However, not all studies have seen such dramatic improvements in diabetes after adenoma removal (143). Clearly additional studies (preferably large randomized trials) are required to better define the prevalence of mild subclinical Cushing’s syndrome in patients with diabetes and whether treating the subclinical Cushing’s syndrome in these patients will improve their glycemic control. For a detailed discussion of autonomous cortisol secretion see the chapter on Adrenal Incidentalomas in the Adrenal section of Endotext (144).

 

Currently, routinely screening patients with T2D for Cushing’s syndrome is not recommended (136). Nevertheless, clinicians should be aware of the possibility of Cushing’s syndrome and screen appropriate patients with T2D (young patients, negative family history of diabetes, patients with physical findings suggestive of Cushing’s syndrome, patients with difficult to control diabetes or hypertension) (136).    

 

Glucocorticoids function as a counter regulatory hormone to insulin and increase in response to hypoglycemia (145). Glucocorticoids disrupt glucose metabolism primarily by inducing insulin resistance in liver and muscle and by stimulating hepatic gluconeogenesis (134,136). The increase in hepatic gluconeogenesis is mediated by several mechanisms including a) directly stimulating the expression of gluconeogenic enzymes b) stimulating proteolysis and lipolysis leading to an increase delivery of amino acids, glycerol, and fatty acids to the liver that provides substrates and energy sources for gluconeogenesis c) inducing insulin resistance and d) enhancing the action of glucagon (134,136). The glucocorticoid induced increase in insulin resistance is due to inhibition of the post-receptor signaling pathway of the insulin receptor, which will result in a decrease in the uptake of glucose by skeletal muscle and adipose tissue (134). In addition to the above glucocorticoids can accelerate the degradation of Glut4 in beta cells, which impairs the ability of beta cells to secrete insulin in response to glucose (146).

 

Treatment of Cushing’s syndrome by removal of a pituitary tumor, an ectopic ACTH producing tumor, or an adrenal lesion result in a marked improvement in glucose metabolism and in many patients a remission of the diabetes (134,136). In patients with persistent Cushing’s syndrome drug therapy may be needed to normalize cortisol levels. Studies have shown that ketoconazole (200–1200 mg/day), metyrapone (250–4500 mg/day), mifepristone (300–2000 mg/day), or cabergoline (1-7mg/day) improves glucose metabolism in patients with Cushing’s syndrome (120,136).  In contrast, pasireotide has been shown to significantly worsen glucose tolerance, despite control of hypercortisolism, in patients with Cushing’s syndrome (120,136). In patients with exogenous Cushing’s syndrome it is important to use as low a dose as possible of glucocorticoids for the shortest period of time to avoid complications of therapy including disrupting glucose homeostasis (147).

 

The management of diabetes in patients with Cushing’s disease is similar to the treatment of other patients with diabetes (120). Since insulin resistance is a key defect in patients with Cushing’s syndrome the use of insulin sensitizers may be especially effective but there are no studies comparing the efficacy of various hypoglycemic agents in patients with Cushing’s syndrome (120). Pioglitazone and rosiglitazone can increase the risk of osteoporosis and it should be noted that patients with Cushing’s syndrome also have a high risk of osteoporosis. As noted above, postprandial glucose levels are preferentially increased in Cushing’s syndrome and therefore drugs that lower postprandial glucose levels, such as DPP4 inhibitors, GLP1 receptor agonists, alpha glucosidase inhibitors, and rapid acting insulin may be very useful (120). In glucocorticoid-treated patients requiring a basal-bolus insulin regimen, a higher requirement of short-acting insulin than basal insulin is frequently required (usually approximately 70% of total insulin dose as prandial and 30% as basal) (120). Because of the insulin resistance in patients with Cushing’s syndrome higher doses of insulin are often required to achieve glycemic control. Patients with Cushing’s syndrome are at higher risk for developing macrovascular disease and therefore the treatment of dyslipidemia and hypertension is required (148,149).  

 

Pheochromocytoma

 

Pheochromocytomas are rare neuroendocrine tumors that secrete norepinephrine, epinephrine, and dopamine (150). In pheochromocytomas the prevalence of diabetes mellitus has been estimated to be between 15-40% and impaired glucose tolerance as high as 50% (151-153). Patients with diabetes were older, had a longer known duration of hypertension, higher plasma epinephrine levels, increased urinary metanephrine excretion, and larger tumors (152,153). Surprisingly the BMI did not differ between patients with and without diabetes perhaps because more active tumors with higher catecholamine levels lead to weight loss (152,153). In most instances the diabetes is relatively mild but in rare instances can be severe with ketoacidosis (154). The association of hypertension with diabetes in a young patient who is not overweight is a clue to the presence of a pheochromocytoma (152).

 

Catecholamines, acting primarily by the beta-adrenergic receptors, stimulate glucose production by the liver by increasing glycogenolysis and increase insulin resistance leading to a decrease in tissue disposal of glucose, which together result in elevations in glucose levels (155-157). In addition, catecholamines acting via the alpha-adrenergic receptors, inhibit insulin secretion by the beta cells and acting via the beta-adrenergic receptors, increase glucagon secretion by the alpha cells (158). A decrease in insulin secretion and an increase in glucagon secretion would facilitate the development of hyperglycemia.

 

With tumor resection the diabetes resolves or markedly improves in the vast majority of patients (>90%) with a pheochromocytoma (152,153). A duration of diabetes of less than 3 years is associated with a remission of diabetes (159). It should be noted that post-surgical removal of a pheochromocytoma, hypoglycemia can occur in approximately 5% of patients (160). Most of these hypoglycemic episodes occur in the first 24 hours and are more likely to occur in patients with large tumors and high urinary metanephrine levels (160). If surgery is unsuccessful the use of alpha and beta blockers may improve insulin resistance and glucose homeostasis (161).

 

Hyperthyroidism 

 

Hyperthyroidism in patients without diabetes leads to an increase in glucose intolerance (162). Whether hyperthyroidism causes frank diabetes is unclear because much of the older literature that purports that hyperthyroidism causes diabetes used criteria for diabetes that differs greatly from current guidelines. For example, the study of Kreines and colleagues reported that 57% of patients with hyperthyroidism had diabetes but the criteria for diabetes was a 1 hour glucose >160mg/dL plus a 2 hour value >120 mg/dL during an oral glucose tolerance test (163). A study from China using oral glucose tolerance tests did not find a major difference in the prevalence of diabetes in patients with Grave’s disease (11.3%) vs controls (10.0%) (164). Additional studies using modern criteria for the diagnosis of diabetes are required to better elucidate the risk of developing diabetes in patients with hyperthyroidism. It should be noted that hyperthyroidism may worsen glycemic control in patients with diabetes by increasing hexose intestinal absorption, decreasing insulin sensitivity, and increasing glucose production (165).

 

T1D and Grave’s disease can occur together as part of the autoimmune polyglandular syndrome (166).

 

Glucagonoma

 

Glucagonomas are extremely rare and are associated with a characteristic rash termed necrolytic migratory erythema (82% of patients), painful glossitis, cheilitis, angular stomatitis, normochromic normocytic anemia (50-60%), weight loss (60-90%), mild diabetes mellitus (68-80%), hypoaminoacidemia, low zinc levels, deep vein thrombosis (50%), and depression (50%) (167,168). Glucagon stimulates hepatic glucose production by increasing gluconeogenesis and glycogenolysis leading to an increase in plasma glucose levels (169). Removal of the tumor results in remission of the diabetes.

 

Somatostatinoma

 

Somatostatinomas are extremely rare tumors that may present with a triad of diabetes mellitus, diarrhea/steatorrhea, and gallstones, but weight loss and hypochlorhydria also occur (170). Approximately seventy-five percent of patients with pancreatic somatostatinomas have diabetes mellitus while diabetes occurs in only approximately 10% of patients with intestinal tumors. Typically, the diabetes is relatively mild and can be controlled with diet, oral hypoglycemic agents or small doses of insulin (170). Somatostatin inhibits insulin secretion which can result in elevations in plasma glucose levels (139). Increased secretion of somatostatin by cells in the pancreas may be in closer proximity to beta cells and more effective in inhibiting insulin secretion. Somatostatin also inhibits glucagon secretion and therefore diabetic ketoacidosis is very unusual but has been reported (171). Additionally, replacement of functional islet cell tissue by pancreatic tumor may also contribute to the development of diabetes in patients with a pancreatic somatostatinoma (170). Removal of the tumor results in remission of the diabetes.

 

Primary Hyperaldosteronism

 

Hypokalemia secondary to hyperaldosteronism can impair insulin secretion and result in diabetes. Potassium replacement will improve glucose homeostasis.

 

Additionally, in a large meta-analysis the risk of diabetes (OR 1.33, 95% CI 1.01–1.74) and the metabolic syndrome (OR 1.53, 95% CI 1.22–1.91) was modestly increased in patients with primary hyperaldosteronism (172). Studies have shown that aldosterone independent of potassium levels reduces insulin secretion and induces insulin resistance via the impairment of post-receptor signaling (173).

 

DRUG- INDUCED DIABETES

 

A large number of different drugs have been shown to adversely affect glucose homeostasis (Table 1). Most of these drug’s act in conjunction with other risk factors for T2D and are usually not the sole cause of diabetes. Drug-induced hyperglycemia is often mild and may be clinically asymptomatic, but in some instances can result in the development of severe hyperglycemia manifesting as diabetic ketoacidosis. There are a number of mechanisms by which drugs induce alterations in glucose metabolism including inducing insulin resistance or inhibiting insulin secretion. In most cases the diabetes remits when the drug is stopped but in some instances the diabetes can be permanent. Use of a rodenticide (N-3 pyridylmethyl-N’4 nitrophenylurea, VacorÒ), structurally related to streptozotocin, was removed from the market in the 1980s because the ingestion of this compound resulted in insulin-dependent diabetes due to beta cell destruction (174). In this section we will focus on drugs that cause major changes in glucose homeostasis or drugs that are commonly used in clinical practice.

 

Antihypertensive Drugs

 

In a meta-analysis the risk of developing diabetes varied between different classes of antihypertensive drugs (175).  The odds ratios were: ARB 0.57; ACE inhibitor 0.67; CCB: 0.75; placebo 0.77; beta blocker 0.90 with the thiazide group set at 1.00. Similarly, in the ALLHAT study the risk of developing diabetes was greater in the thiazide group than in patients treated with an ACE inhibitor or a calcium channel blocker (176). In a meta-analysis of 10 studies of beta-blockers and 12 studies of diuretics in patients without diabetes it was found that beta-blockers increased fasting blood glucose concentrations by 11.5 mg/dL and diuretics by 13.9 mg/dl (177). In a meta-analysis of twelve studies with 94,492 patients beta-blocker therapy resulted in a 22% increased risk for new-onset diabetes compared with nondiuretic antihypertensive agents (178). Thus, both thiazide diuretics and beta blockers increase the risk of developing diabetes while ARBs and ACE inhibitors reduce the risk (179).

 

The hyperglycemia secondary to thiazide diuretics may in some instances be due to decreased insulin secretion secondary to potassium loss, which can be improved with potassium replacement (180). In addition, thiazides may directly affect insulin secretion similar to diazoxide (see below). Finally, thiazides also increase insulin resistance and enhance hepatic glucose production (180).

 

The effect on glucose metabolism differs between different beta-blockers and carvedilol, the third-generation beta-blocker has beneficial effects on glucose metabolism (179,180). A greater inhibition of insulin secretion occurs with non-selective beta-blocking agents (180). Beta blockers decrease insulin secretion and increase insulin resistance (179,180). In addition, beta-blockers increase weight, which could also adversely affect glucose homeostasis (181). Finally, beta-blockers increase the risk of severe hypoglycemia by decreasing the recovery from hypoglycemia and masking the symptoms of hypoglycemia (182).

 

Diazoxide

 

Diazoxide is a non-diuretic benzothiadiazine derivative, which increases plasma glucose levels by inhibiting insulin secretion through opening the potassium/ATP channels in beta cells (183). Diazoxide is used to control hypoglycemia in patients with insulinomas (184).

 

Statins

 

In a meta-analysis of 13 trials with over 90,000 subjects, there was a 9% increase in the incidence of diabetes during follow-up among subjects receiving statin therapy (185). All statins appear to increase the risk of developing diabetes. In comparisons of intensive vs. moderate statin therapy, Preiss et al observed that patients treated with intensive statin therapy had a 12% greater risk of developing diabetes compared to subjects treated with moderate dose statin therapy (186). Older subjects, obese subjects, and subjects with high glucose levels were at a higher risk of developing diabetes while on statin therapy (187). Thus, statins may be unmasking and accelerating the development of diabetes that would have occurred naturally in these subjects at some point in time. In patients without risk factors for developing diabetes, treatment with statins does not appear to increase the risk of developing diabetes.

 

The mechanism by which statins increase the risk of developing diabetes is unknown (187). Studies suggest that the inhibition of HMG-CoA reductase per se may be leading to the statin induced increased risk of diabetes via weight gain (187). However, a large number of studies have now shown that polymorphisms in a variety of different genes that lead to a decrease in LDL cholesterol levels are also associated with an increase in diabetes suggesting that decreases in LDL cholesterol levels per se alter glucose metabolism and increase the risk of diabetes (187). How decreased LDL cholesterol levels effect glucose metabolism is unknown.

 

In some studies statins have been shown to increase insulin resistance (188) and in some studies to decrease insulin secretion (189,190). Clearly further studies are required to understand the mechanisms by which statins increase the risk of developing diabetes.

 

Niacin

 

A recent meta-analysis examined the effect of niacin therapy on the development of new onset diabetes (191). In 11 trials with 26,340 non-diabetic participants, niacin therapy was associated with a 34% increased risk of developing diabetes (RR of 1.34; 95% CIs 1.21 to 1.49). This increased risk results in one additional case of diabetes per 43 initially non-diabetic individuals who are treated with niacin for 5 years (0.47% ten-year risk or 4.7 per 1000 patient years). Results were similar in patients who were receiving niacin therapy in combination with statin therapy. It has been recognized for many years that niacin induces insulin resistance (192). The mechanisms by which niacin induces insulin resistance are unknown but possible mechanisms include a rebound increase in free fatty acids with niacin therapy or the accumulation of diacylglycerol (192).

 

Pentamidine

 

Pentamidine is an antiprotozoal agent known to cause hypoglycemia and hyperglycemia (180). Pentamidine induces a direct cytolytic effect on pancreatic beta cells leading to insulin release and hypoglycemia, which is then followed by beta cell destruction and insulin deficiency resulting in diabetes (193,194).

 

Dilantin

 

Dilantin can cause hyperglycemia and there have been cases of diabetic ketoacidosis (195,196). The adverse effect of dilantin on glucose metabolism is mediated primarily by an inhibition of insulin secretion (180).

 

Alpha Interferon

 

Treatment with alpha interferon in rare instances can cause T1D. Of 987 patients treated with alpha interferon for chronic hepatitis C, 5 patients developed T1D (197). The clinical course is characterized by the abrupt development of severe hyperglycemia at times with ketoacidosis (198). High titers of anti-islet autoantibodies are present and almost all patients require permanent insulin therapy (198). Treatment with interferon alpha facilitates the development of autoimmune disorders including T1D (199). Other autoimmune disorders frequently occur, particularly thyroid dysfunction.

 

Checkpoint Inhibitors

 

There are several checkpoint inhibitors; ipilimumab a cytotoxic T-lymphocyte-associated protein 4 inhibitor (CTLA-4 inhibitor); nivolumab and pembrolizumab, programmed cell death protein 1 inhibitors (PD-1 inhibitors); atezolizumab, avelumab, and durvalumab. programmed cell death 1 ligand inhibitors (PD-L1 inhibitors) (200). Both CTLA-4 and PD-1 play a key role in the maintenance of immunological tolerance to self-antigens thereby preventing autoimmune disorders (200). Immune mediated hypothyroidism, hyperthyroidism, hypophysitis, primary adrenal insufficiency, hypoparathyroidism, and insulin-deficient diabetes have been reported as a complication of the use of these drugs (200,201). In a meta-analysis of 38 randomized clinical trials with 7551 patients, autoimmune diabetes occurred in only 0.2% of the patients and was primarily seen with the use of PD-1 inhibitors (200). In another meta-analysis of 101 studies with 19,922 patients the incidence of autoimmune diabetes was 2.0% (95% CI, 0.7–5.8) for nivolumab and 0.4% (95% CI, 0.2–1.3) for pembrolizumab (201). The occurrence of autoimmune diabetes with other checkpoint inhibitors was rare (201).

 

The onset of diabetes ranges from a few weeks up to one year after initiating therapy and typically presents with polyuria, polydipsia, weight loss and dehydration (201,202). Severe hyperglycemia and ketoacidosis is commonly observed (202). Because of the acute occurrence A1c levels may not be elevated. C-peptide levels are very low and approximately 50% of patients have islet cell antibodies (GAD, ICA, IAA or IA-2; GAD antibodies are the most commonly observed) (201,202). Insulin treatment is required and it is likely that the diabetes will be irreversible (201,202).

 

For additional information on the checkpoint inhibitor associated diabetes see the Endotext chapter “Immune Checkpoint Inhibitors Related Endocrine Adverse Events” in the Disorders that Affect Multiple Organs section (203).

 

Antipsychotic Drugs

 

A large number of studies have linked second generation antipsychotic medications with the development of T2D (Table 5) (204,205). In a meta-analysis of a large number of studies it was reported that olanzapine and clozapine treatment resulted in a greater increase in glucose than aripiprazole, quetiapine, risperidone and ziprasidone (206). Another meta-analysis has further shown that aripiprazole has a reduced risk of T2D compared to other antipsychotic agents (207). With regards to first generation antipsychotic drugs, chlorpromazine has a high risk of disrupting glucose metabolism while haloperidol, fluphenazine, and perphenazine have a low risk (204). It is thought that antipsychotic drugs induce diabetes by multiple mechanisms: (1) they inhibit insulin signalling in muscle cells, hepatocytes and adipocytes thereby causing insulin resistance; (2) they induce obesity, which can also cause insulin resistance; and (3) they cause direct damage to β-cells, leading to dysfunction and apoptosis of β-cells (205,208).

 

Table 5. Risk of Diabetes of Selected First- and Second-Generation Antipsychotics

 

Risk of diabetes*

First-generation antipsychotic

  Chlorpromazine

  Fluphenazine

  Perphenazine          

  Haloperidol  

 

+++

+

+

+

Second-generation antipsychotic

  Clozapine     

  Olanzapine  

  Quetiapine   

  Risperidone 

  Ziprasidone  

  Aripiprazole  

  Paliperidone 

  Lurasidone   

 

+++

+++

++

++

+

+

+

+

*Relative to other antipsychotics. Not all the risk of diabetes or weight gain is related to the antipsychotic. Table modified from (205)

 

Androgen Deprivation Therapy

 

A number of studies have shown that androgen deprivation therapy increases the risk of developing diabetes (209). For example, a study by Tsai reported that androgen deprivation therapy was associated with a 1.61-fold increased diabetes risk (95% CI 1.38-1.88) and the number needed to harm was 29 (210). The androgen deprivation induced diabetes typically develops after a year of treatment (209). Androgen deprivation therapy induces insulin resistance (209). The increase in insulin resistance may be due to an increase in visceral fat mass and/or an increase in pro-inflammatory adipokines such as TNF-a, IL-6, and resistin (209).

 

Immunosuppressive Drugs

 

Immunosuppressive drugs used after organ transplantations increase the risk of diabetes (211). In general, tacrolimus has been associated with a greater risk of developing diabetes compared to cyclosporine (211,212). The calcineurin inhibitors, tacrolimus and cyclosporine, decrease insulin secretion and synthesis (211,212). Additionally, tacrolimus and cyclosporine inhibit glucose uptake in human subcutaneous and omental adipocytes (212).

 

Mechanistic Target of Rapamycin Inhibitors (mTOR inhibitors)

 

mTOR inhibitors, sirolimus and everolimus, can induce diabetes (213).  The adverse effect of mTOR inhibitors on glucose metabolism is due to insulin resistance secondary to a reduction of the post receptor insulin signalling pathway and a reduction of insulin secretion via a direct effect on the pancreatic beta cells (211,213).

 

Asparaginase

 

Hyperglycemia is common with the use of asparaginase treatment ranging from 2.5% to 23% in the pediatric population and as high as 76% in adults with PEG-asparaginase use (214,215). Hyperglycemia usually resolves within 12 days after the last dose (214). Risk factors predisposing to hyperglycemia with asparaginase treatment include a history of impaired glucose tolerance, age >10 years, obesity, family history of diabetes mellitus, and history of Down syndrome (214,215). Diabetic ketoacidosis has been described with asparaginase treatment but is not a common occurrence (214). Decreased insulin secretion, increased insulin resistance, and increased glucagon secretion may contribute to the hyperglycemia observed with asparaginase. Additionally, asparaginase can induce pancreatitis, which can also lead to hyperglycemia (214).

 

Glucocorticoids, Somatostatin, and Glucagon

 

The effects of these hormones on glucose metabolism were discussed in the section on Endocrinopathies.

 

HIV Antiretroviral Therapy

 

The effect of the drugs used to treat patients living with HIV on the development of diabetes is discussed in the Endotext chapter “Diabetes in People Living with HIV” in the Diabetes section (6).

 

IMMUNE-MEDIATED

 

Latent Autoimmune Diabetes in Adults (LADA)

 

LADA is an autoimmune disorder that resembles T1D but shows a later onset and slower progression towards requiring insulin therapy (216-218). The ADA includes LADA as T1D whereas WHO classifies LADA as a hybrid form of diabetes (T1D and T2D) (84) (https://www.who.int/publications/i/item/classification-of-diabetes-mellitus). Epidemiological studies suggest that LADA may account for 2–12% of all cases of diabetes in the adult population (216,217,219). To differentiate LADA from T1D and T2D, the Immunology of Diabetes Society has proposed three criteria: (a) adult age of onset (> 30 years of age); (b) presence of at least one circulating autoantibody (GAD, ICA, IAA or IA-2) and; (c) insulin independence for the first 6 months after the time of diagnosis (216,217). Of the various antibodies associated with autoimmune diabetes GAD antibodies are present in most patients with LADA (216,217,219). Patients with high titers of GAD antibodies progress to requiring insulin more rapidly (218). LADA subjects appear to have a faster decline in C-peptide levels compared to autoantibody negative patients with T2D (219). It should be noted that classic T1D can occur in adults and this is defined as those adult patients with antibodies (GAD, ICA, IAA or IA-2) that require insulin therapy at diagnosis or soon after diagnosis (217,219). In contrast, patients with LADA can often go many years before requiring insulin therapy (217). Whether LADA is just a slowly progressing form of T1D or a hybrid T1D and T2D is unclear (Table 6).

 

Table 6. Comparison of T1D, LADA, and T2D

 

T1D

LADA

T2D

Age

Tend to be young

>age 25

Tend to be adult

Family history

Occasional

Occasional

Usually

C-peptide

Low, often undetectable

Varies

Normal or high

Auto-ab

+

+

-

Weight

Tend to be lean

Tend to be lean

Usually overweight

Metabolic syndrome

No

Varies

Usually

Insulin requirement

Yes

Varies, rapid progression

Varies

Genetic risk

HLA

PTPN22

INS

SH283

PFKFB3

Intermediate between T1D & T2D

TCF7L2

FTO

SLC30A8

 

In a retrospective study, Fourlanos and colleagues pointed out several features that increase the likelihood of a patient with “T2D” having LADA (220). These features include age of onset <50 years of age,  acute symptoms (polyuria, polydipsia, weight loss), BMI <25 kg/m2, personal history of autoimmune disease, and family history of autoimmune disease (220). The presence of at least two of these clinical features indicated a 90% sensitivity and 71% specificity for identifying a patient with LADA (220). As compared to patients with T2D, LADA patients have a lower rate of hypertension, lower total cholesterol levels, higher HDL cholesterol levels, and a decreased frequency of the metabolic syndrome (217,219). HLA-DQB1 risk genotypes have been consistently positively associated and protective genotypes have been negatively associated with LADA (218). However, in addition to genotypes that associate with T1D, patients with LADA also have an increased frequency of genotypes that associate with T2D (TCF7L2, FTO, and SLC30A8) (218).

 

Some have proposed GAD antibody testing all patients with T2D (221) to diagnose LADA but given the given the increased costs and the relatively frequent occurrence of false positive tests compared to true positives in a low-risk population this strategy is not widely accepted (222). The ADA suggests selective testing in adults without traditional risk factors for T2D and/or younger age (84). 

 

In LADA patients initially glycemic control can be achieved with hypoglycemic agents other than insulin but overtime patients progress to requiring insulin therapy. Sulfonylureas seem to accelerate the progress to requiring insulin therapy and therefore should be avoided (223). Because of the progressive loss of beta cell function there is an increased risk of diabetic ketoacidosis with SGLT2 inhibitors and therefore these drugs should be used with caution. Monitoring ketone levels in patient with LADA treated with SGLT2 inhibitors would be prudent. Novel therapies to preserve beta cell function would be ideal in patients with LADA but at this time there are no proven strategies to preserve beta cell function.

 

In a long- term follow-up (median 17.3 years) comparing microvascular outcomes in patients with LADA or T2D it was observed that the risk of renal failure/death, blindness, vitreous hemorrhage, or retinal photocoagulation was decreased in the patients with LADA during the first 9 years (adjusted HR 0.45; p<0.0001), whereas in subsequent years their risk was higher (HR 1·25; p=0.047) (224). This difference was attributed to higher A1c levels in the LADA patients. The prevalence of coronary heart disease and cardiovascular mortality is similar in patients with LADA and T2D (225,226).

 

Autoimmune Polyglandular Syndromes

 

T1D can occur as part of the autoimmune polyglandular syndromes. These disorders are discussed in the Endotext chapter “Autoimmune Polyglandular Syndromes” in the Disorders that Affect Multiple Organs section (4).

 

Stiff-person syndrome

 

Stiff-person syndrome is a rare autoimmune disorder of the nervous system with fluctuating stiffness and spasm of the skeletal muscles that occurs more frequently in females than males (approximately 2/3 women) (227). Muscle involvement is symmetrical and the lower extremities are affected more commonly than the upper extremities and proximal limb and axial muscles are affected more severely than distal muscles (227). Most patients have very high levels of anti-glutamic acid decarboxylase (GAD) antibodies (227). 30-65% of these individuals also develop beta cell destruction and T1D (228). Diabetes may occur several years prior to the development of the stiff person syndrome (60%) or after the development of the stiff person syndrome (227,228). The stiff person syndrome without GAD antibodies is not associated with diabetes (228). Other autoimmune manifestations are also common, particularly thyroid disorders and pernicious anemia (227,228).

 

Autoimmune Insulin Resistance Type B Syndrome

 

Insulin resistance can result from autoantibodies directed against the insulin receptor, which either inhibit insulin from binding to the receptor or stimulate the receptor (229). Thus, they can cause either hyperglycemia or hypoglycemia, even alternating in the same patient. The patients usually present with very high glucose levels and significant weight loss (229). The diagnosis can be confirmed by demonstrating the presence of autoantibodies to the insulin receptor. The prevalence of type B insulin resistance syndrome is unknown but is quite rare (229). Middle-aged women are most often affected and often have other manifestations of autoimmune disease such as SLE or Sjogren’s. However, this disorder can also affect males and younger patients. Patients may have signs of insulin resistance including acanthosis nigricans and ovarian hyperandrogenism. Of note the acanthosis nigricans may involve the lips and the periocular region resulted in a typical facial appearance (229). Serum testosterone levels are often elevated in females (229). Patients often need excessive amounts of insulin (1,000 U or more per day). One can add insulin sensitizers such as metformin and/or thiazolidinediones to try to reduce the insulin dose, which can in some patients be greater than 10,000U per day (229). Treatment includes immunosuppression and/or plasmapheresis to halt the autoantibody production and decrease antibody levels (230). Approximately 1/3 of patients will undergo a spontaneous remission with reversal of the hyperglycemia/hypoglycemia and the clinical manifestations (229).

 

DIABETES OF UNKNOWN CAUSE

 

Ketosis-Prone Diabetes in Adults (Flatbush Diabetes)

 

This syndrome is characterized by the acute onset of severe hyperglycemia with or without ketoacidosis, which after several weeks to months no longer requires insulin therapy and can be treated with diet or oral hypoglycemic agents (231,232). These patients typically have a history of polyuria, polydipsia, and weight loss for less than 4 to 6 weeks indicating an abrupt onset of the disorder in glucose metabolism and no history of an event that could have precipitated the hyperglycemia (232). The initial presentation is suggestive of T1D. While in most patients insulin therapy can be stopped there are some patients who continue to require insulin treatment (231). This syndrome occurs in black populations (African American, African-Caribbean, sub-Saharan African), Hispanic populations, and Asian (Chinese, Indian, and Japanese) populations but is not typically seen in Caucasians (231,232).  The typical patient is male, middle-aged, overweight or modestly obese with a strong family history of diabetes (231,232). Patients are negative when tested for islet cell antibodies (GAD, ICA, IAA or IA-2) (231). Recurrent episodes of ketoacidosis can occur but the clinical course is typical of patients with T2D (231,232). Treatment with hypoglycemic agents reduces the risk of recurrence (232,233). With long-term follow-up many patients eventually require insulin therapy similar to what is observed in patients with T2D (233).

 

During the episode of severe hyperglycemia patients with ketosis-prone diabetes have lost the ability of glucose to stimulate beta cell insulin secretion but nonglycemic pharmacologic agents (glucagon and arginine) can stimulate insulin secretion (231). After restoration of normal glycemia the ability of glucose to stimulate insulin secretion returns towards normal and by 8-12 weeks has maximally improved (231). Usually patients with this syndrome have a modest reduction in stimulated insulin secretion (231). Why these patients temporarily lose the ability for glucose to stimulate insulin secretion is unknown. Additionally, during the acute episode of hyperglycemia the patients are severely insulin resistant, which improves during a period of euglycemia (232,233).

 

Clinically, it is important to recognize this syndrome as some patients presenting with diabetic ketoacidosis, particularly if they are non-Caucasians, may not have T1D but rather have ketosis-prone diabetes. It is estimated that between 20% and 50% of African-American and Hispanic patients with a new diagnosis of diabetic ketoacidosis have ketosis-prone diabetes (232). After restoration of euglycemia the management of these patients is similar to the management of patients with T2D and they frequently do not require permanent insulin treatment.

 

OTHER GENETIC SYNDROMES SOMETIMES ASSOCIATED WITH DIABETES

 

There are a number of inherited monogenic disorders that secondarily can be associated with diabetes. The mechanisms linking these disorders with diabetes is often not clear.

 

Chromosomal Abnormalities

 

DOWN’S SYNDROME

 

Down’s syndrome is due to trisomy of chromosome 21 and occurs in 1 in every 787 liveborn babies (234). Down’s syndrome is often associated with autoimmune disorders like T1D and thyroiditis (234,235). The prevalence rate of T1D in patients with Down's syndrome has been estimated to be between 1.4 and 10.6%, which is higher than in the general population (236). In another study there was a 4-fold increased prevalence of diabetes in patients with Down’s syndrome (237). Diabetes in patients with Down’s syndrome often presents earlier in life with 22% of participants developing diabetes by 2 years of age (238). The presence of diabetes is often associated with other autoimmune disorders, particularly hypothyroidism and celiac disease (235). Anti-glutamic acid decarboxylase antibodies (GAD antibodies) are very frequently present in Down’s syndrome subjects developing diabetes (235). Down’s syndrome patients with diabetes have similar HLA genotypes as non-Down’s syndrome patients with T1D (235). Interestingly, while patients with Down’s syndrome and diabetes are typically treated with simpler regimens their glycemic control tends to be as good or better than the usual patient with T1D, perhaps related to a simpler lifestyle and acceptance of routine (235). The cause of the increased autoimmunity in patients with Down’s syndrome may be due to the abnormal expression of the AIRE gene, which regulates T-cell function and self-recognition and is located on chromosome 21 (21q22.3 region) (234,235).

 

KLINEFELTER SYNDROME

 

Klinefelter syndrome is due to an extra X chromosome in men (XXY) resulting in hypergonadotropic hypogonadism and low testosterone levels (239). The prevalence of Klinefelter syndrome is approximately 1 in 500 to 1 in 1000 males (239). Patients with Klinefelter syndrome are frequently obese, insulin resistant, and at increased risk to develop T2D (240). The prevalence of overt diabetes in Klinefelter syndrome is estimated to be between 10-39% (240,241). Additionally, the prevalence of diabetes is even higher (up to 57%) in patients with the more severe karyotypes (48, or 49 chromosomes) (240). Klinefelter syndrome patients develop diabetes earlier in life (onset around 30 years) and their BMI is lower than what is usually observed in patients with T2D (240). Whether testosterone therapy will be of benefit in preventing or treating diabetes in patients with Klinefelter syndrome is uncertain (241). Given the increased risk of developing T2D patients with Klinefelter syndrome should be periodically screened for diabetes.

 

Interestingly, one study reported an increased prevalence of T1D in patients with Klinefelter syndrome (242). Furthermore, a recent study reported that 8.2% of patients with Klinefelter syndrome had autoantibodies specific to T1D (Insulin Abs, GAD Abs, IA-2 Abs, Znt8 Abs) (243). Additional studies are required to better elucidate whether Klinefelter syndrome increases the risk of developing T1D.

 

TURNER SYNDROME

 

Turner syndrome is the most common chromosomal abnormality in girls, affecting approximately 1:2,500 of female live births (244). The condition is caused by complete or partial deletion of an X chromosome (244). The incidence of both T1D and T2D has been reported to be increased in patients with Turner syndrome (245). However, the link between T1D and Turner syndrome is not well characterized while the link with T2D is clearly established (246,247). For example, in a study of 224 patients with Turner syndrome 56 (25%) had T2D whereas only 1 patient (<0.5%) had T1D (248). Patients with Turner syndrome have an increased risk of autoimmune disorders, particularly hypothyroidism and celiac disease, but the risk of autoimmune T1D is much less (247,249). Four percent of patients with Turner syndrome have been shown to have GAD antibodies, which is greater than the 1% prevalence seen in the general population (249).

 

The prevalence of glucose intolerance is estimated to be from 15-50% while the prevalence of T2D is estimated to be approximately 10-25% (247,248). T2D occurs at a relatively young age in patients with Turner syndrome. Decreased beta cell function and decreased insulin sensitivity was observed in teenagers with Turner syndrome and was accompanied by an increased prevalence of impaired fasting glucose and impaired glucose tolerance compared to controls (250). Increased obesity is common in patients with Turner syndrome, which likely contributes to the abnormalities in glucose metabolism (246). Both insulin resistance and decreased insulin secretion are present in patients with Turner syndrome but the development of hyperglycemia in patients with T2D appears to be driven by decreased insulin secretion (246,247). Because of the high prevalence of diabetes, it is recommended to screen A1c with or without fasting glucose levels annually beginning at 10 years of age (247). Growth hormone therapy does not appear to increase the risk or worsen diabetes (246,247). Growth hormone therapy may lead to a decrease in adiposity and impaired glucose tolerance, which suggests it may actually improve glucose homeostasis (247). Similarly, sex steroid hormone replacement therapy also does not appear to have major adverse effects on glucose metabolism in patients with Turner syndrome (246).

 

WILLIAMS SYNDROME

 

Williams syndrome (Williams-Beuren syndrome) is a multisystem disorder characterized by transient infantile hypercalcemia, distinctive facial dysmorphism, and supravalvular aortic stenosis (251,252). In addition, gastrointestinal problems, dental anomalies, developmental delay/intellectual disability, anxiety disorders, and attention deficit disorder may occur as well as a variety of endocrine abnormalities including reduced statural growth, obesity, dyslipidemia, early pubertal development, hypothyroidism, and decreased bone density (251,253). Williams syndrome is due to a deletion on chromosome 7q, leading to the loss of 25–27 contiguous genes and thus individuals with Williams syndrome have only a single copy of these genes (252). This deletion almost always arises de novo in the affected individual. The estimated prevalence of Williams syndrome is ~1/7,500 and effects both males and females (252).

 

Numerous studies have shown a high prevalence of T2D and impaired glucose tolerance in patients with Williams syndrome (251). The abnormalities in glucose metabolism occur during adolescence and are not necessarily associated with obesity (251). Of note insulin resistance is observed initially followed by a loss of insulin secretion (251). Markers of islet autoimmunity are not observed (251). In a review of 7 studies with 154 participants with Williams syndrome and an average age ranging from 13 to 35 years of age it was observed that 18% had diabetes and 42% impaired glucose tolerance (251). Because of this high risk for diabetes, it is recommended that patients with Williams syndrome be screened for diabetes beginning in adolescence (251). Note-worthy is that A1c was frequently not abnormal and therefore screening should be with fasting glucose levels or an oral glucose tolerance test (251).

 

Diseases of the Endoplasmic Reticulum

 

The endoplasmic reticulum folds and modifies newly formed proteins to make them function properly. Therefore, diseases affecting the endoplasmic reticulum usually affect many organs.

 

WOLFRAM SYNDROME 

 

Wolfram syndrome is a rare autosomal recessive genetic disorder characterized by T1D, diabetes insipidus, optic nerve atrophy, hearing loss, and neurodegeneration (254,255). There are also rare autosomal dominant forms of this disorder (255). This syndrome is sometimes called DIDMOAD (diabetes insipidus, diabetes mellitus, optic atrophy, and deafness). The prevalence is approximately one per 770,000 but varies depending upon the specific population (254,255). The onset of the clinical picture is highly variable in both severity and clinical manifestations (255). This disorder typically has a very poor prognosis with the median age at death being 30 years (254). Diabetes mellitus is usually the first manifestation, typically diagnosed around age 6 (254). The diabetes is not immune mediated but is characterized by insulin deficiency (255). Almost all patients require insulin therapy (255). Residual beta cell function persists and therefore good glycemic control tends to be easier to achieve in Wolfram syndrome than immune mediated T1D (255). However, over time C-peptide levels decrease (256). The development of optic atrophy and hearing loss in children with diabetes are clues to the presence of this syndrome. Until the onset of optic atrophy and hearing loss these patients are usually thought to have typical T1D with an absence of antibodies (255). Confirmation of the diagnosis can be made by identifying mutations in the WFS1 gene (Wolfram syndrome type 1) (254). The WFS1 gene encodes a transmembrane protein (wolframin) localized to the ER (endoplasmic reticulum) and mutations result in ER stress leading to beta cell dysfunction and death (254).

 

Mutations in CISD2 gene cause a similar recessive type of Wolfram syndrome (Wolfram syndrome type 2) with patients exhibiting bleeding from upper intestinal ulcers and defective platelet aggregation without diabetes insipidus and psychiatric disorders (255). CISD2 encodes for a protein that moves between the ER and mitochondrial outer membrane (255).  

 

Unfortunately, there are currently no specific treatments to restore ER function and prevent the complications of this disorder.

 

Base Pair Repeat Syndromes     

 

FRIEDRICHS ATAXIA

 

Friedreich ataxia is a rare recessive disorder caused by triplet repeats (GAA) in the mitochondrial frataxin gene characterized by slowly progressive ataxia associated with dysarthria, muscle weakness, spasticity particularly in the lower limbs, scoliosis, bladder dysfunction, absent lower limb reflexes, and loss of position and vibration sense (257). The onset usually occurs before 25 years of age (257). Cardiomyopathy occurs in 2/3 of patients and up to 30% of patients have diabetes (257). The disorder effects approximately 1 in 30,000 Caucasians.

 

Diabetes occurs in 8-32% of patients with Friedrichs ataxia and an even higher percentage have impaired glucose tolerance (258,259). Hyperglycemia commonly develops approximately 15 years after the manifestation of neurological symptoms often presenting acutely with patients requiring insulin therapy (258,259). In a number of instances patients presented with ketoacidosis (259). Both insulin deficiency and insulin resistance have been reported in patients with Friedreich ataxia (259). It is hypothesized that mutations in frataxin result in alterations in mitochondria function that impair the ability of beta cells to secrete insulin in response to glucose and increase the risk of beta cell death (259).

 

There are no controlled studies comparing different diabetes therapies in patients with Friedreich ataxia. Metformin and thiazolidinediones inhibit the mitochondrial respiratory chain and therefore they should probably be used with caution in patients with mitochondrial disease (259). Additionally, thiazolidinediones increase the risk of congestive heart failure and patients with Friedreich ataxia have a high risk of cardiomyopathies and therefore should be avoided. Insulin is often required to achieve glycemic control.

 

HUNTINGTON’S DISEASE

 

Huntington’s disease is an autosomal dominant disorder that begins in adulthood (usually 30-50 years of age) and has distinctive motor defects (chorea, dystonia, and dyskinesia), psychiatric symptoms (depression and anxiety), and cognitive decline (260). This disorder is due to an unstable expansion of CAG repeats in the first exon of the gene that encodes the protein huntingtin (260). The prevalence of this disorder is approximately 5-12 per 100,000 (260,261). While an early study reported that approximately 10% of patients with Huntington’s disease have diabetes a careful review of recent studies reached the conclusion that the prevalence of diabetes in patients with Huntington’s disease is not increased and might actually be decreased (261,262).

 

MYOTONIC DYSTROPHY

 

Myotonic dystrophy type 1 is an autosomal-dominantly inherited disease characterized by myotonia, distal muscular dystrophy, cataracts, hypogonadism, and frontal hair loss that occurs in middle age (263). The disease is due to a CTG triplet repeat expansion in the myotonic dystrophy protein kinase gene (263). Diabetes is not a characteristic finding in myotonic dystrophy type 1 but the prevalence is increased 2-4-fold in patients with this disorder compared to the general population (263-265). A large study in Korea with 387 patients with myotonic dystrophy type 1 found that 27% had diabetes (266). Patients with myotonic dystrophy type 1 and diabetes have elevated insulin levels suggesting insulin resistance (263,264). Pioglitazone alone and in combination with metformin has been reported to improve glycemic control in patients with myotonic dystrophy and diabetes (267,268).

 

Obesity Syndromes

 

BARDET-BIEDLE SYNDROME (BBS)

 

Bardet-Biedl syndrome (BBS), also earlier referred to as Laurence Moon Biedl syndrome, is a rare autosomal recessive disease with a prevalence of about 1/125,000 (269,270). BBS belongs to the group of ciliopathies characterized by obesity, retinal degeneration, finger anomalies, hypogonadism, renal abnormalities, and intellectual impairment (269,270). It can result from autosomal recessive mutations in at least 22 genes (BBS), which play a key role in structure and function of cilia (269,270). In a study of 152 patients with BBS it was reported that approximately 75% were obese and the average BMI was 35.7kg/m2 (270). Twenty-five of these patients with BBS had diabetes (16.4%) with 24 having T2D and 1 having T1D (270). Of the 24 patients with T2D six patients were diet controlled, eight were taking metformin, and 10 were on insulin therapy. The mean A1c of subjects with T2D was 7.8 (270). In a smaller series of 46 patients, it was reported that 22 had T2D (46%) and the median age of onset of diabetes mellitus was 43 years (217). The risk of developing diabetes increases with age. In the BBS patients without diabetes fasting glucose, insulin levels, and HOMA-IR were significantly increased in the BBS group compared with an age and BMI matched control group (270). The metabolic syndrome was present in 54% of the patients with BBS (270).

 

PRADER WILLI SYNDROME (PWS)

 

PWS is a rare autosomal dominant disorder due to a mutation or deletion of several genes in an imprinting region on chromosome 15 (271). PWS in children is associated with excessive eating and morbid obesity, hypogonadism, low muscle tone, and short stature (271). The hyperphagia that occurs in PWS is believed to be due to a hypothalamic abnormality resulting in lack of satiety. This leads to excessive obesity in children, which is often associated with T2D due to severe insulin resistance (271). Approximately 20- 25% of adults with PWS have T2D with a mean age of onset of 20 years (271,272). Individuals with PWS who develop early diabetes have severe obesity, a high prevalence of psychiatric and metabolic disorders, and a family history of overweight and T2D (272). In recent years the earlier diagnosis and education of parents, use of growth hormone therapy, and the frequency of group homes specific for PWS have led to a reduction in the development of morbid obesity resulting in a decrease in the development of T2D among individuals with PWS (271). Metformin has been shown to be effective in the treatment of PWS patients with diabetes (273). Studies of GLP1 receptor agonists demonstrated lowering of A1c levels but effects on weight loss have been inconsistent (274).

 

ALSTROM SYNDROME

 

Alstrom syndrome is a rare autosomal recessive disorder with a prevalence of less than one per million characterized by retinal dystrophy, hearing loss, childhood truncal obesity, insulin resistance and hyperinsulinemia, T2D, hypertriglyceridemia, short stature in adulthood, cardiomyopathy, and progressive pulmonary, hepatic, and renal dysfunction (275-277). Symptoms appear in infancy and multi-organ pathology leads to a decreased life expectancy (275,276). The syndrome is caused by mutations in ALMS1, which is a ciliary protein and hence many of the features of Alstrom syndrome resemble those seen in the Bardet-Biedl syndrome (275-277). Diagnosis is confirmed by finding biallelic pathogenic variants in ALMS1 gene.

 

Severe insulin resistance secondary to abnormalities in GLUT4 trafficking, hyperinsulinemia, and impaired glucose tolerance frequently present in early childhood and are often accompanied by acanthosis nigricans (275,277). T2D develops early in life with a mean age of onset at 16 years (275). In one study 82% of patients with Alstrom syndrome older than 16 years of age have T2D (278). Weight loss with diet, exercise, and medications is indicated (279). Therapy with oral agents, particularly insulin sensitizing agents, may be effective but insulin therapy may be required (275,279).

 

Miscellaneous

 

PORPHYRIA

 

Porphyria cutanea tarda has been associated with diabetes (280), but given that many patients with this disorder also have iron overload, genes for hemochromatosis, and HCV and HIV infection it is very difficult to tell if porphyria cutanea tarda per se is responsible for the association with diabetes (281,282).

 

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New Osteoporosis / Vertebral Compression Fractures

CLINICAL RECOGNITION

 

Osteoporosis is a prevalent disease characterized by reduced bone mass and architectural deterioration, which leads to structurally weakened bone and an increased risk of fragility fractures. A fragility fracture is defined as a fracture occurring with minimal trauma, such as falling from standing height. These fractures rise exponentially with age and most commonly involve the spine, hip, humerus, and wrist. Vertebral compression fractures are the most common osteoporotic fractures with an estimated 700,000 per year in the United States (1). However, most patients with vertebral fractures are unaware that they have fractured as only ~1/3rd are clinically diagnosed. While there are effective treatments to reduce the risk of fractures, only 23% of patients with fragility fractures receive osteoporosis evaluation and treatment.

 

PATHOPHYSIOLOGY

 

Bone is a dynamic organ with continuous remodeling to maintain a healthy skeleton—osteoclasts resorb bone and osteoblasts form new bone (2). Osteoporosis results from a net increase in bone resorption relative to bone formation. The receptor activator of nuclear factor-kappa β (RANK), RANK ligand (RANKL), and osteoprotegerin (OPG) are key regulators of bone resorption. Interaction between RANKL and RANK stimulates osteoclastic differentiation, while OPG, made by osteoblasts, binds with RANKL and inhibits bone resorption. In addition, the Wnt signaling pathway is a network of proteins that is involved in activating the transcription of genes that direct the differentiation and proliferation of osteoblasts. Sclerostin, produced by the osteocytes embedded in bone, is the product of the SOST gene. Sclerostin reduces the Wnt signaling pathway, thereby, suppressing bone formation by osteoblasts. Some of the key factors that are mechanistically involved in bone turnover are therapeutic targets for osteoporosis treatment. See Table 4 for summary of treatments.

 

Fragility Fractures

 

Vertebral compression fractures are associated with substantial morbidity including: acute and chronic back pain, height loss, kyphosis, restrictive lung disease, early satiety, reduced quality of life, and increased mortality (1). A spine fracture is associated with a 5-fold risk of a subsequent spine fracture and a 2-fold risk of hip and other fractures. Hip fractures are serious fractures that can lead to pain, disability, loss of independence, and high mortality. A Danish registry study published in 2018 found that one-year excess mortality was 20-25% after femur or pelvic, 10% following vertebral, and 5-10% following humerus fractures.

 

There is a high prevalence of low vitamin D levels among hip fracture patients. Since there is a large care gap for patients with fragility fractures, there are critical ongoing efforts to try to implement inter-disciplinary, hospital-based approaches to advance fracture care. It is imperative to ensure timely outpatient follow-up to correct the vitamin D deficiency, evaluate patients for other secondary causes of osteoporosis, and institute osteoporosis treatment. See Treatment section for further description of management of these fractures.

 

DIAGNOSIS and DIFFERENTIAL

 

Assessment of osteoporosis risk factors and measurement of bone mineral density (BMD) by dual energy x-ray absorptiometry (DXA) are important to determine which individuals are at increased risk of fractures. Low bone mass (osteopenia) is present when the BMD is between 1.0 and 2.5 SDs below peak bone density of young, healthy individuals. More than 50% of fragility fractures occur in these patients. Osteoporosis, according to the World Health Organization, is defined as a BMD ≤-2.5 SDs of young normal. BMD testing is typically measured at the proximal femur and lumbar spine, though the 1/3 radius should be measured in patients with hyperparathyroidism (https://www.iscd.org/official-positions/). The Bone Health & Osteoporosis Foundation (BHOF; formerly the National Osteoporosis Foundation) currently recommends that women >65 years, men >70 years, and postmenopausal women and men >50 years with risk factors or fracture after age 50 receive screening DXA scans (3). The BHOF recommends monitoring osteoporosis by an annual measurement of a patient’s height, preferably with a mounted stadiometer, and BMD testing 1-2 years after initiating therapy and every 2 years thereafter. Because spine fractures are often not clinically evident, imaging for spine fractures (vertebral fracture assessment by DXA or X-ray) is recommended, particularly in older adults with osteopenia and after adult-age fracture (>50 years of age), glucocorticoid use, or diagnosis of hyperparathyroidism (See Table 1) (3, 4).

 

The FRAX® calculator was designed to quantify an individual’s absolute fracture risk (http://www.shef.ac.uk/FRAX). In addition to BMD, the following risk factors are included—ethnicity, age, body mass index, prior fracture history, glucocorticoid use, alcohol use, smoking, rheumatoid arthritis, and other secondary causes of osteoporosis. If the 10-year absolute fracture risk is ≥3% for hip fractures or ≥20% for other major osteoporotic fractures, pharmacologic therapy should be considered. Note that the FRAX calculator is not designed for those with osteoporosis on BMD testing but mainly for those with low bone mass.

 

Using a specialized software (incorporated in DXA machines), Trabecular Bone Score (TBS) can be generated from lumbar spine DXA images and is a measure that reflects bone microarchitecture and predicts fracture risk independent of bone density. TBS can now also be incorporated in the FRAX score.

 

Table 1. Imaging Assessment Recommendations

DXA Tests:
Women aged ≥65 and older men aged ≥70
Younger postmenopausal women and men aged 50-69 with risk factors for bone loss or fractures
Adults who have a fracture at age ≥50
Adults with a medical condition or taking a medication associated with bone loss and/or fractures

Vertebral Imaging Tests:
Women aged ≥65 if T-score is ≤ -1.0 at the femoral neck

Women aged ≥70 and men aged ≥80 if T-score is ≤ -1.0 at the lumbar spine, total hip, or femoral neck 
Men aged 70-79 if T-score is ≤ -1.5 at the lumbar spine, total hip, or femoral neck
Postmenopausal women and men aged ≥50 with specific risk factors:

-        Fracture(s) during adulthood (age ≥50) from any cause

-        Historical height loss of ≥1.5 inches (4 cm)

-        Prospective/interval height loss of ≥0.8 inches (2 cm)

-        Glucocorticoid therapy

-        Hyperparathyroidism

 

When the diagnosis of a low bone density is made, a work-up to look for secondary causes of osteoporosis should be considered. See Table 2.

 

Table 2. Secondary Causes of Osteoporosis

Endocrinological Abnormalities

Glucocorticoid excess, hyperthyroidism, hypogonadism, anorexia, prolactinoma, hyperparathyroidism

Hematologic Disorders

Multiple myeloma, mastocytosis, leukemia

Renal Disease

Metabolic bone disease, nephrolithiasis

Connective Tissue Disorders

Osteogenesis Imperfecta, Ehlers-Danlos syndrome

Gastrointestinal Diseases

Celiac disease, inflammatory bowel disease, post-gastrectomy, bariatric surgery

Rheumatological Disorders

Ankylosing spondylitis, rheumatoid arthritis

Medications

Glucocorticoids, cyclophosphamide, aromatase inhibitors, heparin, methotrexate, androgen deprivation therapy, gonadotropin releasing hormone agonists, proton-pump inhibitors, selective serotonin reuptake inhibitors

 

Laboratory evaluation may include the following: calcium, phosphorus, liver tests (including alkaline phosphatase), CBC, 25-hydroxyvitamin D, 24-hour urine calcium, +/- parathyroid hormone, and thyroid stimulating hormone (if clinical evidence of hyperthyroidism or those already on thyroid hormone replacement), and serum testosterone level in men. For select cases one may consider obtaining specialized tests for gastrointestinal disorders (tissue transglutaminase for celiac disease with an IgA level), infiltrative diseases (serum tryptase for mastocytosis), neoplastic (serum and urine protein electrophoresis), or excess glucocorticoid (24-hour urine cortisol, dexamethasone suppression test).

 

TREATMENT

 

Fractures

 

The management of a vertebral compression fracture involves both pharmacologic and non-pharmacologic approaches. The acute pain typically subsides over several weeks, but pain management with non-steroidal anti-inflammatory drugs, neuropathic pain agents, or narcotics may be needed. A 2-4 week course of calcitonin, administered as one spray (200 IU) per day intranasally, may help patients who need additional acute pain management. Spinal bracing may help with pain by limiting movement of bone fragments against one another, and physical therapy may improve mobility and reduce fear of falling. Vertebral fractures are common in older adults and secondary fracture prevention is important. After a vertebral fracture, patients should immediately start osteoporosis treatment to prevent subsequent vertebral fractures, particularly teriparatide, abaloparatide, zoledronic acid, denosumab, or romosozumab, which have been shown to reduce vertebral fracture risk within the first year of treatment.

 

Procedures such as vertebroplasty or kyphoplasty have been thought to be effective for acute fracture pain; however, this finding has not been replicated across studies, especially in those controlled by sham operations. This lack of a clear benefit is also offset by the small but serious risks of these procedures, which include epidural cement leak leading to possible nerve root compression, osteomyelitis, cement pulmonary embolism, and the possibility of subsequent vertebral fractures in adjacent vertebrae. A Cochrane review published in 2018 found no demonstrable clinically important benefits for vertebroplasty compared with placebo (sham procedure), and the results did not differ according to duration of pain (≤6 weeks vs. >6 weeks) (5). A 2019 American Society for Bone and Mineral Research (ASBMR) task force concluded that, for patients with painful vertebral fractures, there was no significant benefit for vertebroplasty compared to placebo or sham procedures and recommended against the use of balloon kyphoplasty (6). If vertebral augmentation is considered in select patients with disabling spine fractures, osteoporosis treatment should be initiated concurrently.

 

Glucocorticoid-induced osteoporosis affects the spine greater than other sites. Glucocorticoids have a major effect on reducing bone formation and also increase bone resorption. Thus, there are two sites for targeted intervention—anabolic and anti-resorptive treatments, respectively. The American College of Rheumatology has recommended starting bone protection therapy for adults ≥40 years taking prednisone at a dose of ≥2.5 mg/day for ≥3 months if at moderate to high risk for fracture (i.e., FRAX 10-year risk of major osteoporotic fracture >10%, FRAX 10-year risk of hip fracture >1%, osteoporosis by bone density criteria, or prior osteoporotic fracture) (7). The Food Drug Administration (FDA) has approved the following anti-resorptive agents — risedronate, alendronate, zoledronic acid, and denosumab — and the anabolic agent teriparatide for glucocorticoid-induced osteoporosis. In a randomized trial, teriparatide was superior to alendronate in preventing BMD declines at the spine and hip.

 

With regards to hip fractures and the use of zoledronic acid once yearly, the timing of this FDA-approved treatment for secondary fracture prevention is important. There is a significant reduction in vertebral and non-vertebral fractures and mortality as well as an increase in hip BMD in those who receive zoledronic acid and supplemental vitamin D between two weeks and 90 days following a hip fracture.

 

Osteoporosis

 

Adequate calcium and vitamin D intake are essential. In 2010, the Institute of Medicine (IOM) set recommendations for daily calcium and vitamin D requirements (8). See Table 3.

 

Table 3. Recommended Daily Intakes of Elemental Calcium (adapted from 2010 IOM report)

Calcium Intake

Women 19 to 50 years / Men 19 to 70 years
Women ≥51 years / Men ≥71 years

1000 mg
1200 mg

Vitamin D Intake

Women and Men < 70 years

Women and Men > 70 years

600U

800U

 

Obtaining calcium through the diet is preferred. However, if taking calcium supplements, for those on proton pump inhibitors, calcium citrate (e.g., Citracal®) is preferred given better absorption over calcium carbonate and can be taken on an empty stomach. Preparations of Citracal® include Maximum Plus (315 mg of calcium per tablet) and Petite (200 mg of calcium per tablet). Calcium carbonate (e.g., Oscal®, Caltrate®), ranging from 500 to 600 mg per tablet, should be taken with food to allow optimal absorption.

 

Vitamin D deficiency is a prevalent problem. The IOM guidelines recommend a daily dose of vitamin D3 of 600 IU for individuals ≤70 years of age and 800 IU daily for those ≥71. Other societies recommend 800-1000 IU of vitamin D for high-risk adults with osteoporosis. Patients with vitamin D deficiency need much higher doses. Although there is debate, the BHOF and other organizations currently recommend a 25-hydroxyvitamin D level ≥30 ng/mL. There are ongoing, population-based studies that are evaluating the effects of supplemental vitamin D on fractures and bone health measures.

 

Recommendations for lifestyle and dietary modification include weight-bearing exercises, balance training, muscle-strengthening, fall prevention interventions, smoking cessation, and moderate alcohol consumption.

 

PHARMACOLOGIC THERAPIES

 

Table 4 lists the currently available osteoporosis drugs approved by the FDA, their dosage, indication, and general efficacy to reduce fractures.

 

Table 4. FDA-approved Treatments for Osteoporosis: Dose, Fracture Indication, Efficacy and Side Effects

Drug

Dose & Administration

Fracture Reduction *

Side Effects

Bisphosphonates

Alendronate

70 mg PO once weekly

V, N, H

Upper GI symptoms, rare bone pain, osteonecrosis of the jaw (rare), atypical femur fracture (rare).

Ibandronate

150 mg PO monthly; 3 mg IV every 3 months

V

Risedronate

35 mg PO once weekly; 150 mg PO once monthly

V, N, H

Zoledronic Acid (ZA)

5 mg IV once yearly

V, N, H

Mild flu like syndrome during and after ZA infusion (pre-treat with acetaminophen); ZA should not be given if severe renal impairment (GFR <35 mL/min). After a hip fracture, vitamin D and ZA should be initiated 2 weeks to 90 days after the fracture.

SERMs (Selective Estrogen Receptor Modulators)

Raloxifene

60 mg PO daily

V

Hot flashes, deep vein thrombosis (rare)

Parathyroid Hormone

PTH
Teriparatide (PTH 1-34)

20 mcg SC daily

V, N

Nausea, hypercalcemia, hypercalciuria, hypotension (rare)

PTHrP
Abaloparatide
(PTHrP 1-34)

80 mcg SC daily (for maximum of 2 years)

V, N

Nausea, hypercalcemia, hypercalciuria, dizziness, osteosarcoma (in rodents)

RANKL inhibitor

Denosumab

60 mg SC every 6 months

V, N, H

Skin infections, other uncommon infections, osteonecrosis of the jaw (rare), atypical femur fractures (rare), bone loss/vertebral fractures upon discontinuation

Sclerostin inhibitor

Romosozumab

210 mg SC every month for 12 months

V, N, H

Injection site reaction, major adverse cardiac events, osteonecrosis of the jaw (rare), atypical femur fracture (rare)

Other

Calcitonin

200 IU nasally or
100 IU subcutaneously every other day

V

Nasal congestion, malignancy

V: vertebral, N: non-vertebral, H: hip

 

CURRENT THERAPEUTIC APPROACH

 

Pharmacologic treatment is indicated for those with osteoporosis by BMD criteria; fragility vertebral or hip fracture regardless of BMD; fragility fracture of the pelvis, proximal humerus, or wrist with osteopenic range BMD; and elevated FRAX scores.

 

The most commonly used therapy is a bisphosphonate, which has long skeletal retention, decreases bone turnover, and reduces the risk of fractures (see Table 4). Alendronate, risedronate, and zoledronic acid decrease vertebral, non-vertebral, and hip fractures, whereas ibandronate decreases vertebral but not hip or non-vertebral fractures. There is concern about the association of its long-term use and risk of atypical femur fractures. These fractures (1) can occur along the subtrochanteric femur, (2) are associated with minimal or no trauma, (3) are in transverse or short oblique configuration, and (4) usually are complete fractures through both cortices. Some patients have prodromal symptoms of thigh or groin pain in the affected leg; bilateral atypical femur fractures may also be present. The incidence of these types of fractures is very low, and the consensus has been that the number of fractures prevented far exceeds the number of these fractures occurring as a result of bisphosphonates. According to the available limited, post-hoc data analyses, continuation of therapy after 3 years for zoledronic acid and 5 years for oral bisphosphonates may be considered in those with hip, spine, or multiple other osteoporotic fractures before or during therapy, osteoporosis at the hip after treatment, or high fracture risk. According to the 2011 FDA review, more data are needed concerning long-term bisphosphonate use. Until these data are available, annual evaluation and follow-up should involve decisions as to whether a 1-2 year or greater bisphosphonate holiday is needed, according to each individual’s risk, or to consider the use of alternative treatments as needed. It is important, however, to follow patients with a history of low bone mass or osteoporosis who are on a bisphosphonate holiday. Another rare complication is osteonecrosis of the jaw, which usually occurs in the setting of an invasive dental procedure. This complication is primarily seen in cancer patients who are receiving zoledronic acid on a monthly basis to prevent cancer-related fractures.

 

Denosumab, FDA approved in June 2010, is a monoclonal antibody that reduces RANKL, inhibiting the cellular mechanisms underlying bone resorption. It decreases the risk of vertebral, non-vertebral, and hip fractures and can be judiciously used in those with renal dysfunction. Denosumab has also been associated with rare cases of atypical femur fractures and osteonecrosis of the jaw. Of note, a drug holiday from denosumab is not recommended due to rebound bone loss and risk of multiple vertebral fractures with discontinuation. If denosumab is to be discontinued, it should be followed by bisphosphonate treatment.

 

Anabolic agents teriparatide (1-34 recombinant PTH) and abaloparatide (1-34 recombinant PTHrP) stimulate overall bone formation, improve bone structure, increase BMD particularly at the spine, and reduce risk of vertebral and non-vertebral fractures. In postmenopausal women with history of vertebral fracture, teriparatide has been shown to reduce incident vertebral and clinical fractures more than risedronate. Abaloparatide appears to be more effective at increasing bone density at the total hip compared to teriparatide and is less likely to cause hypercalcemia. They are administered as daily subcutaneous injections. Due to increased risk of osteosarcoma in rodents, these agents were limited to 2 years in a lifetime. However, due to twenty years of post-surveillance data showing no increased risk of osteosarcoma in humans, use of teriparatide is no longer restricted to 2 years. Use of abaloparatide, which was FDA approved in 2017, continues to be restricted to 2 years. These treatments should not be used in patients with active malignancy, history of radiation therapy, elevated alkaline phosphatase, or Paget’s disease. Anabolic agents should be followed by anti-resorptive therapy to consolidate gains in BMD.

 

Romosozumab, FDA approved in April 2019, is fully human monoclonal antibody that inhibits sclerostin and simultaneously reduces bone resorption and stimulates bone formation. Clinical studies of romosozumab have shown reduced risk of vertebral and nonvertebral, and hip fractures compared to placebo as well as alendronate. However, there were more adjudicated serious cardiovascular events in the romosozumab treatment arm compared to the alendronate arm. Thus, according to the FDA, romosozumab should not be used in patients who have had a myocardial infarction or stroke within the preceding year. A course of romosozumab is 12-months long, as the anabolic effects of romosozumab wane before then. There is no limit of courses. Similar to the parathyroid hormone analogues, romosozumab should also be followed by anti-resorptive therapy.

 

FOLLOW-UP

 

Once an initial bone density is measured, a follow-up BMD should be done 1-2 years after the initial screening and depending on whether pharmacologic therapy was initiated. Biochemical bone turnover markers and collagen breakdown products (e.g., N-telopeptide, C-telopeptide, collected in the morning) at baseline and after 3 months of treatment may be helpful in select patients to determine patient response to a therapeutic intervention. Clinical musculoskeletal evaluation and annual height measurements are important in the identification of spine fractures. Fragility fractures increase exponentially with advancing age, and evaluation and treatment of new fractures are critical for secondary prevention of fractures and healthy aging.

 

GUIDELINES

 

LeBoff M.S., Greenspan S.L, Insogna K.L., Lewiecki E.M., Saag K.G., Singer A.J., Siris, E.S. The Clinician's Guide to Prevention and Treatment of Osteoporosis. Osteoporos Int. In press.

 

Camacho, P. M., Petak, S. M., Binkley, N., Diab, D. L., Eldeiry, L. S., Farooki, A., Harris, S. T., Hurley, D. L., Kelly, J., Lewiecki, E. M., Pessah-Pollack, R., McClung, M., Wimalawansa, S. J., & Watts, N. B. American Association of Clinical Endocrinologists/American College of Endocrinology Clinical Practice Guidelines for the Diagnosis and Treatment of Postmenopausal Osteoporosis – 2020 Update. Endocr Pract. 2020;26(Suppl 1), 1–46..

 

Eastell R., Rosen C.J., Black D.M., Cheung A.M., Murad M.H., Shoback D. Pharmacological Management of Osteoporosis in Postmenopausal Women:  An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2019;104(5):1595-1622.

 

Shoback D., Rosen C.J., Black D.M., Cheung A.M., Murad M.H., Eastell R. Pharmacological Management of Osteoporosis in Postmenopausal Women:  An Endocrine Society Guideline Update. J Clin Endocrinol Metab. 2020;105(3): 587–594

 

REFERENCES

 

  1. Ensrud K.E., Schousboe J.T. Clinical practice. Vertebral fractures. N Engl J Med. 2011;364(17):1634–42. [PubMed]
  2. Rosen CJ. The Epidemiology and Pathogenesis of Osteoporosis. In: De Groot LJ, Chrousos G, Dungan K, Feingold KR, Grossman A, Hershman JM, Koch C, Korbonits M, McLachlan R, New M, Purnell J, Rebar R, Singer F, Vinik A, editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-2017 Feb 21.
  3. LeBoff M.S., Greenspan S.L, Insogna K.L., Lewiecki E.M., Saag K.G., Singer A.J., Siris, E.S. The Clinician's Guide to Prevention and Treatment of Osteoporosis. Osteoporos Int. In press.
  4. Chou S.H., LeBoff M.S. Vertebral Imaging in the Diagnosis of Osteoporosis: a Clinician's Perspective. Curr Osteoporos Rep. 2017;15(6):509–520. [PubMed]
  5. Buchbinder R., et al. Percutaneous vertebroplasty for osteoporotic vertebral compression fracture. Cochrane Database Syst Rev. 2018;4:CD006349. [PMC free article] [PubMed]
  6. Ebeling P., et al. The Efficacy and Safety of Vertebral Augmentation: A Second ASBMR Task Force Report. J Bone Miner Res. 2019; 34(1):3-21.
  7. Buckley L, Guyatt G, Fink H, Cannon M, Grossman J, Hansen K, Humphrey HB, Lane NE, Magrey M, Miller M, Morrison L, Rao M, Robinson AB, Saha S, Wolver S, Bannuru RR, Vaysbrot E, Osani M, Turgunbae M, Miller A, McAlindon T. 2017 American College of Rheumatology Guideline for the Prevention and Treatment of Glucocorticoid-Induced Osteoporosis. Arthritis Rheumatol. 2017;69(8):1521-1537.
  8. Institute of Medicine. In Ross AC, Taylor CL, Yaktine AL, Del Valle HB e, eds. Dietary reference intakes for calcium and vitamin D. Washington, DC: National Academy of Sciences; 2011.

Age-Related Changes in the Male Reproductive System

ABSTRACT   

In male mammals, changes at all levels of the hypothalamic-pituitary-testicular axis, including alterations in the GnRH pulse generator, gonadotropin secretion, and testicular steroidogenesis, in addition to alterations of feed-forward and feed-back relationships contribute to the age-related decline in circulating testosterone concentrations. The rate of age-related decline in testosterone levels is affected by the presence of chronic illness, adiposity, medication, sampling time, and the methods of testosterone measurement. Epidemiologic surveys reveal an association of low testosterone levels with changes in sexual function, body composition, physical function and mobility, and increased risk of diabetes, late life persistent depressive disorder (dysthymia), unexplained anemia of aging, osteoporosis and bone fractures. Age-related decline in testosterone should be distinguished from classical hypogonadism due to known diseases of the hypothalamus, pituitary, and the testis. In young hypogonadal men who have a known disease of the hypothalamus, pituitary, and testis, testosterone therapy is generally beneficial and has been associated with a low frequency of adverse events. However, neither the long-term benefits in improved health outcomes nor the long-term risks of testosterone therapy are known in older men with age-related decline in testosterone levels. Well-conducted randomized trials have found that testosterone replacement of older men with unequivocally low testosterone levels improves sexual desire, erectile function, and overall sexual activity; lean body mass, muscle strength and some measures of physical function and mobility; areal and volumetric bone density and bone strength; depressive symptoms; and corrects anemia of aging. Testosterone treatment does not worsen lower urinary tract symptoms but the effects of long-term testosterone treatment on the risk of prostate cancer and major adverse cardiovascular events remain unknown. Although testicular morphology, semen production, and fertility are maintained up to a very old age in men, there is clear evidence of decreased fecundity with advancing age and an increased risk of specific genetic disorders related to paternal age among the offspring of older men. Thus, reproductive aging of men is emerging as an important public health problem whose serious societal consequences go far beyond the quality-of-life issues related to low testosterone levels.

INTRODUCTION

Aging of male mammals is a very recent evolutionary event observed mostly in humans and animals in captivity. Most animal species in the wild with few exceptions [e.g., short-finned pilot whales, killer whales and some fish (1)] do not live beyond their reproductive years; during periods of food deprivation, many small animals may not even live beyond puberty. Even among humans, only the men and women of the past three generations have enjoyed a life expectancy of greater than fifty years. With increasing life expectancies of human populations across the globe, today, most men and women can expect to spend a substantial proportion of their lifespan past their procreative years.

 

The historical transition towards aging of human populations has profoundly influenced the health and wellbeing of older adults in their post-reproductive years as well as the size, health, vitality, and economies of human societies (2). At an individual level, many conditions related to reproductive aging, including sexual dysfunction, subfertility or infertility, conditions related to sex-steroid deficiency, genitourinary disorders, pelvic floor disorders, and cancers of the reproductive and accessory organs motivate middle-aged and older men and women to seek medical care. At a societal level, reproductive aging poses a potential threat to the reproductive capacity, health, and welfare of the current and future generations (2,3). Birth-rates in the United States, which had been declining since the turn of the nineteenth century - except during a short baby boom period after World War II - have trended below replacement levels since 1971 (Figure 1) (3-6). Several factors have contributed to this trend, including a growing proportion of couples having their first child after age 30, and an increasing proportion postponing pregnancy beyond age 35 (Figure 1) (4,7).  Societal developments underpinning these trends include the availability of contraceptives that enable couples to separate their sexual and procreative lives; increased work force participation and changing career expectations of women; and a higher age of the male and female partners at reproductive union (2,3). Postponement of childbearing to an older age increases the risk of involuntary childlessness because of the adverse effects of advanced maternal and paternal age per se on fecundity, increased risk of comorbidities associated with advancing age that may indirectly affect fecundity, and the age-related changes in reproductive behaviors (4,8-11).

Figure 1. Birth rates, mean age of mother at first childbirth, and the proportion of infants born in the United States to women >35 years of age since 1970. Legend. The birth rate per 1000 population declined from 18.4 in 1970 to 11.8 in 2017. The mean age of mothers at first child birth increased from 21.4 years in 1970 to 26.6 in 2016. The proportion of all infants born in the USA to mothers > 35 years increased from 4.6% in 1970 to 14.9% in 2012. Birth rates are per 1,000 population estimated as of July 1 for each year except in 1970 and 1980, which were estimated as of April 1. Reproduced with permission from Bhasin S, Kerr C, Oktay K, Racowsky C. The Implications of Reproductive Aging for the Health, Vitality and Economic Welfare of Human Societies. J Clin Endocrinol Metab. 2019 Apr 16:jc.2019-00315. doi: 10.1210/jc.2019-00315. Epub ahead of print. PMID: 30990518. The original figure was based on data derived from: Centers for Disease Control and Prevention. National Vital Statistics System: birth data. Available at: www.cdc.gov/nchs/nvss/births.htm. Accessed 24 June 2021.

The health issues related to reproductive aging of women have been the subject of intense research for nearly 50 years and are covered in other sections of this textbook (12-15). This chapter focuses only on the reproductive aging of men, which has recently begun to garner considerable attention as reflected by the opening of hundreds of men's health clinics across the United States, and in the growing sales of testosterone and erectile dysfunction products.

 

The aging of men is associated with functional alterations at all levels of the reproductive axis that affect both the steroidogenic and gametogenic compartments (16-19). As discussed in this chapter, there is agreement that serum testosterone levels decline with age, a decline that is exacerbated by the accumulation of comorbidities (20,21); however, the long-term effects of testosterone supplementation on health-related outcomes in older men have not been fully examined. Long-term safety data on the effects of testosterone supplementation on the risk of prostate cancer and major adverse cardiovascular events are also lacking. The recent publication of several well-conducted placebo-controlled trials of testosterone in middle-aged and older men has greatly advanced our understanding of the effects of testosterone treatment on sexual function, mobility, vitality, lower urinary tract symptoms and atherogenesis progression (22-28). However, in the absence of long-term, adequately-powered randomized trials of the effects of testosterone on hard patient-important health outcomes – fractures, falls, physical disability, progression from prediabetes to diabetes, remission of depressive disorders, wellbeing, and progression to dementia - the risks and benefits of long-term testosterone replacement in older men remain incompletely understood. The first section of this chapter reviews the pathophysiology and health consequences of age-related decline of testosterone levels and offers a patient-centric individualized approach to the treatment decisions. The second section describes the age-related alterations in the gametogenic compartment of the testes.

 

CHANGES IN THE STEROIDOGENIC COMPARTMENT OF THE TESTIS

 

 

Many studies suggest that aging per se affects the gonadal axis independently of the co-morbidities that accrete with aging, but there remains controversy about the relative contributions of the aging and the accumulation of co-morbidities to the age-related decline in testosterone levels. A few studies of older men have reported preservation of normal testosterone concentrations and its circadian rhythm in healthy older men (29,30). However, many other cross-sectional studies have shown that even after accounting for the potential confounding factors such as time of sampling, concomitant illness and medications, and technical issues related to hormone assays, serum total testosterone levels are lower in older men in comparison to younger men (31-52). Several longitudinal studies (31-34)also have confirmed a gradual but progressive decrease in serum testosterone concentrations from age 20 to 80. Adiposity, chronic illness, weight gain, lifestyle factors, medications, and genetic factors affect testosterone levels and the trajectory of the age-related decline in testosterone levels in men (29,32,35,53-56).  The rate of age-related decline is greater in older men with chronic illness and adiposity than in healthy, non-obese older men (35,53,54). In the European Male Aging Study, adiposity and comorbidities were more strongly associated with low testosterone levels than age (57).

 

In contrast to the sharp reduction in ovarian estrogen production at menopause, the age-related decline in men does not start at a discrete coordinate in old age; rather, total testosterone concentrations, after reaching a peak in the second and third decade, decline inexorably throughout a man’s life (Figure 2). Because of the absence of an identifiable inflection point at which testosterone levels begin to decline abruptly or more rapidly, many investigators have questioned the validity of the concept of “andropause”, which misleadingly implies an abrupt cessation of androgen production in men (39,58). The term ‘late-onset hypogonadism’ has been proposed to reflect the view that in some middle-aged and older men (> 65 years), the age-related decline in testosterone concentration is associated with a cluster of symptoms and signs in a syndromic constellation which resembles in some aspects that observed in men with classical hypogonadism (47,59).

Figure 2. The distribution of total and free testosterone levels by decades of age in male participants of the Framingham Heart Study, the European Male Aging Study (EMAS) and the Study of Osteoporotic Fractures in Men (MrOS). Means and standard deviations are shown. To convert total testosterone from ng/dL to nmol/L, multiply concentrations in ng/dL with 0.0347. To convert free testosterone from pg/mL to pmol/L, multiply concentrations in pg/mL with 3.47. Reproduced with permission from Bhasin et al, J Clin Endocrinol Metab. 2011 Aug;96(8):2430-9.

Sex-hormone binding globulin concentrations are higher in older men than younger men (32,43,48). Thus, the age-related decline in free testosterone levels is of a greater magnitude than that in total testosterone levels. Similarly, there is a greater percent decline in bioavailable testosterone concentrations (the fraction of circulating testosterone that is not bound to SHBG) than in total testosterone concentrations.

 

An Expert Panel of the Endocrine Society defined androgen deficiency as a syndrome resulting from reduced production of testosterone and characterized by a set of signs and symptoms in association with unequivocally low testosterone levels (16). Many epidemiologic studies have defined androgen deficiency solely in terms of serum testosterone concentrations below the lower limit of the normal range for healthy, young men leading to inaccurate estimates of the prevalence of androgen deficiency in older men. Additionally, serum testosterone levels in most studies were measured using direct immunoassays, whose accuracy in the low range has been questioned. Not surprisingly, the estimates of the prevalence of androgen deficiency in older men have varied greatly among different studies. In the Baltimore Longitudinal Study of Aging (BLSA) (31), 30% of men over the age of 60 and 50% of men over the age of 70 had total testosterone concentration below the lower limit of normal range for healthy young men (325 ng/dL, 11.3 nmol/L). The prevalence was even higher when these investigators used a free testosterone index to define androgen deficiency (31). In contrast, more recent studies that have used liquid chromatography tandem mass spectrometry found the prevalence of androgen deficiency to be significantly lower than that observed in the MMAS and BLSA (39,40,47-50). Although 10–15% of men aged ≥65 years have low total testosterone levels (Table 1) (47-50), the prevalence of late-onset hypogonadism defined by symptoms and a total testosterone level <8 nmol/L in the EMAS was 3.2% for men aged 60–69 years and 5.1% for those aged 70–79 years (47). The Healthy Man Study in Australia found no significant age-related decline in testosterone or dihydrotestosterone in men who reported being in good health (60). The authors of the Health Man Study have argued that ill health, rather than aging itself, is the major contributor to androgen deficiency in older men. A Finnish cross-sectional study also demonstrated very low prevalence of low serum testosterone concentrations in older men who were healthy (39).

 

Table 1. Percent of Community-Dwelling Older Men with Unequivocally Low Testosterone Level in Population Studies

Study

Principal Investigator

Number of Men with Age > 65 years

% Men with Testosterone <250 ng/dL

Framingham Heart Study (FHS)

Bhasin

1870

12.1%

Osteoporotic Fractures in Men Study (MrOs)

Orwoll

2623

10%

European Male Aging Study (EMAS)

Wu

1080

7.3%

Cardiovascular Health Study (CHS)

Hirsch

639

14.3%

Data derived from Bhasin et al, JCEM 2011; Orwoll et al, JCEM 2009; Wu et al, NEJM 2010; Hirsch et al, JCEM 2009.

 

 

Circulating testosterone concentrations are a function of testosterone production and clearance rates; the age-related decline in serum testosterone concentrations is primarily a consequence of decreased production rates in older men (20,21,43-45,48). Plasma clearance rates of testosterone are, in fact, lower in older men than in younger men (54,55). The decline in testosterone production in older men is the result of abnormalities at all levels of the hypothalamic-pituitary-testicular axis (42-44,60-71).

GONDADOTROPIN-RELEASING HORMONE SECRETION AND REGULATION IN OLDER MEN           

 

Pulsatile GnRH secretion is attenuated in older men. In addition, there are disturbances of the feedback and feed-forward relationships between testosterone and LH secretion (63,71,72). Thus, the sensitivity of pituitary LH secretion to androgen-mediated feedback inhibition is increased; in addition, the ability of LH to stimulate synchronously testicular testosterone secretion (feedforward) is attenuated (63,71,72). Veldhuis has shown that the orderliness of LH pulses and the synchrony between LH and testosterone pulses are decreased in older men (63,71,72); in addition, there is greater variability in LH pulse frequency, amplitude, and secretory mass in older men, in comparison to younger men (71,72).

 

GONADOTROPIN SECRETION AND REGULATION IN OLDER MEN     

 

There is considerable heterogeneity in circulating LH and FSH concentrations in individual older men; both hypogonadotropic and hypergonadotropic hypogonadism have been reported (54,59). As a group, serum LH and FSH concentrations are higher in older men than in young men (32,33). However, serum LH concentrations do not increase in proportion to the age-related decline in circulating testosterone levels, due to the impairment of GnRH secretion and alterations in gonadal steroid feedback and feedforward relationships (60-71).

 

In the EMAS, secondary hypogonadism (low testosterone and low or normal LH) was more prevalent (nearly 12%) than primary hypogonadism (low testosterone and elevated LH, 2%) (57). Secondary hypogonadism was associated with obesity and comorbid conditions, while primary hypogonadism was associated predominately with age (57). Nearly 10% of men in EMAS had normal testosterone levels and elevated LH; these men with elevated LH tended to be older and in poor health and were at increased risk of developing low testosterone and other comorbid conditions (73).

 

The data on LH response to GnRH are somewhat inconsistent across studies. Urban et al (65) used an interstitial cell bioassay to measure serum concentrations of bioactive LH and found that although basal bioactive LH concentrations were similar in this sample of young and older men, older men demonstrated diminished LH response to GnRH administration. However, in a subsequent study, Zwart et al (66) found greater gonadotropin responsiveness to GnRH in older men than younger men; the maximal and incremental LH and FSH secretory masses in response to graded doses of GnRH were significantly higher in healthy, older men than in younger men. The estimated half-lives of LH, FSH, or alpha-subunit did not significantly differ between young and older men (66).

 

The Brown Norway rat has been widely used as a model of reproductive aging. In this experimental model, the prepro-GnRH mRNA content and the number of neurons expressing prepro-GnRH mRNA are lower in older male rats in comparison to young rats (67,68). The GnRH content of several hypothalamic areas is also lower in intact older rats than younger rats (67). Older Brown Norway rats exhibit significant reductions in glutamate and -aminobutyric acid (GABA) levels in the hypothalamus compared to young rats (68). These observations suggest that the decreased hypothalamic excitatory amino acid expression and the reduced responsiveness of GnRH neurons to N-methyl-D-aspartate may contribute to the altered LH pulsatile secretion observed in old rats (68).

 

Infusions of testosterone and DHT are associated with greater reductions in mean serum LH and FSH levels and the frequency of LH pulses in older men in comparison to young men (69). Winters et al (64) reported that the degree of LH inhibition during testosterone replacement of older, hypogonadal men was significantly greater than in young, hypogonadal men suggesting that older men are more sensitive to the feedback inhibitory effects of testosterone on LH. Deslypere et al (69) also found decreased LH pulse frequency and a greater degree of LH inhibitory response to estradiol administration in older men than young controls. Age-related increase in FSH levels is not associated with a progressive or proportionate decrease in inhibin B levels (70). Thus, the mechanistic basis of FSH increase with advancing age is not fully understood, although the lack of change in inhibin B levels suggests that Sertoli cell function is relatively preserved in older men.

 

TESTICULAR TESTOSTERONE PRODUCTION IN OLDER

 

Testosterone secretion in healthy, young men exhibits a diurnal rhythm characterized by higher concentrations in the morning and lower concentrations in the late afternoon. The diurnal rhythm of testosterone secretion is dampened in older men (41,51). Testosterone response to LH and human chorionic gonadotropin is decreased in older men, compared to younger men (42-44).

 

 

Many physiological changes that occur with advancing age, such as the loss of bone and muscle mass, increased fat mass, impairment of physical and sexual functions, loss of body hair, and decreased hemoglobin levels, are similar to those associated with androgen deficiency in young men. Aging is associated with loss of skeletal muscle mass (Figure 3), muscle strength and power, and progressive impairment of physical function (74-98). Epidemiological studies of older men have reported associations between low testosterone levels and some age-related conditions, although these associations are weak. For instance, in a number of epidemiologic studies, such as the St. Louis Inner City Study of Aging Men (77), the Olmsted County Epidemiological Study (76), and the New Mexico Elderly Health Study (79,80), low bioavailable testosterone levels (unbound and albumin-bound testosterone) were associated with low appendicular skeletal muscle mass. Low bioavailable testosterone levels also have been associated with decreased strength of upper as well as lower extremity muscles (77,78) and decreased performance in self-reported as well as performance-based measures of physical function (99-103). Low free testosterone levels have also been associated with the development of mobility limitation and the frailty syndrome (104-107).

Figure 3. A schematic diagram of the age-related changes in body composition in 7265 men. Lines represent the longitudinal changes in body weight (black line), fat mass (red line) and fat-free. mass (blue line) components from age 20 years. The estimated mass values at age 20 years were as follows: body mass, 72.72 kg; fat mass, 9.14 kg; fat-free mass, 64.09 kg. Figure adapted with permission from Jackson et al. Br J Nutr. 2012;107(7):1085-91.

The association of testosterone levels with sexual dysfunction has been inconsistent across studies because of the heterogeneity and variable quality of instruments used to assess sexual dysfunction, problems of testosterone assay quality, and failure to distinguish among various categories of sexual dysfunction (108-113). Androgen deficiency and erectile dysfunction are two independently distributed clinical disorders and because both disorders are prevalent in middle-aged and older men, they can often co-exist (112,113). Low testosterone levels were associated with low sexual desire in the MMAS (108). Among men enrolled in the testosterone trials, free and total testosterone levels were independently associated with sexual desire, erectile function, and sexual activity scores (114).

 

In the EMAS, total and free testosterone levels were associated with overall sexual function in middle-aged and older men (47). This relationship was observed more robustly at testosterone concentrations <8 nmol/L, but not at higher testosterone concentrations (115). Men deemed to have low total and free testosterone levels in EMAS were more likely to report decreased morning erections, erectile dysfunction, and decreased frequency of sexual thoughts than those with normal testosterone levels (48). In another study of men over the age of 50 who had benign prostatic hyperplasia, sexual dysfunction was reported only by men with serum total testosterone levels less than 225 ng/dL (110).

 

Aging of humans is attended by a decline in several aspects of cognitive function; of these multiple domains of cognition that decline with aging, declines in verbal memory, visual memory, spatial ability, and executive function are associated with the age-related decline in testosterone (109-113,115-124).

 

The relationship of testosterone levels with depression has been inconsistent across epidemiologic studies (125-129). Low testosterone levels in older men are associated more with late-onset low grade persistent depressive disorder (dysthymia) but not with major depression (128-130). In general, testosterone levels are lower in older men with dysthymic disorder than in those without any depressive symptoms (129).

 

Several epidemiologic studies of older men (131-135), including MrOS (131), Rancho Bernardo Study (132), Framingham Heart Study (133), and the Olmsted County Study (134) - have found bioavailable testosterone levels to be associated with bone mineral density, bone geometry, and bone quality (135); the associations are stronger with bioavailable testosterone and estradiol levels than with total testosterone levels. In the MrOS Study, the odds of osteoporosis in men with a total testosterone less than 200 ng/dL were 3.7-fold higher than in men with normal testosterone level (131); free testosterone was an independent predictor of prevalent osteoporotic bone fractures (136).

 

Several studies have evaluated the association of testosterone levels and mortality (137-141). Some, but not all, studies found higher all-cause mortality and cardiovascular mortality in men with low testosterone levels than in those with normal testosterone levels. In a meta-analyses of epidemiologic studies of community-dwelling men, low testosterone levels were associated with an increased risk of all-cause and CVD death (Figure 4) (142,143). However, the strength of the inferences of these meta-analyses was limited by considerable heterogeneity in study populations; it is possible that effects may have been driven by differences in the age distribution and the health status of the study populations (142-146).

Figure 4. The relationship of low testosterone level with all-cause mortality in a meta-analysis of epidemiologic studies of community-based men. Eleven studies which enrolled 16,184 subjects were included in this meta-analysis. There was considerable heterogeneity of the age distribution, health status, and other subject characteristics. Reproduced with permission from Araujo et al, J Clin Endocrinol Metab 2011;96:3007-19.

Testosterone levels are not correlated with aging-related symptoms assessed by the Aging Male Symptom (AMS) score or with lower urinary tract symptoms assessed by the IPSS/AUA prostate symptom questionnaire (144). Some cross-sectional studies found no difference in serum testosterone levels between men who had coronary artery disease and those who did not have coronary artery disease; other studies have reported testosterone levels to be lower in men with coronary artery disease than in men without coronary artery disease (145-150).

 

Epidemiologic studies, especially cross-sectional studies, can only demonstrate associations; causal relationships are difficult to establish from these studies. Furthermore, the associations between testosterone levels and health-related outcomes are generally weak. The inferences are further confounded by the co-linearity of aging-related co-morbid conditions, low testosterone levels, and age-related changes in body composition and inflammatory markers. Although epidemiologic studies have reported associations between the age-related changes in circulating testosterone levels and skeletal muscle mass, muscle strength and physical function; sexual and cognitive functions; areal and volumetric bone density and fracture risk; and mood, long-term randomized trials are needed to determine whether these relations are causal.

 

Potential Beneficial Effects of Testosterone Treatment in Older Men with Low Testosterone Levels

 

It has been hypothesized that increasing serum testosterone concentrations in older men with low testosterone levels into a range that is mid-normal for healthy, young men would improve physical function and mobility, some domains of sexual and cognitive functions, energy and sense of wellbeing, and reduce the risk of falls and fractures, and improve overall quality of life. A number of randomized trials have demonstrated improvements in measures of sexual function, lean and fat mass, and areal and volumetric bone mineral density; however, there has been a paucity of long-term, placebo-controlled, randomized trials that are adequately powered to detect clinically meaningful changes in health outcomes such as fracture rates, physical disability, progression to dementia, remission of late onset low grade persistent depressive disorder (dysthymia), progression from prediabetes to diabetes, and overall quality of life. Furthermore, none of the previously published studies had sufficient power to address the long-term risks of prostate and cardiovascular disease. 

 

The following section describes the effects of testosterone supplementation on multiple organ systems focusing on physical function, sexual function, vitality, bone health, mood, wellbeing, and depression, and cognitive function.

 

EFFECTS OF TESTOSTERONE SUPPLEMENTAION ON MUSCLE MASS AND PERFORMANCE AND PHYSICAL FUNCTION IN OLDER MEN WITH LOW TESTOSTERONE LEVELS 

        

 

Sarcopenia, the loss of muscle mass and function, is an important consequence of aging (75-79). The principal component of the decrease in fat-free mass is the loss of muscle mass; there is little change in non-muscle lean mass (81-87). Between 20 and 80 years of age, the skeletal muscle mass decreases by 35-40% in men (85), in part due to decreased muscle protein synthesis (92). Although there is a loss of both type I and type II fibers, there is a disproportionate decrease in the number of type II muscle fibers that are important for the generation of muscle power (93,94). In spite of the significant depletion of skeletal muscle mass, body weight does not decrease, and may even increase because of the accumulation of body fat (81-87) (Figure 3).

 

The loss of skeletal muscle mass that occurs with aging is associated with a reduction in muscle strength (95-98). There is a substantial decrease in muscle strength and power between 50 and 70 years of age, primarily due to muscle fiber loss and selective atrophy of type II fibers (93-98). The loss of muscle strength is even greater after the age of 70; 28% of men over the age of 74 could not lift objects weighing more than 4.5 kg (97). With increasing age, there is a progressive reduction in muscle power (151,152), the speed of strength generation, and fatigability, the ability to persist in a task.

 

Loss of muscle mass and strength leads to impairment of physical function, as indicated by the impaired ability to arise from a chair, climb stairs, generate gait speed, and maintain balance (151-154). The impairment of physical function contributes to loss of independence, and increased risk of physical disability, falls and fractures in older men.

 

Anabolic Effects of Testosterone in Humans: Testosterone Trials in Healthy, Hypogonadal Men, Men with Chronic Illness, and Older Men

 

The anabolic effects of testosterone on the muscle have been a source of controversy for over sixty years. The athletes and recreational bodybuilders use large doses of androgenic steroids with the belief that these compounds increase muscle mass and strength. Until recently, the academic community was skeptical about such claims because of the problems of study design. However, a large number of studies in healthy young men, healthy hypogonadal men, men with chronic illness, and in healthy older men have established that testosterone administration improves skeletal muscle mass, maximal voluntary strength, leg power, aerobic capacity, and some measures of physical performance and mobility (154-165). In a systematic review of testosterone trials in healthy, hypogonadal men, testosterone therapy increased fat-free mass and body weight (Figure 5) (154-161).

Figure 5. The effects of testosterone therapy on body composition, muscle strength, and sexual function in intervention trials. The point estimates and the associated 95% confidence intervals are shown. Panel A shows the effects of testosterone therapy on, grip strength, fat mass and lean body mass in a meta-analysis of randomized trials (data derived from Bhasin et al. Nat Clin Pract Endocrinol Metab. 2006;2(3):146-59; figure reproduced with permission from Spitzer et al. Nat Rev Endocrinol. 2013;9(7):414-24). Panel B shows the effects of testosterone therapy on sexual function in a meta-analysis of randomized trials (figure adapted with permission from Ponce et al. J Clin Endocrinol Metab. 2018;103(5):1745-54).

The anabolic effects of testosterone on fat-free mass, muscle size, and maximal voluntary strength are related to the administered testosterone dose and the circulating testosterone concentrations (166-168) (Figure 6). The administration of supraphysiologic doses of testosterone in eugonadal men increases fat-free mass, muscle size, and maximal voluntary strength (166-169).

Figure 6. Testosterone Dose Response Relationship in Young and Older Men. In this study, healthy, young men (18-34 years of age) and healthy older men (60-75 years of age) were treated with a long-acting GnRH agonist plus graded doses of testosterone enanthate for 20 weeks. Shown are mean (±SEM) changes from baseline in fat free mass (upper left), skeletal muscle mass (upper right), fat mass (lower left), and leg press strength (lower right) in young (black bars) and older (lightly shaded bars) men. Adapted with permission from Bhasin et al. J Clin Endocrinol Metab. 2005 Feb;90(2):678-88.

Testosterone effects on muscle performance are domain-specific: testosterone administration increases maximal voluntary strength and leg power, but does not affect fatigability or specific tension (167). The gains in maximal voluntary strength during testosterone administration are proportional to the increase in muscle mass; unlike resistance exercise training, testosterone does not improve the contractile properties of the human skeletal muscle (167).

 

Resistance exercise training augments the anabolic response to androgens; thus, men receiving testosterone and resistance exercise training together experience greater gains in fat-free mass and muscle strength than those receiving either intervention alone (169). The anabolic effects of testosterone are also augmented by concomitant recombinant growth hormone administration (170). Although it has been speculated in the sports medicine literature that increasing the protein intake can enhance the muscle mass and strength gains in response to anabolic stimuli such as resistance exercise training or androgens, the evidence supporting such speculation is weak. In a recent controlled feeding study, increasing the daily protein intake to a level (1.3 g/kg/day) higher than the recommended daily allowance (0.8 g/kg/day) for six months did not increase lean body mass or maximal muscle strength more than that associated with the daily intake of the recommended daily allowance of 0.8 g/kg/day (171) . The higher level of daily protein intake (1.3 g/kg/day) also did not augment the gains in lean body mass and muscle strength in response to testosterone administration above that observed in participants eating the recommended dietary allowance for protein (171,172). 

 

Testosterone replacement of young, hypogonadal men has been reported to increase muscle protein synthesis (158,173,174). The effects of testosterone replacement on muscle protein degradation need further investigation.

 

Systematic reviews (155,175,176) of randomized, placebo-controlled trials in HIV-infected men with weight loss (176-181) have revealed that testosterone therapy for 3 to 6 months was associated with greater gains in lean body mass than placebo administration (difference in lean body mass change between placebo and testosterone arms 1.22 kg, 95% CI 0.23-2.22 for the random effect model). In two (176,180) out of three trials that measured muscle strength (176,180,181), testosterone administration was associated with significantly greater improvements in maximal voluntary strength than placebo. Testosterone therapy had a moderate effect on depression indices (-0.6, 95% CI -1.0, -0.2) (182) and fatigue (183), but did not improve overall quality of life (182,183). Changes in CD4+ T lymphocyte counts, HIV copy number, PSA, plasma HDL cholesterol, and adverse event rates were not significantly different between the placebo and testosterone-treatment groups (176-183). Overall, short-term (3-6 months) testosterone use in HIV-infected men with low testosterone levels and weight loss can induce modest gains in body weight and lean body mass with minimal changes in quality of life and mood. This inference is weakened by inconsistency of results across trials, and heterogeneity in inclusion and exclusion criteria, disease status, testosterone formulations and doses, treatment duration, and methods of body composition analysis (155). Data on testosterone effects on physical function, risk of disability, or long-term safety in HIV-infected men are limited.

 

Testosterone administration increases fat-free mass and decreases fat mass in older men with low testosterone levels. Meta-analyses (155,183) of randomized trials (184-188) that included middle-aged and older men with low or low normal testosterone levels, and that used testosterone or its esters in replacement doses for >90 days, have confirmed that testosterone administration is associated with a significantly greater increase in whole body and appendicular fat-free mass and a greater reduction in whole body and appendicular fat mass than placebo (Figure 5). The average gains in fat-free mass generally were greater in trials that used injectable testosterone esters than in those which used transdermal testosterone gel, presumably because of the higher doses of testosterone delivered by the injectable formulations than by transdermal gel formulations. The change in body weight did not differ significantly between the testosterone and placebo groups.

 

Testosterone administration improves stair climbing speed and power, and self-reported physical function, as assessed by the Medical Outcomes Study Short Form 36 (MOS SF36) questionnaire. Testosterone’ Effects on Atherosclerosis Progression in Aging Men Trial (The TEAAM Trial), a randomized trial conducted in healthy community-dwelling older men without functional limitations and low to low-normal testosterone levels, showed that testosterone replacement for 3-years was associated with modest improvements in leg-press and chest-press power and the stair-climb power (163). Changes in gait speed generally have been modest and inconsistent across randomized trials (25,185,188,189). Testosterone administration is associated with small improvements in aerobic capacity and attenuation of the age-related decline in VO2peak (Figure 7) (164,165).

Figure 7. Effects of testosterone administration on measures of muscle performance and physical function in randomized testosterone trials in older men. Panel A shows the mean (SD) change from baseline to maximal voluntary strength in the leg press and chest press exercises and on loaded stair climbing power at either the end of the intervention period or at the last measurement performed in who dropped out before study completion in the testosterone in older men with mobility limitation (The TOM Trial). The minimal clinically important difference (MCID) for each outcome was determined using an anchor-based method within the trial. The proportion of men (percent) whose change from baseline either equaled or exceeded the MCID is shown below the figure along with the P-value for the comparison of placebo and testosterone groups (figure adapted with permission from Spitzer et al. Nat Rev Endocrinol. 2013;9(7):414-24). Panel B shows the long-term effects of testosterone administration on aerobic capacity in older men participating in the TEAAM trial. Data points represent mean changes from baseline and error bars are 95% CI in VO2peak (L/min) and in peak work rate. P values indicate the overall effect of the testosterone intervention over time (figure reproduced with permission from Traustadóttir et al, J Clin Endocrinol Metab. 2018;103(8):2861-2869).

One reason for the variable improvements in physical function in various testosterone trials is that the measures of physical function used in previous studies had low ceilings. Another confounder of the effects of anabolic interventions on muscle function is the learning effect. For instance, subjects who are unfamiliar with weightlifting exercises often demonstrate improvements in measures of muscle performance simply because of increased familiarity with the exercise equipment and technique. Because of the considerable test-to-test variability in tests of physical function, it is possible that previous studies did not have adequate power to detect meaningful differences in measures of physical function between the placebo and testosterone-treated groups. It is also possible that neuromuscular adaptations needed to translate strength gains into functional improvements require a lot longer than the 3 to 6-month duration of most of the previous trials. The measures of physical function that are more robustly related to lower extremity muscle strength, such as stair climbing speed and power, have shown more consistent improvements in testosterone trials than walking speed (22,23,190).

 

Only a few testosterone trials have been conducted in older men with functional limitations (22,26,28,190-192). In a trial of pre-frail or frail men (28), administration of 50 mg testosterone gel daily for 6 months induced greater improvements in lean mass, knee extension peak torque, and sexual symptoms than did placebo gel (28). Performance-based measures of physical function did not differ significantly between groups, but they improved in the subgroup of frail elderly men (28). In Testosterone in Older Men (TOM) Trial, older men with mobility limitation were randomly assigned to either placebo or 10 g testosterone gel daily for 6 months (22,190). The testosterone dose was adjusted to achieve testosterone levels between 17.4 nmol/l and 34.7 nmol/L (500 to 1000 ng/dL). The improvements in leg-press strength, chest-press strength and power, and loaded stair-climbing speed and power were significantly greater in men assigned to testosterone arm than in those receiving placebo (Figure 7). A greater proportion of men in the testosterone arm improved more than the minimal clinically important difference for leg-press and chest-press strength and stair-climbing speed than that in the placebo arm. Because of a higher frequency of cardiovascular-related events in the testosterone arm compared with the placebo arm, the trial’s data and safety monitoring board stopped further administration of study medication (22,190). The findings of the TOM trial and other epidemiologic studies have heightened the concern that frail elderly men with a high burden of chronic co-morbidities may be at an increased risk of adverse events (22), providing the impetus to develop, strategies to achieve increased selectivity and a more favourable risk to benefit ratio (22).

 

The Testosterone Trials were a coordinated set of seven randomized double-blind, placebo-controlled trials designed to determine the benefits of testosterone therapy in older men 65 years and older with low testosterone levels and clinical symptoms of androgen deficiency on a variety of androgen-dependent outcomes (192). To participate in these trials, the men had to be eligible for at least one of the three main trials (the Sexual Function Trial, the Physical Function Trial, or the Vitality Trial). The men were assigned to testosterone or placebo gel for 1 year and the dose was adjusted to maintain testosterone concentrations within the normal range for healthy young men.  The Physical Function Trial of the TTrials recruited older men with self-reported difficulty walking or climbing stairs and walking speed less than 1.2 m/s and an average of two morning fasting testosterone levels less than 275 ng/dL (162). The 6-minute walking distance improved significantly more in the testosterone than in the placebo group among all men in the TTrials, but not in those who were enrolled in the PFT (162). The self-reported physical function assessed using the physical component of the Medical Outcomes Study Short Form-36 questionnaire, improved more in the testosterone group than in the placebo group in all men in TTrials and in men enrolled in the PFT (162). The men in the testosterone group were more likely to report improvement in their walking ability than men in the placebo group. The changes in 6-minute walking distance were significantly associated with changes in testosterone, free testosterone, dihydrotestosterone, and hemoglobin levels, and to baseline gait speed and self-reported mobility limitation (162). Thus, testosterone treatment of older men with mobility limitation consistently improved self-reported walking ability, modestly improved 6-minute walking distance (162). The number of falls was similar in the testosterone and placebo arms (162).

 

Innovative strategies to translate gains in muscle mass and strength induced by testosterone into functional improvements are needed (18). Resistance exercise training augments the anabolic effects of androgens on muscle mass and performance and physical function (193). Thus, adjunctive exercise training might be required to induce the neuromuscular and behavioural adaptations that are necessary to translate the gains in muscle mass and strength into clinically-meaningful functional improvements (18). In addition, there is some evidence that the anabolic response of skeletal muscle to dietary protein is attenuated with age (194,195). These findings have raised the question whether the current recommended dietary allowance (RDA) for protein (0.8 g/kg/day) is adequate to preserve lean body mass and physical function in older adults. However, In a recent controlled feeding study in functionally-limited older men with usual protein intake less than or equal to the RDA for protein (171) higher protein intake exceeding the RDA did not increase lean body mass, muscle performance or physical function nor augmented the anabolic response to testosterone. However, higher protein intake was associated with lower whole body and visceral abdominal fat, although no significant changes in metabolic biomarkers (fasting glucose, fasting insulin, HOMA-IR, leptin, adiponectin, IL-6, and hs-CRP) were observed (196). These findings suggest that the current RDA for protein is adequate to maintain lean body mass and higher protein intake above the RDA does not promote additional gains in muscle mass or physical function with or without testosterone supplementation.

 

Mechanisms of Androgen Action on Muscle

 

Testosterone-induced increase in muscle mass is associated with hypertrophy of both type I and II muscle fibers (197). The absolute number and the relative proportion of type I and type II fibers do not change during testosterone administration. Testosterone-induced muscle fiber hypertrophy is associated with dose-dependent increases in myonuclear number and satellite cell number (198), suggesting that testosterone administration increases the number of muscle progenitor cells.

 

Testosterone administration has been shown to increase fractional muscle protein synthesis and improve the reutilization of amino acids (173,174). The effects of testosterone on muscle protein degradation have not been well studied. However, the muscle protein synthesis hypothesis does not explain the reciprocal decrease in fat mass or the increases in myonuclear and satellite cell number that occur during testosterone administration (198). Testosterone promotes the differentiation of mesenchymal multipotent muscle progenitor cells into the myogenic lineage and inhibits the differentiation of these progenitor cells into the adipogenic lineage (199,200). Thus, testosterone promotes the formation of myosin heavy chain II positive myotubes in multipotent cells and up-regulates markers of myogenic differentiation, such as MyoD and myosin heavy chain (199,200). Testosterone and DHT inhibit adipogenic differentiation and downregulate markers of adipogenic differentiation, such as PPAR-¡ and C/EBPµ (201). 

 

Testosterone’s effects on myogenic differentiation are mediated largely through its binding to the classical androgen receptor, which induces a conformational change in the androgen receptor protein, promoting its association with its co-activator, beta-catenin, causing the complex to translocate into the nucleus (200,202). The androgen receptor – beta-catenin complex associates with TCF-4 and activates a number of Wnt target genes (200,202), including follistatin. Follistatin cross-communicates the signal from the AR-beta- catenin pathway to the TGF-beta signaling pathway, blocking signaling through the TGF-beta / Smad 2/3 (201,203). Follistatin plays an essential role in mediating the effects of testosterone on myogenic differentiation (203,204). Jasuja et al (204) found that the administration of recombinant follistatin selectively increased muscle mass and decreased fat mass but had no effect on prostate growth. Recombinant follistatin and testosterone each regulated the expression of a large number of common genes in the skeletal muscle, but they differed substantially in the expression profile of genes activated in the prostate (204). Among the genes activated differentially by testosterone but not by follistatin in the prostate, Jasuja et al (204) identified polyamine pathway as an important signaling pathway. The polyamine pathway has been known to be involved in regulating prostate growth. Administration of testosterone in combination with an inhibitor of ornithine decarboxylase-1, a key enzyme in the polyamine pathway, to castrated male mice restored levator ani muscle mass but not prostate mass, indicating that ODC1 plays an important role in mediating the effects of testosterone on the prostate (Figure 8) (204). Therefore, combined administration of testosterone plus ODC1 inhibitor provides a novel approach for achieving selectivity of testosterone’s anabolic effects on the muscle while sparing the prostate (204).

Figure 8. Testosterone Plus Ornithine Decarboxylase 1 Inhibitor as a Selective Prostate Sparing Anabolic Therapy. Intact and castrated adult male mice were treated for 2-weeks with vehicle or testosterone with and without α-difluoromethylornithine (DFMO), a specific Odc1 inhibitor, as follows: Intact, castrated (Cx), castrated + 15µg/day T (Cx+T), castrated +15µg/day T+ 15µg/day DFMO (Cx+T+DFMO). Levator ani weights (right panel) in mice treated with testosterone plus DFMO were similar to those in intact controls and testosterone-treated castrated mice. Prostate weights in castrated mice were lower than in intact controls and were restored by testosterone administration to levels seen in intact mice (left panel). Mice treated with testosterone plus DFMO had significantly lower prostate weights than intact controls or castrated mice treated with testosterone alone, but not significantly different from those in castrated mice treated with vehicle alone. Thus, testosterone plus ODC1 inhibitor could serve as prostate-sparing selective anabolic therapy. Reproduced with permission from Jasuja et al. Aging Cell. 2014 Apr;13(2):303-10.

The Role of Steroid 5-Alpha Reductase and DHT in Mediating Androgen Effects in the Muscle

 

Although the enzyme steroid 5-alpha-reductase is expressed at low concentrations within the muscle (205,206), we do not know whether conversion of testosterone to dihydrotestosterone is required for mediating testosterone's effects on the muscle. Men with benign prostatic hypertrophy who are treated with a 5-alpha reductase inhibitor do not experience muscle loss (207). Similarly, individuals with congenital 5-alpha-reductase deficiency have normal muscle development at puberty (207). These data suggest that 5-alpha reduction of testosterone to DHT is not obligatory for mediating its effects on the muscle. However, all the kindred with steroid 5-alpha reductase deficiency that have been published to-date have had mutations of type 2 isoform of the enzyme. Similarly, finasteride is a weak inhibitor of only the type 2 isoform of the enzyme. The circulating concentrations of DHT in male patients with congenital mutation of type 2 steroid 5-alpha reductase enzyme or in men treated with finasteride are lower than eugonadal men; however, these patients still produce significant amounts of DHT and their circulating DHT concentrations are often in the lower end of the range in healthy young men. Long-term administration of dutasteride, a dual and potent inhibitor of both 5-alpha reductase isoforms, has not been associated with significant reductions in bone mineral density (207). This issue is important because if 5-alpha reduction of testosterone to DHT were not obligatory for mediating its anabolic effects on the muscle, then it might be beneficial to administer testosterone with an inhibitor of steroid 5-alpha reductase or to develop selective androgen receptor modulators that do not undergo 5-alpha reduction.

 

To determine whether testosterone’s effects on muscle mass and strength, sexual function, hematocrit, prostate, sebum production, and lipids are attenuated when its conversion to DHT is blocked, we administered to healthy men, 21-50 years, a long-acting GnRH-agonist to suppress endogenous testosterone. We randomized them to placebo or dutasteride (dual inhibitor of steroid 5-alpha reductase type 1 and 2) 2.5-mg daily, plus 50, 125, 300, or 600-mg testosterone enanthate weekly for 20-weeks (208). Changes in lean and fat mass, leg-press and chest-press strength, were related to testosterone dose but did not differ between placebo and dutasteride groups (208). The relation between testosterone concentrations and the changes in lean body mass, maximum voluntary muscle strength, hematocrit, and sebum production was similar between dutasteride and placebo arms (Figure 9) (208). Changes in sexual-function scores, bone markers, prostate volume, and PSA did not differ between groups (208). These data indicate that testosterone’s conversion to DHT is not essential for mediating its effects on muscle mass and strength, sexual function, hematocrit, or sebum production in men over the range of testosterone concentrations achieved in this trial (208). These data are consistent with studies that have reported that administration of steroid 5α-reductase inhibitors has little or no effect on muscle or bone mass (209-211). The isoforms of steroid 5α reductase enzyme also catalyze the 5α reduction of cortisol, progesterone, bile acids and other metabolites. In the central nervous system, 5α-reductase is the rate-limiting enzyme in the conversion of progesterone to allopregnanolone that serves as a positive allosteric modulator of gamma-aminobutyric acid (GABA) A receptors to modulate neural pathways that regulate mood, affect, and  cognition (212-214). Low levels of allopregnanolone have been implicated in the pathogenesis of some forms of depressive and anxiety disorders (215). An intravenous preparation of allopregnanolone was found to be efficacious and approved for the treatment of postpartum depression (216,217) and is being investigated for the treatment of other depressive disorders. Steroid 5α-reductase enzymes are also involved in cortisol metabolism and in the pathogenesis of metabolic disease (218).

Figure 9. The Role of 5-alpha-Dihydrotestosterone in Men. In this randomized trial, healthy men, 18-50 years, received a long-acting GnRH-agonist to suppress endogenous testosterone. They were then randomized to either placebo or dutasteride (dual inhibitor of steroid 5-alpha reductase types 1 and 2) 2.5-mg daily, plus 50, 125, 300, or 600-mg testosterone enanthate weekly for 20-weeks (535). Changes in fat-free mass (upper panel) and leg-press strength (lower panel), were related to testosterone dose but did not differ between placebo and dutasteride groups (535). The relationship between change in total testosterone (TT) levels and change in fat-free mass and leg press strength (right panels) did not differ between men assigned to placebo or dutasteride arms. Reproduced with permission from Bhasin et al, JAMA. 2012 Mar 7;307(9):931-9.

The Role of CYP19A1 (Aromatase) in Mediating Testosterone’s Effects on the Muscle

 

Studies of aromatase knockout mice have revealed higher fat mass and lower muscle mass in mice that are null for the P450-linked CYP19A1 aromatase gene (219). Similarly, humans with CYP19A1 mutations have decreased muscle mass and increased fat mass, and they exhibit insulin resistance (220). Data from these experiments of nature suggest that aromatization of testosterone to estradiol is important in mediating androgen effects on body composition. Finkelstein et al (221) have recently examined the relative roles of testosterone and estradiol in regulation of muscle and fat mass, and sexual function. These investigators found that testosterone’s effects on lean mass, muscle size, and strength were not significantly attenuated when its conversion to estradiol was blocked by administration of an aromatase inhibitor (221).

 

REGULATION OF FAT MASS, FAT DISTRIBUTION, AND METABOLISM BY TESTOSTERONE         

 

Testosterone is an important regulator of fat mass and distribution. Lowering testosterone concentrations by administration of a GnRH agonist increases fat mass, and testosterone administration in hypogonadal men decreases whole body fat mass (159,222-224). The loss of fat mass during testosterone administration occurs both in the appendices as well as the trunk and is distributed evenly between the superficial subcutaneous and deep intra-abdominal and intermuscular compartments (166,223). The effects of testosterone on whole body fat mass are related to the administered testosterone dose and the circulating testosterone concentrations (166,223).

 

Mechanisms of Testosterone’s Effects on Fat Mass and Metabolism  

 

The effects of testosterone on fat mass are mediated through its conversion to estradiol by the aromatase enzyme encoded by CYP19A1 (221).  Men with inactivating mutations of CYP19A1 are characterized by increased fat mass, metabolic syndrome, hepatic steatosis, and insulin resistance (225-227). Estradiol replacement of male aromatase knockout mice reverses the adiposity and metabolic abnormalities associated with estrogen deficiency (228).

 

Testosterone regulates adipose tissue mass and metabolism through multiple mechanistic pathways. Androgens inhibit adipogenic differentiation of multipotent mesenchymal progenitor cells; these effects are blocked by androgen receptor blocker, bicalutamide (200,201,229). Testosterone regulates fat oxidation but does not appear to affect triglyceride secretion over short durations (230).

 

Testosterone, after its aromatization to estradiol, acts through the estrogen receptors in specific brain regions to regulate eating behavior, energy expenditure, and adipose tissue metabolism. The deletion of estrogen receptor α (ER-α) in specific brain regions is associated with adiposity, hyperphagia, and hypometabolism (231); estradiol acting through ER-α regulates eating behavior and energy expenditure differentially through actions on different hypothalamic neurons (231).  Activation of estrogen receptor β (ER-β) by selective agonists inhibits weight gain, adiposity, increases energy expenditure and thermogenesis, and reverses hepatic steatosis in mice through direct effects on xenobiotic and bile acid receptors in the liver (232).

 

TESTOSTERONE AND SEXUAL FUNCTION IN OLDER MEN     

 

Regulation of Sexual Function by Testosterone  

Sexual function in men is a complex process that includes central mechanisms for regulation of sexual desire and arousal, and local mechanisms for penile tumescence, orgasm, and ejaculation (233). Primary effects of testosterone are on sexual interest and motivation (233-238). Testosterone replacement of young, androgen deficient men improves a wide range of sexual behaviors including frequency of sexual activity, sexual daydreams, sexual thoughts, feelings of sexual desire, attentiveness to erotic stimuli, and spontaneous erections (233-241).  Kwan et al (237)demonstrated that androgen-deficient men have decreased frequency of sexual thoughts and lower overall sexual activity scores; however, these men can achieve erections in response to visual erotic stimuli. Hypogonadal men have lower frequency and duration of the episodes of nocturnal penile tumescence; testosterone replacement increases both the frequency and duration of sleep-entrained, penile erections (239-241). Although both orgasm and ejaculation are believed to be androgen-independent, hypogonadal men have decreased ejaculate volume and their orgasm may be delayed.

 

Although hypogonadal men can achieve erections, it is possible that achievement of optimal penile rigidity might require physiologic testosterone concentrations. Testosterone regulates nitric oxide synthase activity in the cavernosal smooth muscle (242). Testosterone administration in orchiectomized rats increases penile blood flow and has trophic effects on cavernosal smooth muscle (243-245).

 

In male rodents, all measures of mating behavior are normalized by relatively low testosterone levels that are insufficient to maintain prostate and seminal vesicle weight (246,247). Similarly, in men, sexual function is maintained at relatively low normal levels of serum testosterone (221,238,248). Testosterone’s effects on libido are mediated through its conversion to estradiol (221).

Total and free serum testosterone levels are positively associated with sexual desire, erectile function and sexual activity in older men with unequivocally low testosterone levels and symptoms of sexual dysfunction (114). These findings suggest that low testosterone levels may contribute to impaired sexual functioning in older men.

 

Erectile dysfunction and androgen deficiency are two common but independently distributed, clinical disorders that sometimes co-exist in the same patient (112,113,233,249). Hypogonadism is a clinical syndrome that results from androgen deficiency (16); in contrast, erectile dysfunction is usually a manifestation of a systemic vasculopathy, often of atherosclerotic origin. Thus, androgen deficiency and erectile dysfunction have distinct pathophysiology. Eight to ten percent of middle-aged men presenting with erectile dysfunction have low testosterone levels (113,249-251).

 

Clinical Trials of the Effects of Testosterone Therapy on Sexual Function of Older Men with Low Circulating Testosterone Concentrations

 

In open-label trials, testosterone treatment has been shown to improve sexual function in young men with classical hypogonadism due to disorders of the hypothalamus, pituitary, or testes (159,252). However, previous trials evaluating the benefits of testosterone therapy in men 60 years and older with age-related decline in testosterone levels on sexual functioning have yielded inconsistent results (253), with some studies showing improvement (254,255), while others have suggested no clear benefit (23). The inconsistencies in these previous studies are due to several factors, including small sample sizes, inclusion of men who were not clearly hypogonadal or did not have sexual symptoms, inclusion of men with heterogeneous sexual disorders, variable treatment durations, and the use of outcomes assessment tools that had not been rigorously validated.

 

In a small number of placebo-controlled trials of testosterone that have been conducted in men with sexual symptoms and low testosterone levels (24,26,161), testosterone replacement has been associated with a small but significant increase in overall sexual activity, sexual desire, erectile function, and sexual satisfaction.  A meta-analysis of these placebo-controlled trials found that testosterone replacement of hypogonadal men is associated with a small but significant increase in sexual desire [standardized mean difference (SMD): 0.17; 95% CI, 0.01, 0.34], erectile function (SMD: 0.16; 95% CI, 0.06, 0.27), and sexual satisfaction (SMD: 0.16; 95% CI, 0.01, 0.31) (256). 

 

The Sexual Function Trial of the TTrials determined the efficacy of testosterone treatment for 1-year on sexual function in symptomatic, community-dwelling, older men ≥65 years with low testosterone levels (26). Testosterone administration for 1-year to raise testosterone concentrations into a range that is mid-normal for healthy young men was associated with significant improvements in sexual activity, desire, and erectile function (257). The treatment effects tended to wane over time, and the effect on erectile function was substantially smaller than that reported with phosphodiesterase 5 inhibitors (258). The magnitude of increase in testosterone levels was related to the improvements in sexual activity and desire, but not erectile function (257). There was no clear testosterone threshold level of effect.

 

Testosterone does not improve sexual function in middle-aged and older men who have normal testosterone levels and do not have any sexual symptoms (23). Testosterone replacement therapy does not improve ejaculatory function in men with ejaculatory disorder (259).

 

It had been speculated that testosterone administration might improve erectile response of men with ED to selective phosphodiesterase inhibitors (260-262). To determine whether the addition of testosterone to a phosphodiesterase-5-inhibitor improves erectile response, we conducted a randomized, placebo-controlled trial (263), in men, 40-to-70 years, with erectile dysfunction and low total testosterone< 11.5 nmol/L (330ng/dL) and/or free testosterone <173.5 pmol/L (50 pg/mL). All participants were initially started on sildenafil alone and the sildenafil dose was optimized based on their response during a 3 to 7-week run-in period (263). The participants were then randomized to 10-g testosterone or placebo gel for 14-weeks in combination with the optimized sildenafil dose (263). The administration of sildenafil alone was associated with substantial increases in erectile function domain (EFD) score and total and satisfactory sexual encounters (263). However, the change in EFD score in men assigned to testosterone plus sildenafil did not differ significantly from that in men assigned to placebo plus sildenafil (263). Changes in total and successful sexual encounters, quality-of-life, and marital-intimacy did not differ between testosterone and placebo groups. Even among the subsets of men with baseline testosterone <250 ng/dL or those without diabetes, there were no significant differences in EFD scores between the two arms (263). Another placebo-controlled trial of men with erectile dysfunction who were non-responders to tadalafil also did not show a greater improvement in erectile function in men assigned to the testosterone arm than in those assigned to the placebo arm (262). Thus, in randomized trials, the addition of testosterone to PDE5Is has not been shown to improve erectile function in men with erectile dysfunction (262,263).

Synopsis of The Effects of Testosterone on Sexual Function

 

In older hypogonadal men with low sexual desire, testosterone treatment improves sexual desire, erectile function, and overall sexual activity. Androgen deficiency is an important cause of low sexual desire disorder (233). Therefore, serum testosterone concentrations should be measured in the diagnostic evaluation of hypoactive sexual desire disorder as well as erectile dysfunction, recognizing that low sexual desire is often multifactorial; systemic illness, relationship and differentiation (the ability of individuals in a relationship to maintain their distinct identities) issues, depression, and many medications can be important antecedents or contributors to low sexual desire and sexual dysfunction.

 

TESTOSTERONE EFFECTS ON BONE MINERAL METABOLISM        

 

The Effects of Androgen Deficiency on Bone Mass

 

Testosterone deficiency is associated with a progressive loss of bone mass (264-267). In one study performed in sexual offenders (264), surgical orchiectomy was associated with a progressive decrease in bone mineral density of a magnitude similar to that seen in women after menopause. Similarly, androgen deficiency induced by the administration of a GnRH agonist, surgical orchiectomy, or an androgen antagonist for the treatment of prostate cancer leads to loss of bone mass (265-267) and an increase in fracture risk (268,269), which is related to the dose of GnRH agonist and the degree of testosterone suppression (270). In male rats, surgical orchiectomy or androgen blockade by administration of an androgen receptor antagonist is associated with loss of bone mass (271).

 

Androgen deficiency that develops before the completion of pubertal development is associated with reduced cortical and trabecular bone mass (272,273). During the pubertal years, bone accretion, and bone length and thickness is regulated by sex steroids. During puberty, sex hormones slow long bone growth and accelerate axial growth. Prepubertal sex hormone deficiency allows continued long bone growth and slows axial growth resulting in longer limbs and a shorter trunk (eunuchoidal proportions)  (274). Sex differences in bone width are also established during pubertal development. Men increase bone width by periosteal bone formation and women mostly by endocortical apposition  (275). Young men with constitutional delay of puberty have lower bone mineral density (276), which does not improve spontaneously 2 years later (277). Indeed, men with hip fractures have been shown to have smaller femoral head diameters, which may potentially be related to delayed puberty (278). Therefore, individuals with sex-steroid deficiency before or during peri-pubertal years may end up with suboptimal peak bone mass and increased lifetime fracture risk. Similarly, men with acquired androgen deficiency have lower bone mineral density than age-matched controls (155).

 

Clinical Trials on The Effects of Testosterone Therapy on Bone in Young, Hypogonadal Men

 

Testosterone therapy of healthy, young, hypogonadal men is associated with significant increases in vertebral bone mineral density (156,279-283). However, bone mineral density is typically not normalized after 1-2 years of testosterone replacement therapy (156). Some hypogonadal patients included in these testosterone trials had panhypopituitarism and also suffered from growth hormone deficiency. It is possible that concomitant GH replacement might be necessary for restoration of normal bone mineral density. Excessive glucocorticoid replacement might also contribute to bone loss in these patients. In addition, some participants had experienced testosterone deficiency before the onset and completion of pubertal development; the individuals who develop androgen deficiency during the critical pubertal developmental window of bone accretion, may end up with decreased peak bone mass, and testosterone administration may not be able to restore bone mass to levels seen in eugonadal age-matched controls. Many testosterone replacement trials were less than 3 years in duration, and it is possible that a longer period of testosterone administration might be necessary to achieve maximal improvements in bone mineral density. Indeed, Behre et al (279) reported that bone mineral density in some hypogonadal men continued to increase even after many years of testosterone treatment using a scrotal transdermal patch and reached the levels expected for age-matched eugonadal controls.

 

Cross-Sectional Studies of the Relationship Between Sex-Hormone Concentrations and Osteoporosis in Older Men 

 

The age-related decline in sex hormones is associated with age-related changes in bone mineral density and increased risk of osteoporotic fractures (131-136,284,285). Older men with hip fractures have lower testosterone levels than age-matched controls (286). Bioavailable testosterone levels have been found to be better predictors of fracture risk than total testosterone levels (287). Interestingly, a U-shaped association between endogenous testosterone concentrations and incident fractures was recently observed, with midrange plasma testosterone levels being associated with lower incidence of any fracture and with hip fracture compared to lower or higher testosterone (288). Men with osteoporosis have been found to have lower DHT levels than those without osteoporosis (289). In the Cardiovascular Health Study, in which testosterone and DHT levels were measured by liquid chromatography–tandem mass spectrometry, circulating DHT, but not testosterone, was found to be negatively associated with hip fracture risk in men (290).

 

In epidemiologic studies, estradiol levels are more strongly associated with bone mineral density of the spine, hip, and distal radius than total testosterone levels (132,134,135,285). Men with low bioavailable estrogen have increased risk of non-vertebral fracture which is increased further in those with low bioavailable estrogen, low bioavailable testosterone as well as high SHBG (287) suggesting a complex interplay of these hormones in fracture resilience.  Mendelian randomization analysis have found that increased genetically determined estradiol levels are associated with increase in lumbar spine bone mineral density (291) and lower fracture risk (292).  The CYP19A1 alleles associated with higher estradiol levels are associated with higher bone mineral density (291).

 

Clinical Trials of the Effects of Testosterone Therapy on Bone of Middle-Aged and Older Men with Low Circulating Testosterone Concentrations 

 

Earlier studies of testosterone replacement of relatively healthy older men that examined the effects of testosterone on bone mineral density reported inconsistent results (188,189,293,294). One study found greater increases in vertebral bone mineral density in the testosterone arm of the trial than in the placebo arm, while another study did not find any significant differences between the change in vertebral or femoral bone mineral density between testosterone and placebo groups (294). A meta-analysis of randomized trials found a significantly greater increase in lumbar bone mineral density but not in femoral bone mineral density in the testosterone arms of trials that used intramuscular testosterone than in placebo arms (Figure 10) (295); transdermal testosterone had no significant effect.

Figure 10. The effects of testosterone therapy on bone health in intervention trials. Panel A shows the effects of testosterone therapy on lumbar and femoral bone mineral density in a meta-analysis of randomized trials (data derived from a meta-analysis by Tracz et al, J Clin Endocrinol Metab. 2006;91(6):2011-6.; figure adapted with Spitzer et al. Nat Rev Endocrinol. 2013;9(7):414-24). Panels B and C show the effects of testosterone replacement for 12 Months on volumetric bone mineral density and estimated bone strength of trabecular, peripheral, and whole bone of the spine and hip, as assessed by quantitative computed tomography (figure reproduced with permission from Snyder et al. JAMA Intern Med. 2017;177(4):471-479).

The Bone Trial of the TTrials determined the effects of testosterone replacement for 1-year in men 65 years or older with low testosterone levels on volumetric bone mineral density and bone strength using quantitative computed tomography (296). This trial found significantly greater increases in volumetric bone mineral density and estimated bone strength in the testosterone arms compared to placebo; specifically, these increases were most prominent in the spine than hip and more in trabecular than peripheral bone (Figure 10). The treatment effects on volumetric bone density and bone strength observed in the TTrials compare favorably with those reported in trials of bisphosphonates and some selective estrogen receptor modulators.

 

The T4Bone substudy of the T4DM trial determined the effect of 24 months of testosterone treatment on bone microarchitecture and bone mineral density of men aged 50 years or older enrolled in a community-based lifestyle program using high resolution-peripheral quantitative computed tomography (297). Compared to placebo, testosterone treatment increased cortical and total bone mineral density of the tibia and radius, as well as cortical area and thickness at both sites. Testosterone treatment also increased areal thickness at the lumbar spine (297).

 

Future studies are needed to determine whether these improvements from testosterone treatment are associated with reduced fracture risk in older men with low testosterone levels.

 

Mechanisms of Androgen Action on the Bone 

 

Testosterone increases bone mass by several mechanisms (298). Short-term studies of androgen replacement have shown inconsistent increases in markers of bone formation, but a more consistent reduction in markers of bone resorption (283,298-300). These observations suggest that testosterone increases bone mineral density in part through its aromatization to estrogen, which inhibits bone resorption. Estrogen deficiency contributes to increased bone resorption and remodeling by multiple mechanisms. Estrogens regulate the activation frequency of bone functional basic multicellular units, the duration of the resorption phase and the formation phase, and osteoclast recruitment (301). The protective effects of estrogen on bone in both male and female mice during growth and maturation are mediated largely through estrogen receptor-alpha (302-308). 17β-estradiol has also been shown to increase connexin-43 based intracellular communication which may modulate the bone response to mechanical loading in osteocytes (309). Dias and colleagues found that treatment with testosterone for 12-months improved lumbar spine bone mineral density compared to placebo but not in men treated concomitantly with anastrozole, suggesting that aromatization of testosterone to estrogen may be required for maintaining bone mineral density (310). Similarly, another recent study found significant reduction in spine bone mineral density in men treated with testosterone and anastrozole for 16-weeks that was independent of testosterone dose (311). In addition, treatment with lower testosterone doses were associated with greater increases in bone turnover markers; an effect that was significantly greater in combination with anastrozole. DHT suppresses osteoclast formation in vitro via NF-kB ligand (RANKL) mediated effects, comparable to estradiol (312); the clinical significance of DHT in suppressing bone resorption is incompletely understood.

 

Testosterone also directly stimulates osteoblastic bone formation. Androgen receptors have been demonstrated on osteoblasts and on mesenchymal stem cells (313). Testosterone stimulates cortical bone formation (314). Sclerostin is secreted by osteocytes and inhibits osteoblast differentiation. Sclerostin was found to be negatively related to total and free testosterone in men with idiopathic osteoporosis (312). Hypogonadal men have higher serum sclerostin levels than eugonadal men, and DHT directly suppresses sclerostin production in cultured human osteocytes through an AR-mediated mechanism (315). Testosterone also stimulates the production of several growth factors within the bone, including IGF-1; these growth factors may contribute to bone formation (316). Leydig cells in the testis secrete insulin-like peptide 3 (INSL3) in addition to testosterone. INSL3 has been reported to have a negative association with sclerostin in specific populations and INSL3 downregulates sclerostin protein expression in cultured osteocytes (317). Osteocalcin secreted by osteoblasts acts on Leydig cells through the GPRC6A receptor, suggesting a possible feedback mechanism for bone-testis crosstalk (318). Testosterone increases muscle mass, which may indirectly increase bone mass by increased loading. Testosterone might inhibit apoptosis of osteoblasts through non-genotropic mechanisms (319,320). In addition to its effects on bone mineral density, testosterone might reduce fall propensity because of its effects on muscle strength and reaction time.

 

We have shown that testosterone has dose dependent effects on erythropoiesis (321) possibly through increased erythropoietin and reduced hepcidin (322). Hematopoietic cells and bone cells are interdependent and support each other at different stages in development   (323,324). Androgen deficiency seems to favor hematopoietic precursor differentiation to an osteoclast fate (312), which is consistent with the decreased bone resorption observed with testosterone supplementation (275,287-289). We have shown that older men in the MrOS study with accelerated bone density loss (>0.5%/year) have increased risk of anemia (325), and that anemia increases the risk of non-spine fractures independent of bone density (326). In a prospective analysis of the Cardiovascular Health Study, we have recently shown that men with anemia and, separately, men with decreasing hemoglobin were at increased risk of hip fracture (327). Low endogenous testosterone levels have been associated with lower hemoglobin (328), which is reversible with testosterone supplementation (329). It is possible that testosterone sufficiency is required for a healthy hematopoietic niche in men, which is then able to support a favorable microenvironment for bone health.

 

In men androgens and estrogens both play independent roles in regulating bone resorption (301). Estradiol levels above 10 pg/ml are generally believed to be sufficient to prevent increases in bone resorption and decreases in BMD in men (311).

 

Synopsis of The Effects of Testosterone on Bone 

 

Testosterone replacement has been shown to increase vertebral and femoral bone mineral density, and bone strength in older men with unequivocally low testosterone levels (16). Testosterone increases bone mass by multiple mechanisms. Testosterone’s aromatization to estrogen plays an important role in regulating bone health in men. Testosterone’s effects on fracture risk have not been studied. 

 

TESTOSTERONE EFFECTS ON COGNITIVE FUNCTION        

           

Cross-Sectional and Longitudinal Studies Correlating Sex-Hormone Levels and Cognitive Function

 

Several lines of evidence suggest that testosterone regulates several domains of cognition, sexually dimorphic behaviors, mood, and affect, and the neuropathology of Alzheimer’s Disease (AD). Testosterone is aromatized to estrogen in the brain, and some effects of testosterone on cognition might be mediated through its conversion to estradiol. Additionally, androgen receptors are expressed in the brain (330), and androgen effects on brain organization during development (331,332) are mediated through androgen receptor. Androgens increase neurite arborization, facilitating intercellular communication (331-334). Testosterone is metabolized in neurons as well as in glial cells to DHT, which is further converted reversibly in some cell types such as type 1 astrocytes to 5α-androstane-3α,17β-diol (335), which is a potent modulator of GABA on GABAA receptors but a weak ligand for AR and ER (336). The 3β isomer of androstanediol, 5α-androstan-3β,17β-diol, is also synthesized in the brain; this steroid is a ligand for ERβ (337).  Thus, testosterone treatment may potentially expose the brain to a range of biologically active metabolites, all of which may contribute to the observed responses. Testosterone also affects serotonin, dopamine, acetylcholine (333), and calcium signaling (334). Thus, testosterone could influence cognitive function and the development and progression of AD neuropathology through multiple mechanistic pathways.

 

The age-related decline in serum testosterone levels has been associated with impairment in cognitive function (338). Androgens effects on cognitive function are domain-specific. For instance, observations that men outperform women in a variety of visuo-spatial skills suggest that androgens enhance visuo-spatial skills (339). In !Kung San hunter-gatherers of Southern Africa, testosterone, but not estradiol, levels correlated with better spatial ability and with worse verbal fluency (340). Women with congenital adrenal hyperplasia with high androgen levels score higher on tests of spatial cognition than their age- and gender-matched siblings (341). 46, XY rats with androgen insensitivity perform worse on tests of spatial cognition than their age-matched controls (342). Other studies have reported a complex relationship between androgen levels and spatial ability (123,343-345). Circulating levels of dihydrotestosterone, a metabolite of testosterone that is not converted to estrogen, positively correlated with verbal fluency (340). Barrett-Conner et al (122) found positive associations between total and bioavailable testosterone levels, and global cognitive functioning and mental control, but not with visuospatial skills. In the Baltimore Longitudinal Study of Aging (346), higher free testosterone index was associated with better scores on visual and verbal memory, visuospatial functioning, and visuomotor scanning. Men with low testosterone levels had lower scores on visual memory and visuospatial performance (346); however, some studies have shown no association of serum testosterone levels with domains of visual and verbal memory, and executive function in older men (347,348). In the Concord Health and Aging in Men Project, the authors found that changes in serum testosterone levels over time, rather than baseline testosterone levels, were predictive of cognitive decline (338).

 

The Potential Role of Testosterone in the Pathobiology of Alzheimer's Disease

 

A large body of preclinical and epidemiologic data shows that testosterone acts as a negative regulator of endogenous Ab amyloid accumulation in the brain, attenuates tau phosphorylation, reduces neuro-inflammation, exerts neuronal protective effect in response to injury and disease, and promotes neuronal regeneration and connectivity. However, the randomized trials data generated largely in community dwelling middle-aged and older adults without cognitive deficits or Alzheimer's Disease neuropathology have been inconclusive for reasons that are discussed below.   

 

Testosterone acts as a negative regulator of endogenous Ab amyloid accumulation in the brain through multiple mechanisms. Surgical orchiectomy of male rats is associated with increased accumulation of Ab amyloid in the brain; the accumulation of Ab amyloid in surgically orchiectomized rats is prevented by DHT administration but not by estradiol administration (349-351).  In male Brown-Norway rats, age-related decreases in testosterone and DHT are associated with increased brain levels of Ab amyloid (350). Testosterone promotes the conversion of amyloid precursor protein (APP) to soluble APP-alpha rather than A beta amyloid. Consistent with these findings, prolonged treatment of cultured cortical neurons and neuroblastoma cell lines with testosterone resulted in increased production of soluble sAPP-a and decreased production of Ab amyloid (349). This effect of testosterone on the processing of APP is mediated in part through its aromatization to estradiol (352). However, there is strong evidence of mediation through a direct androgen receptor (AR)-mediated pathway as well (350-353). 

 

In 3XTg-AD mouse model of Alzheimer’s Disease, orchiectomy at age 3 months is associated with significantly increased accumulation of Ab amyloid in hippocampus CA1, amygdala, and subinculum at age 6 months (352,353).  DHT treatment of orchiectomized mice prevents the accumulation of Ab amyloid as well as deterioration of spontaneous alternation behavior (353).  DHT also reduces Tau-phosphorylation in orchiectomized triple transgenic mouse model of AD (352).  Androgens upregulate the expression of neprilysin, the enzyme that catalyzes the degradation and clearance of Ab amyloid in neuronal cells (354) and decrease Ab amyloid accumulation (355). 

 

Testosterone also attenuates AD-like tau pathology. In gonadectomized mice, testosterone as well as estradiol reduce tau phosphorylation (356,357).  Androgens also reduce tau phosphorylation induced by acute heat shock and injury in male rats independent of estradiol (358).

 

Testosterone exerts neuroprotective effects in many brain regions. Androgens promote neuronal viability during neural development as well as in adult brain following mechanical injury and disease-related toxicity  (359-361).  Testosterone protects motor neurons in the spinal cord following axotomy (359-361).  In this experimental model, testosterone treatment accelerates the rate of nerve regeneration and attenuates neuronal loss (359,360,362-364).

 

Testosterone exerts neuroprotective effects across the lifespan in brain areas susceptible to neurodegeneration in AD (365-368). Thus, in cultured neurons, testosterone reduces neuronal apoptosis induced by oxidative stress and Abamyloid (369-371).

 

Testosterone promotes neuronal growth, connectivity, and functioning. Testosterone increases neurite arborization, and synapse formation facilitating intercellular communication (372-375).  Testosterone also has nongenomic effects, and affects serotonin, dopamine, acetylcholine and calcium signaling (376-378).  Androgen receptors are expressed in the brain, and androgen effects on organization of the brain during development are likely mediated directly through AR. Some additional effects of testosterone are mediated through its conversion to estradiol.

 

Testosterone also exerts protective effects against neuroinflammation. Orchiectomy as well as obesogenic diet are each associated with increased expression levels of proinflammatory cytokines TNF-alpha and IL-1beta in the cerebral cortex in middle-aged male rats (370). The castration-induced upregulation of proinflammatory cytokines TNFa and IL-1b effect is prevented by testosterone supplementation (370). Similar stimulatory effects of testosterone on the expression of proinflammatory cytokines were observed in mixed glial cell cultures in vitro (370).  5aDihydrotestosterone inhibits interleukin-1a or tumor necrosis factor a-induced proinflammatory cytokine production via androgen receptor-dependent inhibition of nuclear factor kB activation (371). Low testosterone also is associated with increased macrophage infiltration in sciatic nerve in castrated male rats (371). The mechanisms of these protective effects of testosterone on neuroinflammation are incompletely understood but appear to require both androgen receptor and estrogen receptor-mediated pathways (371).

 

Epidemiological Data on the Association of Testosterone Levels with Cognitive Function and AD Pathology

 

Some but not all epidemiologic studies have found an association between low circulating testosterone levels and AD (346,379-384); the relation appears to be stronger between free testosterone levels and the risk of AD than between total testosterone and AD (346). The strength of the association between testosterone and AD is affected by apolipoprotein ε4 genotype, a genetic risk factor for AD (380); men with one or more ε4 alleles have lower testosterone levels and a higher risk of AD than men without an ε4 allele (380).

 

In longitudinal follow-up of male participants of the Baltimore Longitudinal Study on Aging (346), the men who were healthy at baseline and developed a clinical diagnosis of AD had significantly lower free testosterone levels than those who did not develop AD.  The age-related decline in circulating free testosterone levels preceded the clinical diagnosis of AD by nearly 10 years (346). 

 

Rosario et al. (384) found that low brain levels of testosterone were associated with increased risk of AD in men. In human postmortem brain tissue from neuropathologically normal men, tissue levels of testosterone but not E2 showed an age-related decline (384). The brain tissue levels of testosterone were significantly lower in AD cases as compared with neuropathologically normal cases after controlling for age (384).

Epidemiologic investigations of the association of circulating testosterone levels with age-related changes in cognitive function are in agreement that androgens effects on cognitive function are domain-specific. Generally, men with low testosterone levels perform less well than those with normal testosterone levels on tests of verbal fluency, visuospatial abilities, verbal memory, and executive function (121,122,385-388). Some inconsistency in findings across studies is likely related to heterogeneity of study populations, lack of standardization of cognitive assessments across studies, inaccuracy and imprecision of testosterone immunoassays, and the use of variable thresholds of testosterone levels to define "low". Some studies have suggested a curvilinear relation between testosterone levels and cognitive function; both low and high testosterone levels are associated with worse function suggesting that there may be an optimal level at which cognitive performance is optimized (386).

 

Clinical Trials Data

 

No adequately powered randomized placebo-controlled trials of testosterone replacement have been conducted in men with AD (389-392). The clinical trials data on the effects of testosterone on cognition have provided conflicting results; these trials were limited by their small size, inclusion of men who were not clearly hypogonadal and who did not have cognitive impairment or AD neuropathology, and use of outcomes that were not directly related to AD phenotype. Some studies have reported improvements in verbal memory and visuospatial skill while others found no effect (389,391-393).

 

The Testosterone Trials, a set of 7 coordinated trials of community-dwelling older men with unequivocally low testosterone levels, measured using liquid chromatography tandem-mass spectrometry (LC-MS/MS), showed no significant effect on delayed paragraph recall – the primary outcome of the trial (26,390). Post hoc analysis of the TTrials data showed small but significant improvement in executive function (390). The TTrials had many attributes of good trial design - prospective allocation of participants, parallel groups, blinding, and high retention rates, but progression of AD was not a primary aim of the trial (26). The trial’s duration of one year was not long enough to evaluate effects on clinically meaningful measures of cognitive function or AD pathology. The trial did not include any measures of AD pathology, including A beta amyloid or Tau-protein or blood or CSF markers of AD. The participants in this well conducted trial were not selected prospectively based on cognitive deficits or risk of AD. A few small testosterone supplementation studies (sample size varying from 11 to 47) of 6 weeks to 6-month duration in men with cognitive impairment of AD have reported modest improvements in verbal and spatial memory but the small sample sizes, short intervention durations, variable eligibility criteria, and inclusion of men without confirmed AD, and inclusion of men with normal testosterone levels limit the interpretability of these data (391,392).      

 

In a double-blind randomized placebo-controlled trial, Huang et al investigated the effect of testosterone administration for 3-years on multiple domains of cognitive function in a large cohort (n=280) of men 60 years and older with low or low-normal testosterone levels (394). In this trial of older, cognitively healthy men, testosterone administration was not associated with significant improvement in any domain of cognitive function (Figure 11). These findings are similar to another recent placebo-controlled clinical trial conducted in older men 65-years and older (n=493) with low testosterone levels, which showed that treatment with testosterone for 1-year was not associated with improved cognitive function or memory (Figure 11) (390). Sensitivity analysis that was limited to men with minimal cognitive impairment also did not find significant differences in measures of cognition between the testosterone and placebo groups (390).

Figure 11. Effects of testosterone therapy on cognition domains in older men. Left panels show the long-term effects of testosterone therapy in visual and verbal memory, spacial ability and executive function in the TEAAM trial (36 months of treatment). Data displayed as baseline and post-randomization cognitive function test scores by group and study visit. Error bars are 95% CIs for mean scores and p-values are for the estimated difference between treatment effects, controlling for baseline values, age, and education (figure adapted from Huang et al. J Clin Endocrinol Metab. 2018;103(4):1678-1685.) Right panels show adjusted mean change from baseline to 6 months and 12 months for men with age-associated memory impairment by treatment group in cognition domains in the Cognitive Function Trial of the TTrials (12 months duration; figure adapted from Resnick et al. JAMA. 2017;317(7):717-727).

Synopsis of the Effects of Testosterone on Cognition

 

In spite of the robust preclinical data that testosterone acts as a negative regulator of endogenous Ab amyloid accumulation in the brain, attenuates tau phosphorylation, reduces neuro-inflammation, exerts neuronal protective effect in response to injury and disease, and promotes neuronal regeneration and connectivity and some epidemiologic evidence that decline in testosterone levels increases the risk of incident clinical AD, the randomized trial data on the effects of testosterone on cognition is highly equivocal. The randomized clinical trials in community dwelling older adults without cognitive deficit or AD neuropathology have not found clinically meaningful improvements in cognitive function. The inconsistency in findings cannot yet be interpreted as conclusive evidence that there is no effect. Limitations of previous studies include limited sample sizes, inclusion of men with no clear cognitive deficit or AD neuropathology, the use of a variety of neuropsychological tests that are not clinically meaningful in the context of AD or dementia; the use of differing protocols in clinical trials. The effects of testosterone therapy on clinically important outcomes in men with cognitive impairment have not been studied. The efficacy of testosterone replacement in men with cognitive impairment, such as in patients with Alzheimer’s disease, needs further investigation in larger randomized controlled trials.

 

TESTOSTERONE EFFECTS ON MOOD, ENERGY, AND HEALTH-RELATED QUALITY OF LIFE      

 

Circulating testosterone concentrations have not been consistently associated with major depressive disorder in men (128,129,395-398). Rather, testosterone levels appear to be associated with a late-life low grade persistent depressive disorder (dysthymia) (128,129,395-398). Intervention trials have failed to demonstrate statistically significant or clinically meaningful improvements in patients with major depressive disorder (399). Placebo-controlled trials of testosterone in men with refractory depression also have not consistently shown a beneficial effect of testosterone (399-402). A meta-analysis of randomized trials reported modest improvements in depressive symptoms in testosterone-treated men compared to placebo-treated  men (403),  but there is no convincing evidence that testosterone treatment can induce remission in men with major depressive  disorder (404). Two small trials in men with dysthymia have reported greater improvements in depressive symptoms in testosterone-treated men than in placebo-treated men (405,406). Adequately powered long-term randomized trials are needed to determine whether testosterone replacement therapy can induce remission in older hypogonadal men with late-onset, low grade persistent depressive disorder (dysthymia).  

 

There is anecdotal evidence that androgens improve energy and reduced sense of fatigue (407). Testosterone administration increases hemoglobin and red cell mass, stimulates 2, 3 DPG concentrations thereby shifting the oxygen – hemoglobin dissociation curve favorably to improve greater oxygen delivery, and induces muscle capillarity (322,408,409). Additionally, testosterone stimulates mitochondrial biogenesis and mitochondrial quality (410). All of these adaptations would be expected to improve net oxygen delivery to the muscle, improve aerobic performance, and reduce fatigability. The effects of testosterone on fatigue and vitality have been studied in some randomized trials. Endogenous levels of total and free testosterone are not significantly associated with vitality in older hypogonadal men with sexual dysfunction, diminished vitality, and/or mobility limitation (114). In the Vitality Trial of the TTrials, testosterone treatment for 1-year did not improve vitality in older men with low vitality measured using the Functional Assessment of Chronic Illness Therapy (FACIT)-scale but men receiving testosterone did report a small but statistically significant improvement in mood.  These findings are consistent with other randomized controlled studies (23,171,190), showing no clear benefit on fatigue and health-related quality of life with testosterone therapy.

 

Supraphysiologic doses of androgenic steroids such as those abused by athletes and recreational bodybuilders have been associated with aggressive responses to provocative situations (411), increased scores on Young’s manic scale, and with affective and psychotic disorders in some individuals (412); these adverse effects have not been reported with physiologic testosterone replacement.

By improving some aspects of physical and sexual function, testosterone supplementation might be expected to improve health-related quality of life. However, only a few small trials have evaluated the effects of testosterone on health-related quality of life. A systematic review of a small number of randomized trials has not revealed a significant improvement in composite health-related quality of life scores, but testosterone therapy improves scores on the physical component of MOS SF-36 (16,159). 

 

Risks of Testosterone Administration in Older Men

 

Short-term testosterone administration in healthy, young, androgen-deficient men with classical hypogonadism is associated with a low frequency of relatively mild adverse effects such as acne, oiliness of skin, and breast tenderness. However, the long-term risks of testosterone supplementation in older men are largely unknown. There are several unique considerations in older men that may increase their risks of testosterone administration. Serum total and free testosterone concentrations are higher in older men than young men at any dose of testosterone therapy, due to decreased testosterone clearance in older men (61). Older men exhibit greater increments in hemoglobin and hematocrit in response to testosterone administration than young men (321), adjusting for testosterone dose. Altered responsiveness of older men to testosterone administration might make them susceptible to a higher frequency of adverse events, such as erythrocytosis, or to unique adverse events not observed in young hypogonadal men. The baseline prevalence of disorders such as prostate cancer, benign prostatic hypertrophy, and cardiovascular disease that might be exacerbated by testosterone administration is high in older men; therefore, small changes in risk in either direction could have enormous public health impact. Furthermore, the clustering of co-morbid conditions in the frail elderly might render these men more susceptible to the adverse effects of testosterone therapy than healthy young hypogonadal men.

 

The contraindications for testosterone administration include history of prostate or breast cancer (16). Benign prostatic hypertrophy by itself is not a contraindication, unless it is associated with severe symptoms, as indicated by IPSS symptom score of greater than 21. Testosterone should not be given without prior evaluation and treatment to men with baseline hematocrit greater than 50%, severe untreated sleep apnea, or congestive heart failure with Class III or IV symptoms (16). Testosterone suppresses spermatogenesis and should not be prescribed to men who are considering having a child in the near future.  

 

The risks of testosterone administration include acne, oiliness of skin, erythrocytosis, induction or exacerbation of sleep apnea, leg edema, transient breast tenderness or enlargement, and reversible suppression of baseline spermatogenesis (16) (Table 2). Abnormalities of liver enzymes, hepatic neoplasms, and peliosis hepatis that have been reported previously with orally administered, 17-alpha alkylated androgens, have not been observed with replacement doses of transdermal or injectable testosterone formulations. The two major areas of concern and uncertainty are the effects of long-term testosterone administration on prostate cancer and major adverse cardiovascular events.

 

Table 2. Potential Adverse Effects of Testosterone Replacement in Older Men

Adverse Events for Which There is Evidence of Association with Testosterone Administration

1.         Erythrocytosis

2.         Acne and oily skin

3.         Detection of subclinical prostate cancer

4.         Growth of metastatic prostate cancer

5.         Reduced sperm production and fertility

Potential Adverse Events for Which There is Weak Evidence of Association with Testosterone Administration

1.         Gynecomastia

2.         Male pattern balding (familial)

3.         Growth of breast cancer

4.         Induction of worsening of obstructive sleep apnea

Formulation Specific Adverse Effects

1.     1.         Oral Tablets (not recommended)

·                          - Effects on liver enzymes and HDL cholesterol (methyltestosterone)

1.     2.          Pellet Implants

¨                        -. Infection, extrusion of pellet

2.     3.          Intramuscular Injections

¨                         - Fluctuations in mood or libido

¨                         - Pain at injection site

¨                         - Coughing episodes immediately after injection

3.     4.          Transdermal Patches

¨                         - Skin reaction at the patch application site

4.     5.          Transdermal Gel

¨                         - Potential risk of transference to partner

¨                         - Skin irritation and odor at application site

¨                         - Stickiness, slow drying, dripping

5.     6.           Buccal Testosterone Tablets

¨                         - Alterations in taste

¨                         - Irritation of gums

Adapted with permission from the Endocrine Society Guideline for Testosterone Therapy in Men with Hypogonadism in: Bhasin et al J Clin Endocrinol Metab 2018;103(5):1715-1744.

 

TESTOSTERONE EFFECTS ON THE RISK OF ATHEROSCLEROTIC HEART DISEASE       

 

The long-term consequences of testosterone supplementation on the risk of heart disease remain unknown and have been the subject of debate (145,413-417). Some known effects of testosterone such as increase in hematocrit, suppression of plasma HDL cholesterol, and salt and water retention, might be expected to increase cardiovascular risk. Some other effects such as testosterone’s vasodilator effect on coronary arteries resulting in increased coronary blood flow, reduction of whole body and abdominal fat mass, and improved brachial reactivity might be perceived as beneficial. Testosterone’s effects on coagulation are complex; testosterone administration is associated with stimulation of both anti-coagulant and pro-coagulant proteins.

 

Androgen Effects on Plasma Lipids 

 

Cross-sectional studies of middle-aged men found a positive relationship between serum testosterone levels and plasma HDL-cholesterol concentrations (415,418-421). Lower testosterone levels in men are associated with higher levels of dense LDL particles (418), triglycerides (421,422) and prothrombotic factors (423).

 

The effects of androgen supplementation on plasma lipids depend on the dose, the route of administration (oral or parenteral), the type of androgen (aromatizable or not), and the subject population (whether young or old, and hypogonadal or not). Supraphysiological doses of testosterone and non-aromatizable androgens frequently employed by bodybuilders undoubtedly decrease plasma HDL-cholesterol levels (424-427). However, administration of replacement doses of testosterone in older men has been associated with only a modest decrease or no change in plasma HDL-cholesterol (16,22,23,25,184,186-189,428-430), and without a significant effect on cholesterol efflux capacity from macrophages (431), suggesting preserved HDL function.

 

Androgens and Other Cardiovascular Risk Factors 

 

Cross-sectional studies have found a positive association between circulating testosterone concentrations and tissue plasminogen activator activity (432), and a negative relationship between testosterone and plasminogen activator inhibitor-1 activity, fibrinogen, and some other prothrombotic factors (432), suggesting an antithrombotic effect of testosterone. However, testosterone increases hematocrit (433), as well as neutrophil, monocyte and platelet counts (434). In men, higher neutrophil counts – even within the normal range - are associated with cardiovascular disease (435). Similarly, higher monocyte counts within the normal range have been suggested as a risk-factor for coronary artery plaque formation and cardiovascular mortality (436). Additionally, testosterone administration increases thromboxane A2 receptor density on human platelets, increasing platelet aggregability ex vivo (437,438). Observational studies have not found a consistent relationship between testosterone treatment and the risk of venous thromboembolism (439-443), although one study reported a small increase in VTE risk in the first few months after starting testosterone treatment (439).

 

Cross-sectional studies have reported conflicting findings on the association of endogenous testosterone levels and inflammatory markers (444-449). Intervention trials of testosterone generally have not found a significant effect of testosterone on inflammatory markers (430,450). Even supraphysiological doses of testosterone have been found not to affect C-reactive protein (451). Similarly, a prospective cohort study did not find meaningful changes in inflammatory markers in men with prostate cancer receiving androgen deprivation therapy (452).

 

Androgens and Coronary Artery Disease 

 

Whether variation of testosterone within the normal range is associated with risk of coronary artery disease remains controversial. Of the 30 cross-sectional studies reviewed by Alexandersen (145), 18 reported lower testosterone levels in men with coronary heart disease, 11 found similar testosterone levels in controls and men with coronary artery disease and 1 found higher levels of DHEAS. Prospective studies have failed to reveal an association of total testosterone levels and coronary artery disease (146-150,453-455). The common carotid artery intimal media thickness, a marker of generalized atherosclerosis, is negatively associated with circulating testosterone levels (150).

 

One interventional study (456), reported that testosterone undecanoate given orally improved angina pectoris in men with coronary heart disease. Testosterone infusion acutely improves coronary blood flow in a canine model and in men with coronary artery disease (457-463). Short-term administration of testosterone induces a beneficial effect on exercise-induced myocardial ischemia in men with coronary artery disease (462). This effect may be related to a direct vasodilator effect of testosterone on the coronary arteries resulting in increased coronary blood flow. Testosterone replacement has been shown to increase the time to 1-mm ST-segment depression (460). However, in another study, there were no differences between the placebo or testosterone groups in peak heart rate, systolic blood pressure, maximal rate pressure product, perfusion imaging scores, or the onset of ST-segment depression (462). Yue et al (463) reported that testosterone induces endothelium-independent relaxation of rabbit coronary arteries via potassium conductance. Testosterone is a potent vasodilator; it induces nitric oxide synthesis in human aortic endothelial cells in vitro (464). Testosterone has been shown to be an inhibitor of L-type Ca2+ channel. In human cells transfected with α1C subunit of the human cardiovascular L-type Ca2+ channel, testosterone inhibits these calcium channels with a potency that is similar to that of dihydropyridine calcium channel blockers (465).

 

Effects of Testosterone Supplementation on Atherosclerosis Progression

 

In some animal models, orchiectomy accelerates and testosterone administration retards atherogenesis progression (466). The protective effect of testosterone on aortic atherogenesis in this preclinical model is mediated through its conversion to estradiol by the CYP19A1 in the blood vessel wall (466).

 

Two large placebo-controlled trials have evaluated the effects of testosterone treatment on atherogenesis progression in middle-aged and older men. The Testosterone’s Effects on Atherosclerosis Progression in Aging Men (TEAAM) Trial determined the effects of testosterone therapy on progression of subclinical atherosclerosis in the common carotid artery using sonographic measurement of common carotid artery intima-media thickness (CCA-IMT) and the coronary artery calcium scores measured using MDCT. The participants in the TEAAM Trial were 308 men, 60 years and older, with total testosterone between 100 and 400 ng/dL or free testosterone below 50 pg/mL (23). Men were randomized to receive either 75 mg of transdermal testosterone gel or placebo gel daily and received for 3 years. Neither the progression of CCA-IMT nor coronary artery calcium scores differed between the men randomized to the testosterone and placebo groups (Figure 12) (23).

Figure 12. Effects of testosterone administration on atherosclerosis progression. Panels A and B show data from the TEAAM trial (Basaria et al. JAMA. 2015;314(6):570-81; figure reproduced with permission from JAMA) Panel C shows data from the Cardiovascular Trial of the TTrials (data from Budoff et al. JAMA. 2017;317(7):708-716; figure adapted from Gagliano-Jucá & Basaria, Asian J Androl. 2018;20(2):131-137).

In the cardiovascular trial of the TTrials, 138 men with serum total testosterone below 275 ng/dL received either testosterone gel or placebo gel for one year and were evaluated by coronary computed tomographic angiography for progression of non-calcified and calcified coronary artery plaque volume, as well as coronary artery calcium score (467). Consistent with the findings of the TEAAM Trial, the changes in coronary artery calcium scores did not differ between the testosterone and placebo groups over one year of intervention. However, the increase in non-calcified plaque volume (primary endpoint) was significantly greater in men assigned to the testosterone arm than in those assigned to placebo arm (Figure 12) (467); there were baseline differences in non-calcified plaque volume between the two groups. The clinical implications of these findings to cardiovascular risk remain to be established.

 

Testosterone and Cardiac Arrhythmias

 

Testosterone has important effects in cardiac electrophysiology (468); it increases potassium currents derived from the human ether-a-go-go related gene (hERG) (469), and inhibits the depolarizing delayed calcium current (ICaL) (470), with its effects on ICaL being more meaningful than on hERG (471). These effects lead to shortening of ventricular cardiomyocyte repolarization time, which can be seen in the electrocardiogram as shortening of the heart-rate corrected QT interval (QTc). Indeed, cross-sectional studies have observed a negative association between serum testosterone levels and QTc duration (472). Additionally, in randomized trials of testosterone replacement to men with low testosterone levels, testosterone treatment shortened QTc duration in community-dwelling older men (473) and in men with chronic heart failure (474). Similarly, in a prospective cohort study, androgen deprivation therapy in men with prostate cancer was associated with QTc prolongation compared with men with prostate cancer not receiving the therapy (475). As QTc prolongation is associated with an increased risk of ventricular tachyarrhythmias (torsades de pointes) and sudden cardiac death (476-478), it is not surprising that androgen deprivation therapy is associated with a higher risk of arrhythmia, cardiac conduction disturbances and sudden death (479,480). A small case series study and analysis of the European pharmacovigilance database concluded that “conditions or drugs leading to male hypogonadism were associated with torsades de pointes”, and “correction of hypogonadism with testosterone replacement therapy can treat or prevent torsades de pointes” (481).

 

Cross-sectional studies have also linked low androgen levels in men to an increased risk of atrial fibrillation (482-484), and normalization of testosterone levels with testosterone replacement is associated with a decreased incidence of atrial fibrillation compared with untreated hypogonadal men (485). These findings need corroboration in randomized trials.

 

The Effects of Testosterone on Major Adverse Cardiovascular Events (MACE)

 

To-date, no randomized trials have been large enough or of sufficiently long duration to determine the effects of testosterone treatment on MACE (416). The frequency of MACE reported in randomized testosterone trials has been low—even lower than that expected for the age and comorbid conditions of the participants (18,486,487). A randomized trial of testosterone in older men (The TOM Trial) with mobility limitation was stopped early due to a higher frequency of cardiovascular-related events in men assigned to testosterone than in those assigned to placebo (22), heightening concern about the cardiovascular safety of testosterone in frail older men. In contrast to many other testosterone trials in older men, which recruited relatively healthy older men, the participants in the TOM trial had a high prevalence of chronic conditions, such as heart disease, diabetes mellitus, obesity, hypertension, and hyperlipidaemia (22). Men, 75 years of age or older, and men with high on-treatment testosterone levels seemed to be at the greatest risk of cardiovascular-related events. In secondary analyses, these events were found to be associated with changes in serum free testosterone and estradiol levels (488). The dose of testosterone used in the TOM trial was higher than that used in some previous trials, but not dissimilar from or lower than that used in some other trials. The cardiovascular events were small in number and of variable clinical significance. The TOM trial was not designed for cardiovascular events; therefore, the cardiovascular events were not a pre-specified endpoint, and were not collected in a standardized manner, nor adjudicated prospectively. Additionally, many of the cardiovascular events were not MACE.

 

The higher cardiovascular adverse event incidence in testosterone-treated older men observed in the TOM trial was not reproduced in two larger trials of longer duration published more recently; in the TEAAM trial, the incidence of major adverse cardiac events throughout the 3 years of intervention was similar between groups (23). Similarly, in the TTrials, the number of MACE (myocardial infarction, stroke or death related to cardiovascular disease) during the one year of treatment was similar in the two groups, with seven men in each group experiencing an event (26). The number of MACE in the TEAAM and Ttrials were too few to permit strong inferences on the effects of testosterone treatment on MACE.

 

The Hormonal Regulators of Muscle and Metabolism in Aging (HORMA) trial reported a significantly greater increase in blood pressure in men treated with testosterone than in those treated with placebo (489). Testosterone administration causes salt and water retention (490), which can induce edema and worsen pre-existing heart failure.

 

Several meta-analyses of randomized testosterone trials have been published (413,486,487,491,492); however, these meta-analyses are limited by the small size of most trials, heterogeneity of study populations, poor quality of adverse-event reporting, and short treatment duration in many trials. None of the testosterone trials to date was sufficiently powered to adequately assess safety outcomes. The rigor of adverse-event reporting varied greatly among studies. The MACE was not ascertained rigorously nor adjudicated in most trials except in the TTrials.

 

The meta-analyses of randomized testosterone trials (487,491-494) and retrospective analyses of electronic medical records data (495-498) have also yielded inconsistent findings. These meta-analyses and pharmacovigilance studies have suffered from many limitations that are inherent in retrospective analysis of electronic medical records data. These studies included heterogeneous populations, and differed in the duration of intervention and study design. They used variable definitions and ascertainment of cardiovascular outcomes. The cardiovascular events were not prespecified, not collected prospectively and were not adjudicated. Treatment indications, treatment regimens, on-treatment testosterone levels and exposure differed among studies. These studies also suffered from a potential for residual confounding in that the patients assigned to testosterone therapy differed from comparators in baseline cardiovascular risk factors. Because of these inherent limitations and inconsistency of findings, these epidemiologic studies do not permit strong inferences about the relation between testosterone therapy and mortality and cardiovascular outcomes.

 

Synopsis of the Effects of Testosterone on Cardiovascular Risk 

 

The long-term effects of testosterone replacement therapy on MACE remain unknown. The FDA conducted an extensive review and concluded “the studies...have significant limitations that weaken their evidentiary value for confirming a causal relationship between testosterone and adverse cardiovascular outcomes”. Nevertheless, the FDA directed the pharmaceutical companies to add in the drug label information about a possible increased risk of cardiovascular events with testosterone use. An independent review conducted by the European Medicines Agency also found no consistent evidence of an increased risk of coronary heart disease associated with testosterone treatment of hypogonadal men. Long-term randomized trials of the effects of testosterone replacement on MACE are needed and are particularly important because even small changes in incidence rates could have significant public health impact.

 

A large randomized, placebo-controlled trial to study the effects of testosterone replacement therapy on the incidence of major adverse cardiovascular events in men 45 to 80 years of age with low testosterone levels and one or more symptoms of testosterone deficiency, who are at increased risk for cardiovascular events is currently underway (The TRAVERSE Trial, NCT03518034). The intervention duration is up to 5 years in this trial of over 6,000 men. The efficacy outcomes include adjudicated clinical fractures, remission of low-grade persistent depressive disorder (dysthymia), progression from pre-diabetes to diabetes, correction of anemia, and overall sexual activity, sexual desire, and erectile function. This randomized, placebo-controlled trial offers an historical opportunity to advance our understanding of the cardiovascular safety and long-term efficacy of testosterone replacement in middle-aged and older hypogonadal men.

 

TESTOSTERONE, DIA

Figure 13. Circulating Concentrations of SHBG, but not total or free testosterone, were associated prospectively with risk of incident diabetes in the Massachusetts Male Aging Study (MMAS). In a prospective analysis of data from the Massachusetts male Aging Study, total testosterone (left panel) and free testosterone (middle panel) were not associated significantly with risk of incident diabetes. Only SHBG concentrations were associated with incident diabetes in longitudinal analysis. Reproduced with permission from Lakshman et al J Gerontol A Biol Sci Med Sci. 2010;65(5):503-9.

 

BETES, AND METABOLIC SYNDROME      

 

Spontaneous (156) and experimentally induced (222) androgen deficiency is associated with increased fat mass, and testosterone replacement decreased fat mass in older men with low testosterone levels (16). In epidemiologic studies, low testosterone levels are associated with higher levels of abdominal adiposity (499,500). Testosterone administration promotes the mobilization of triglycerides from the abdominal adipose tissue in middle-aged men (501). Surgical castration in rats impairs insulin sensitivity; physiologic testosterone replacement reverses this metabolic derangement (502). However, high doses of testosterone impair insulin sensitivity in castrated rats (502), suggesting a biphasic relationship in which both low and high testosterone levels impair insulin resistance. Androgens increase insulin-independent glucose uptake (503) and modulate LPL activity in a region-specific manner (504).

 

Testosterone levels are lower in men with type 2 diabetes mellitus compared with controls (505-510). Low total testosterone levels have been associated with lower insulin sensitivity (505,511) and increased risk of type 2 diabetes mellitus and metabolic syndrome in community dwelling men both cross-sectionally and longitudinally (508-510,512-520). However, the association of free testosterone and type 2 diabetes mellitus has been inconsistent; some studies have reported a weak relationship (509,510,512) while others have failed to find any relationship (508,514). Circulating sex hormone binding globulin (SHBG) and some SHBG polymorphisms also have been associated negatively with the risk of type 2 diabetes (508-510,512-516,521-524). For instance, individuals with the rs6257and rs179994 variant alleles of the SHBG single nucleotide polymorphism (SNP) have lower plasma SHBG levels and a higher risk of type 2 diabetes (521-524). Similarly, individuals with the rs6259 variant have higher SHBG levels and lower type 2 diabetes risk (524). We performed longitudinal analyses of men participating in the Massachusetts Male Aging Study (525), a population-based study of men aged 40-70 years (Figure 13) to evaluate whether SHBG is an independent predictor of T2DM (526). After adjustment for age, body mass index, hypertension, smoking, alcohol intake and physical activity, the hazard ratio  for incident type 2 diabetes was 2.0 for each one SD decrease in SHBG and 1.29 for each one SD decrease in total testosterone (525). Free testosterone was not significantly associated with type 2 diabetes. The strong association of T2DM risk with SHBG persisted even after additional adjustment for free testosterone. The association of SHBG polymorphisms with type 2 diabetes suggests a potential role of SHBG in the pathogenesis of type 2 diabetes. In a Mendelian randomization analysis of the UK Biobank data, genetically determined free testosterone levels were associated with the risk of type 2 diabetes mellitus in a sexually dimorphic manner after adjusting for SHBG levels; men with low genetically determined free testosterone levels had increased risk of type 2 diabetes while women with low genetically determined free testosterone levels had reduced risk of type 2 diabetes (527).

Figure 13. Circulating Concentrations of SHBG, but not total or free testosterone, were associated prospectively with risk of incident diabetes in the Massachusetts Male Aging Study (MMAS). In a prospective analysis of data from the Massachusetts male Aging Study, total testosterone (left panel) and free testosterone (middle panel) were not associated significantly with risk of incident diabetes. Only SHBG concentrations were associated with incident diabetes in longitudinal analysis. Reproduced with permission from Lakshman et al J Gerontol A Biol Sci Med Sci. 2010;65(5):503-9.

HbA1c levels did not differ between groups. Homeostasis model assessment of insulin resistance (HOMA-IR), a marker of insulin resistance, improved modestly in men who were assigned to testosterone compared with placebo (532). Dhindsa et al reported improvement of insulin sensitivity with testosterone replacement for 24 weeks in hypogonadotropic hypogonadal men with type 2 diabetes (511). Overall, studies have failed to show improvements in diabetes outcomes or consistent changes in measures of insulin sensitivity (18,430,451,532-536) even though interventional trials have found a consistent reduction in whole body fat as well as abdominal fat (18,123,343). Indeed, in the TEAAM trial, 3 years of testosterone supplementation decreased fat mass in community-dwelling older men with low or low-normal serum testosterone concentrations, but did not improve insulin sensitivity (Figure 14) (537). The T4DM trial, one of the largest testosterone trials conducted to-date, evaluated the effects of 2 years of testosterone treatment in men aged 50 to 74 years with impaired glucose tolerance or newly diagnosed type 2 diabetes and a serum testosterone concentration below 404 ng/dL enrolled in a lifestyle program. Compared to placebo, testosterone treatment in conjunction with a life style program was associated with a lower proportion of participants with type 2 diabetes (538). It is important to note, however, that the men enrolled in the T4DM trial were not hypogonadal.

Figure 14. Long-term effects of testosterone therapy on insulin sensitivity in older men. Change in insulin sensitivity over time measured by the octreotide insulin suppression test and estimated as the mean concentration of glucose at equilibrium (SSPG). Figure adapted from Huang et al. J Clin Endocrinol Metab. 2018;103(4):1678-1685.

TESTOSTERONE AND PROSTATE RISK      

 

There is no evidence that testosterone causes prostate cancer (539). A retrospective analysis of the Registry of Hypogonadism in Men (RHYME) (540) and several meta-analyses of randomized controlled trials (486,541,542) did not find an increased risk of prostate cancer in men receiving testosterone. Also, there is no consistent relationship between endogenous serum testosterone levels and the risk of prostate cancer (16,18,123,343,542-545). A meta-analyses of prospective cohort studies did not find a significant association between endogenous total testosterone levels and prostate cancer (542). Conversely, an analysis of 20 prospective studies found that men in the lowest tenth of free testosterone concentration had a lower risk of prostate cancer (OR=0.77, 95%CI= 0.69 to 0.86; p<0.001) compared with men with higher concentrations (545). Similarly, in the male participants in the UK Biobank followed for a mean of 6.9 years, higher genetically determined free testosterone was associated with a higher risk of prostate cancer, while total testosterone was not associated with prostate cancer risk (Figure 15) (546). The men with Klinefelter Syndrome have lower risk of prostate cancer than the general population. Taken together, these data suggest that life-long exposure to testosterone treatment in hypogonadal men could potentially increase the risk of prostate cancer.

Figure15. Mendelian Randomization: Genetically Determined Bio-Testosterone Associated with Increased Prostate Cancer Risk. Legend: UK Biobank Study: Genetic determinants of bioavailable testosterone were positively associated with risk of prostate cancer in men and ER+ breast cancer and endometrial cancer in women. Reproduced with permission from: Ruth KS, et al. Nat Med. 2020;26(2):252‐258.

Prostate cancer is an androgen–dependent tumor, and testosterone treatment is known to promote the growth of metastatic prostate cancer (544,547). Testosterone administration has been historically contraindicated in men with history of prostate cancer (16,543). The prevalence of subclinical, microscopic foci of prostate cancer in older men is high (548-555). There is concern that testosterone administration might make these subclinical foci of cancer grow and become clinically overt. In addition, older men with low testosterone levels may have prostate cancer (556,557). Morgentaler et al (556,557) reported a high prevalence of biopsy-detectable prostate cancer in men with low total or free testosterone levels despite normal PSA levels and normal digital rectal examinations. However, this study did not have a control group, and we do not know whether sextant biopsies of age-matched controls with normal testosterone levels would yield a similarly high incidence of biopsy-detectable cancer. Therefore, this study should not be interpreted to conclude that there is a higher prevalence of prostate cancer in older men with low testosterone levels, or that low testosterone levels are an indication for performing prostate biopsy.

 

Effects of Testosterone Therapy on Prostate Events

 

None of the testosterone trials in middle-aged or older men had sufficient power or intervention duration to detect meaningful differences in the incidence of prostate cancer between testosterone and placebo-treated men. Testosterone treatment of hypogonadal men increases PSA levels (16,558), which may lead to urological referral for prostate biopsy. A systematic review of randomized testosterone trials in middle-aged and older men found (413) that men treated with testosterone in clinical trials were at significantly higher risk for undergoing prostate biopsy than placebo-treated men (413). Because of the high prevalence of subclinical prostate cancer in older men, the higher number of prostate biopsies in testosterone-treated men could lead to increased detection rates of subclinical prostate cancer in comparison with placebo-treated men. Thus, testosterone therapy of middle-aged and older men is associated with a higher risk of prostate biopsy and a bias towards detection of a higher number of prostate events (18,413).

 

Administration of exogenous testosterone or suppression of circulating levels of testosterone by administration of a GnRH antagonist is not associated with proportionate changes in intra-prostatic testosterone or DHT concentrations. For instance, in a randomized controlled trial, Marks et al (559) measured intraprostatic testosterone and DHT levels in older men treated with placebo or testosterone. Surprisingly, intraprostatic DHT concentrations were not significantly higher in testosterone-treated men than in placebo-treated men (559). Similarly, the expression levels of androgen-dependent genes in the prostate were not significantly altered by testosterone administration (559). In separate studies, lowering of circulating testosterone levels by administration of a GnRH antagonist was not associated with changes in intraprostatic androgen concentrations (560,561).  

 

Effects of Testosterone Replacement on Serum PSA Levels

 

Serum PSA levels are lower in androgen–deficient men and are restored to normal following testosterone replacement (16,558,562-570). Lowering of serum testosterone concentrations by withdrawal of androgen therapy in young, hypogonadal men is associated with a decrease in serum PSA levels. Similarly, treatment of men with benign prostatic hyperplasia with a 5-alpha reductase inhibitor, finasteride, is associated with a significant lowering of serum and prostatic PSA levels (570,571). However, serum PSA levels do not increase progressively in healthy hypogonadal men with replacement doses of testosterone. The increase in PSA levels during testosterone replacement might trigger evaluation and biopsy in some patients (16,543).

 

More intensive PSA screening and follow-up of men receiving testosterone replacement might lead to an increased number of prostate biopsies and the detection of subclinical prostate cancers that would have otherwise remained undetected (16,543). Serum PSA levels tend to fluctuate when measured repeatedly in the same individual over time (572-574). There is considerable test-retest variability in PSA measurements (572-574). Some of this variability is due to the inherent assay variability, and a significant portion of this variability is due to unknown factors. Fluctuations are larger in men with high mean PSA levels. Variability can be even greater if measurements are performed in different laboratories that use dissimilar assay methodology (572-574).

 

An important issue is what increment in PSA level should warrant a prostate biopsy in older men receiving testosterone replacement. To address this issue, we conducted a systematic review of published studies of testosterone replacement in hypogonadal men (543). This review indicated that the weighted effect size of the change in PSA after testosterone replacement in young, hypogonadal men is 0.68 standard deviation units (95% confidence interval 0.55 to 0.82). This means that the effect of testosterone replacement therapy is to increase PSA levels by an average 0.68 standard deviations over baseline. Because the average standard deviation was 0.47 in this systematic analysis, the standard deviation score of 0.68 translates into an average increase in serum PSA levels of about 0.30 ng/ml in young hypogonadal men (543). The average change in serum PSA levels after testosterone replacement in studies of older men was 0.43 ng/mL (543). The data from the Proscar Long-Term Efficacy and Safety Study (PLESS) demonstrated that the 90% confidence interval for the change in PSA values measured 3 to 6 months apart is 1.4 ng/mL (570). Therefore, a change in PSA of >1.4 ng/ml between any two values measured 3 to 6 month apart in the same patient is unusual (16,543).  In the TTrials, 2.4% of men receiving testosterone had increases above 1.4 ng/mL at 3 months, and 4.7% at 12 months (26).

 

Carter et al, based on the analysis of PSA data from the Baltimore Longitudinal Study of Aging, reported that PSA velocity, defined as the annual rate of change of PSA, is different in men who develop prostate cancer than in those who do not (575-577). Thus, PSA velocity greater than 0.7 ng/ml/year was unusual in men without prostate cancer whose baseline PSA was between 4 and 10 ng/ml (575-577). However, most men being considered for testosterone replacement will have baseline PSA less than 4 ng/ml. In a subsequent analysis, the same group reported that the PSA velocity in men with baseline PSA between 2 and 4 ng/ml was 0.2 ng/ml/year (577). Because test-to-retest variability in PSA measurement is far greater than this threshold, it is likely that the use of this threshold of 0.2 ng/ml/year to select men for prostate biopsy would lead to many unnecessary biopsies.

 

In eugonadal, young men, administration of supraphysiological doses of testosterone does not further increase serum PSA levels (166,169,578). These data are consistent with dose response studies in young men that demonstrate that maximal serum concentrations of PSA are achieved at testosterone levels that are at the lower end of the normal male range; higher testosterone concentrations are not associated with higher PSA levels (166,169).

 

In summary, these data suggest that the administration of replacement doses of testosterone to androgen-deficient men can be expected to produce a modest increment in serum PSA levels. Increments in PSA levels after testosterone supplementation in androgen-deficient men are generally less than 0.5 ng/mL and increments in excess of 1.4 ng/mL over a 3–6-month period are unusual. Nevertheless, administration of testosterone to men with baseline PSA levels between 2.6 and 4.0 ng/mL will cause PSA levels to exceed 4.0 ng/mL in some men. Increments in PSA levels above 4 ng/mL will trigger a urological consultation and many of these men will be asked to undergo prostate biopsies. However, considering the controversy over prostate cancer screening and monitoring, the decision to monitor PSA levels during testosterone treatment and the decision to refer a patient for consideration of prostate biopsy should be made only after informing him of the risks and benefits of prostate cancer screening and monitoring and engaging the patient in a shared decision-making process.

 

Monitoring PSA Levels in Older Men Receiving Testosterone Replacement (Tables 3 and 4)

 

Older men considering testosterone supplementation should undergo evaluation of risk factors for prostate cancer; the Endocrine Society guideline suggest a baseline PSA measurement and a digital prostate examination (16). Prostate cancer screening has some risks; therefore, initiation of prostate monitoring should be a shared decision, made only after a discussion of the risks and benefits of prostate cancer monitoring. Men with history of prostate cancer, should not be given androgen supplementation and those with palpable abnormalities of the prostate or PSA levels greater than 3 ng/ml should undergo urological evaluation. After initiation of testosterone replacement therapy, PSA levels should be repeated at 3 months and annually thereafter (16). Although measurements of free PSA and PSA density have been proposed to enhance the specificity of PSA measurement, long term data, especially from studies of testosterone replacement in older men, are lacking. Considering the interassay variability and the longitudinal change in PSA previously discussed, an Endocrine Society Expert Panel recently suggested that men receiving testosterone replacement should be referred to urological consultation if: 1) PSA increases more than 1.4 ng/mL in the first 12 months of treatment; 2) a PSA above 4 ng/mL is confirmed; or 3) a prostatic abnormality is detected on digital rectal examination (16). After 12 months of treatment, prostate monitoring should follow standard guidelines for prostate cancer screening taking into account the age and race of the patient (16).

 

Table 3. Recommendations for Monitoring of Men Receiving Testosterone Therapy

A. Explain the potential benefits and risks of monitoring for prostate cancer and engage the patient in shared decision making regarding the prostate monitoring plan.

B. Evaluate the patient at 3–12 months after treatment initiation and then annually to assess whether symptoms have responded to treatment and whether the patient is suffering from any adverse effects

C. Monitor testosterone concentrations 3–6 months after initiation of therapy:

·       --Therapy should aim to raise testosterone into the mid-normal range.

·       --Injectable testosterone enanthate or cypionate: measure testosterone midway between injections. If midinterval T is >600 ng/dL (24.5 nmol/L) or <350 ng/dL (14.1 nmol/L), adjust dose or frequency.

·       --Transdermal gels: assess testosterone 2–8 h following the gel application, after the patient has been on treatment for at least 1 week; adjust dose to achieve testosterone in the mid-normal range.

·       --Transdermal patches: assess testosterone 3–12 h after application; adjust dose to achieve concentration in the mid-normal range.

·       --Buccal T bioadhesive tablet: assess concentrations immediately before or after application of fresh system.

·       --Testosterone pellets: measure concentrations at the end of the dosing interval. Adjust the number of pellets and/or the dosing interval to maintain serum T concentrations in the mid-normal range.

·       --Oral T undecanoate: monitor serum T concentrations 3–5h after ingestion with a fat-containing meal.

·       --Injectable testosterone undecanoate: measure serum T levels at the end of the dosing interval just prior to the next injection and aim to achieve nadir levels in low-mid range.

D. Check hematocrit at baseline, 3–6 months after starting treatment, and then annually. If hematocrit is >54%, stop therapy until hematocrit decreases to a safe level; evaluate the patient for hypoxia and sleep apnea; reinitiate therapy with a reduced dose.

E. Measure BMD of lumbar spine and/or femoral neck after 1–2 year of testosterone therapy in hypogonadal men with osteoporosis, consistent with regional standard of care.

F. For men 55–69 years of age and for men 40–69 years of age who are at increased risk for prostate cancer who choose prostate monitoring, perform digital rectal examination and check PSA level before initiating treatment; check PSA and perform digital rectal examination 3–12 months after initiating testosterone treatment, and then in accordance with guidelines for prostate cancer screening depending on the age and race of the patient.

G. Obtain urological consultation if there is:

·       An increase in serum PSA concentration.1.4 ng/mL within 12 months of initiating testosterone treatment

·       A confirmed PSA > 4 ng/mL at any time

·       Detection of a prostatic abnormality on digital rectal examination

·       Substantial worsening of lower urinary tract symptoms

Adapted with permission from the Endocrine Society Guideline for Testosterone Therapy in Men with Hypogonadism in: Bhasin et al J Clin Endocrinol Metab 2018;103(5):1715-1744.

 

Table 4. Indications for Urological Consultation in Men Receiving Testosterone Replacement

1.     1) An increase in serum or plasma PSA concentration >1.4 ng/mL within any 12-month period after initiating testosterone treatment

2.     2) A PSA >4.0 ng/mL

3.     3) Detection of a prostatic abnormality on digital rectal examination

4.     4) An AUA/IPSS prostate symptom score of >19

Adapted with permission from the Endocrine Society Guideline for Testosterone Therapy in Men with Hypogonadism in: Bhasin et al J Clin Endocrinol Metab 2018;103(5):1715-1744.

 

Testosterone and Benign Prostatic Hypertrophy  

 

Testosterone replacement can be administered safely to men with benign prostatic hypertrophy who have mild to moderate symptom scores. The severity of symptoms associated with benign prostatic hypertrophy can be assessed by using either the International Prostate Symptom Score (IPSS) or the American Urological Association (AUA) Symptom questionnaires. Androgen deficiency is associated with decreased prostate volume and androgen replacement increases prostate volume compared to  age–matched controls (559,562,566,567). Meta-analyses of testosterone trials have not found statistically significant difference in lower urinary tract symptoms scores in hypogonadal men receiving testosterone replacement compared to placebo (Figure 16) (256,579). However, in patients with pre–existing, severe symptoms of benign prostatic hypertrophy, even small increases in prostate volume during testosterone administration may exacerbate obstructive symptoms. In these men, testosterone should either not be administered or administered with careful monitoring of obstructive symptoms.

Figure 16. Adverse events associated with testosterone therapy in randomized trials. The relative risk and 95% CI for development of erythrocytosis (RR= 8.14; 95%CI= 1.87 to 35.40) and lower urinary tract symptoms (LUTS; RR= 0.38; 95%CI= -0.67 to 1.43) in randomized testosterone trials derived from meta-analyses published by Ponce et al., 2018 are shown. The figure was adapted with permission from Ponce et al. J Clin Endocrinol Metab. 2018;103(5):1745-54.

ERYTHROCYTOSIS

 

Testosterone replacement is associated with increased red cell mass and hemoglobin levels (Figure 16) (256,329,580-585). Therefore, testosterone replacement should not be administered to men with baseline hematocrit of 52% or greater without appropriate evaluation and treatment of erythrocytosis (16) (Table 3). Administration of testosterone to androgen–deficient young men is typically associated with a small increase in hemoglobin levels. Clinically significant erythrocytosis is uncommon in young hypogonadal men during testosterone replacement therapy, but can occur in men with sleep apnea, significant smoking history, or chronic obstructive lung disease. Testosterone administration in older men is associated with greater increments in hemoglobin than observed in young, hypogonadal men (321). The magnitude of hemoglobin increase during testosterone therapy appears related to the testosterone dose, the increase in testosterone concentrations during testosterone therapy, and age (321). Testosterone replacement by means of a transdermal system has been reported to produce a lesser increase in hemoglobin levels than that associated with intramuscular testosterone enanthate and cypionate presumably because of the substantially higher testosterone dose and average circulating testosterone levels achieved with testosterone esters (586).

 

Testosterone increases hemoglobin and hematocrit by multiple mechanisms (322,408,409,587). Testosterone administration stimulates iron-dependent erythropoiesis by suppressing hepcidin transcription and increasing iron availability for erythropoiesis (322,408,409,587). Additionally, testosterone stimulates erythropoiesis by a direct effect on bone marrow hematopoietic progenitors and increasing the numbers of myeloid progenitors. Testosterone also stimulates erythropoietin and alters the set-point of the relationship between erythropoietin and hemoglobin (322). Testosterone supplementation can correct anemia in older men with unexplained anemia of aging and anemia of inflammation (322,329,409). Suppression of testosterone secretion in men receiving androgen deprivation therapy reduces hematocrit and hemoglobin levels by slowing erythropoiesis independently of changes in erythropoietin levels (588).     

 

Monitoring Hematocrit During Testosterone Replacement Therapy (Table 3)

 

Hematocrit levels should be measured at baseline and 3 months after institution of testosterone replacement or after increase in dosage, and every 12 months thereafter. It is not clear what absolute hematocrit level or magnitude of change in hematocrit warrants discontinuation of testosterone administration. Plasma viscosity increases disproportionately as hematocrit rises above 50%. Hematocrit levels above 54% may be associated with increased risk of neuro-occlusive events. Therefore, testosterone dose should be withheld if hematocrit rises above 54%; once hematocrit falls to a safe level, testosterone therapy may be re-initiated at a reduced dose or with a different formulation (16).  

 

SLEEP APNEA   

 

Circulating testosterone concentrations are related to sleep rhythm and are generally higher during sleep than during waking hours (589-592). Testosterone secretory peaks coincide with the onset of rapid-eye movement sleep. Aging is associated with decreased sleep efficiency, reduced numbers of REM sleep episodes, and altered REM sleep latency, which may contribute to lower circulating testosterone concentrations (590-594).  The degree of sleep-disordered breathing increases with age and is associated with reduced overnight plasma bioavailable testosterone. Thus, changes in sleep efficiency and architecture are associated with alterations in testosterone levels in older men (590-594). Sleep apnea and disordered sleep are often associated with low testosterone levels (595), particularly in patients with more severe cases of OSA (i.e. severe hypoxemia) (596). Some potential mechanisms by which OSA may decrease endogenous testosterone levels include disruption of pulsatile luteinizing hormone secretion from restricted sleep and/or recurrent nocturnal hypoxia (597,598), which is further exacerbated by obesity. OSA treatment with continuous positive airway pressure has been demonstrated to increase serum testosterone levels (599).

 

Testosterone can induce or exacerbate sleep apnea in some individuals, particularly those with obesity or chronic obstructive lung disease (589-594,600). This appears to be due to direct effects of testosterone on laryngeal muscles. Testosterone administration depresses hypercapnic ventilator drive and induces apnea in primate infants (594). Short-term administration of high doses of testosterone shortens sleep duration and worsens sleep apnea in older men (601). The frequency of sleep apnea in randomized testosterone trials in older men has been very low (16,486) and no randomized trial has reported an increased incidence of OSA or OSA worsening in men randomized to the testosterone arm compared to the placebo arm. 

 

Testosterone should not be given to men with severe untreated OSA without evaluation and treatment of sleep apnea. Several screening instruments can be used to detect sleep apnea. A history of loud snoring, and daytime somnolence, in an obese individual with hypertension increases the likelihood of having sleep apnea; such patients should be referred for a sleep study.

 

BREAST ENLARGEMENT AND TENDERNESS

 

Testosterone administration can induce breast tenderness; however, gynecomastia is an uncommon complication of testosterone replacement therapy. Even with administration of supraphysiological doses of testosterone enanthate, less than 4% of men in a contraceptive trial developed detectable breast enlargement (580). Breast cancer is listed as a contraindication for testosterone replacement therapy primarily because of concern that increased estrogen levels during testosterone treatment might exacerbate breast cancer growth. There are, however, few case reports of breast cancer occurring as a complication of testosterone treatment. Men with Klinefelter’s syndrome have a higher risk of breast cancer than the general population (602).

 

An Individualized, Patient-Centric Approach to Shared Decision Making in the Evaluation and Treatment of Older Men with Low Testosterone Levels

 

Recent large randomized clinical trials, especially the TTrials, have substantially expanded our understanding of the efficacy and short-term safety of testosterone in older men with low testosterone levels. However, none of the trials has been long enough or large enough to determine the effects of testosterone treatment on major adverse cardiovascular events and prostate cancer risk. Furthermore, the long-term efficacy of testosterone treatment in improving hard outcomes – physical disability, fractures, falls, progression to dementia, progression from prediabetes to diabetes, remission of late-life low grade persistent depressive disorder (dysthymia) - remains to be established. Adherence with testosterone treatment is poor and in one survey, nearly 50% of men prescribed testosterone, discontinued treatment within 3 months.  Population level screening of all older men for androgen deficiency is not justified (16) because of the lack of agreement on a case definition, the paucity of data on the performance characteristics of the screening instruments (e.g., the ADAM questionnaire (603), the Aging Male Symptoms questionnaire (604), and the MMAS questionnaire (605)) and the lack of clarity on the public health impact of the androgen deficiency syndrome in the general population.

 

Recognizing the lack of evidence of the long-term safety and efficacy of testosterone therapy in older men with symptomatic androgen deficiency, the expert panel of the Endocrine Society recommended against testosterone therapy of all men 65 years or older with low testosterone levels (16). Instead, the panel suggested that “in men >65 years who have symptoms or conditions suggestive of testosterone deficiency (such as low libido or unexplained anemia) and consistently and unequivocally low morning testosterone, clinicians offer testosterone therapy on an individualized basis after explicit discussion of the potential risks and benefits” (16).

 

The decision to offer testosterone treatment to older men with low testosterone levels should be guided by an individualized assessment of potential benefits and risks  (Figure 17) (606).  Determine whether the patient has clear evidence of testosterone deficiency recognizing the imprecision and inaccuracy of many available immunoassays and the substantial overlap in the symptoms of hypogonadism and aging per se. Perform a careful general health evaluation to identify conditions, such as prostate cancer, erythrocytosis, heart failure, or a hypercoagulable state that could increase the risk of harm. Weigh the burden of patient's symptoms and conditions associated with testosterone deficiency against the potential benefits and the uncertainty of long-term harm. Evaluate prostate cancer risk recognizing that prostate cancer screening and monitoring has some risks. Weigh the bother and distress associated with symptoms of testosterone deficiency and patient's values and risk tolerance against the uncertainty of benefits and long-term risks, and the burden, cost, and risks of treatment and monitoring. The participation of the patient who is well informed of the potential benefits and risks in the shared decision to initiate testosterone treatment can enable a more thoughtful treatment plan and increased adherence with the treatment and monitoring (606).

Figure 17. An evidence-based, individualized approach to testosterone therapy in older men with testosterone deficiency. The decision to offer testosterone treatment to older men with low testosterone levels should be guided by an individualized assessment of potential benefits and risks. Testosterone deficiency needs to be evaluated using reliable assays for the measurement of total and free testosterone levels. Patients should also be evaluated for conditions that are likely to respond to testosterone replacement therapy (TRT) as well as conditions that could be adversely impacted, such as prostate cancer, erythrocytosis, heart failure, or a hypercoagulable state. It is important to consider each patient’s burden of symptoms, individual preferences, and risk tolerance against the uncertainty of long-term benefits and risks, the burden and risks of monitoring, and the cost. Reproduced with permission from Bhasin S. 2021. J Clin Invest. 2021;131(4):e146607.

Testosterone therapy can be instituted using any of the available approved formulations based on considerations of pharmacokinetics, patient convenience and preference, cost, and formulation-specific adverse effects (16).  The men receiving testosterone therapy should be monitored using a standardized monitoring plan to facilitate early detection of adverse events and to minimize the risk of unnecessary prostate biopsies (Table 2), as recommended by the Endocrine Society expert panel (Table 3) (16).

 

CHANGES IN THE SPERMATOGENIC COMPARTMENT OF THE TESTIS

 

Women are more fertile below the age of 40, and fecundity decrease after age 35 and fertility ceases at the inception of menopause, around age 50. Increasing age in women confers greater risk for infertility, spontaneous abortion, and genetic and chromosomal defects among offspring. In contrast, there is no critical age at which sperm production or function, and fertility cease in men (607-614).  Although serum testosterone concentrations decrease below the normal range in a significant minority of older men, men over the age of 60 years commonly father children; the oldest father on record was 94-years old (607,609). Even though many older men are fertile, the overall fertility and fecundity decline with aging. The interpretability of data on the effects of aging on male fertility is limited by the small size of the studies and the low overall event rates.

 

Paternal age is associated with an increased risk of germ line mutations in FGFR2, FGFR3, and RET genes and inherited autosomal dominant diseases, such as Apert's syndrome, achondroplasia, and Costello Syndrome, respectively, in the offspring of older men (614-623). These monogenic disorders have been referred to as paternal age effect (PAE) disorders. Approximately one third of babies with diseases due to new autosomal dominant mutations are fathered by men aged 40 years or older (624).

 

Some other disorders such as schizophrenia, autism, and bipolar disorder have also been linked to paternal age (Figure 18) (615,616,622,623). The rate of de novo mutations increases with paternal age (622), which may contribute to the increase risk of neurodevelopmental diseases such as schizophrenia and autism (622).

Figure 18. Impact of paternal age on incidence of schizophrenia and early-onset bipolar disorder. Increasing paternal age at conception increases the relative risk of having an offspring with schizophrenia (panel A; figure adapted from Malaspina et al. Arch Gen Psychiatry. 2001 Apr;58(4):361-7.) and the odds ratio of having a child with early-onset bipolar disorder (compared to fathers aged 20 to 24 years; panel B; data derived from Frans et al. Arch Gen Psychiatry. 2008 Sep;65(9):1034-40)

The accumulation of these de novo germ line mutations with increasing paternal age has been explained by the “selfish spermatogonial selection" hypothesis (618,619).  According to this hypothesis, the somatic mutations in male germ cells that enhance the proliferation of germ cells could lead to within-testis expansion of mutant clonal lines (620,621), thus favoring the propagation of germ cells carrying these pathogenic mutations, and increasing the risk of mutations in the offspring of older fathers (620,621). Interestingly, the risk of autism has also been associated with the age of the father as well as the grandparent (623). These concerns have prompted the American Society of Reproductive Medicine to state in their guidelines that semen donors should be younger than 40 years of age so that potential hazards related to aging are diminished (610).

 

Some cardiac defects have also been attributed to aberrant genetic input from older men.  For instance, a case-control study of 4,110 individuals with congenital heart defects born between 1952 and 1973 in British Columbia, found a general pattern of increasing risk with increasing paternal age among cases relative to controls for ventricular septal defects, atrial septal defects, and patent ductus arteriosus (617,624). The risk of schizophrenia has also been reported to increase with paternal age (618) and possible loci affecting this risk have been identified (625). In addition, a modest proportion of preeclampsia, normally associated with increased maternal risk factors including age, might be attributable to an increase in paternal age although no gene loci have been identified (626). These observations need further corroboration.

 

Although there is a positive association between paternal age and incidence of aneuploidy, it has been difficult to dissociate the effect of paternal age from the confounding influence of the advanced maternal age. After accounting for various confounders, there does not appear to be a major independent effect of increased paternal age on the incidence of autosomal aneuploidies (608,609,615,616,627,628). The existence of a paternal age effect on Down syndrome is controversial. Earlier studies from the 1960s and 1970s found no correlation between Down syndrome and paternal age (e.g.,(629)). However, a study in New York from 1983 to 1997 found a significant greater numbers of mothers and fathers 35 years of age and older, respectively, among parents of patients with Down’s syndrome (630). Among the cases of Down syndrome evaluated, paternal age had a significant effect only when the mothers were 35 years of age or older, and was the highest when both the Mother and Father were older than 40 years in which case the risk of Down Syndrome was 6 times that observed among couples younger than 35 years of age (630).

 

Changes in Fertility of Older Men  

 

A review of studies examining fertility at different ages demonstrated significant age-related differences in fertility rates in men; men older than 50 have lower pregnancy rates, increased time to pregnancy, and subfecundity compared to younger men (608,609,615,631,632). Some changes in fertility rates might be related to age-related decrease in sexual activity.  A literature review found no significant change in sperm concentration with aging when comparing men under the age of 30 to those greater than 50 years (613). However, in general, semen volume, sperm motility, and the number of morphologically normal sperm decrease with advancing age (Table 5; (45,608-614,622,627,628,631,633)). A number of these studies, however, did not control for important confounding variables. Of the 21 studies in which sperm densities were compared among men of different age groups (613), only four studies adjusted for the duration of abstinence, well known to affect sperm concentration. In addition, there is significant heterogeneity in the populations studied; most of the studies examined data from semen of sperm donors while others examined men from infertility clinics. Sperm donors might represent a healthier group of men than the general population; conversely men in infertility clinics might be more likely to have abnormalities of sperm number or function. Even studies that have controlled for abstinence as well as alcohol and tobacco use have shown an age-related decrease in semen volume. In one study of men whose partners had bilateral tubal obstruction or absence of both tubes and who were treated by conventional IVF, the odds ratio of failure to conceive was higher for men 40 years of age or older (634).

 

Table 5: Changes in Semen Quality and Fertility in Men with Age

Parameter

Age comparison

Change

Semen volume

30 versus ³50 years

3-22% decrease

Sperm concentration

Varying

None

Abnormal sperm morphology

£30 versus ³ 50 years

4-18% increase

Time to pregnancy

<30-35 versus >30-50 years

5-20% increase

Pregnancy rates

<30 versus > 50 years

23-38% decrease

Subfecundity

Varying

11-250% increase

From a Literature Review by Kidd et al., 2001 (501)

CHANGES IN THE GERM CELL COMPARTMENT

 

In a comparison of younger men (21-25 years) with older men (>50) referred for andrological evaluation, the ejaculate volume, progressive sperm motility, and sperm morphology were lower in older men than younger men after adjustment for duration of sexual abstinence (635). The older men also had a higher frequency of sperm tail defects, suggesting epididymal dysfunction (636). In addition, the fructose content was significantly lower in older men suggesting a defect in the seminal vesicle contribution to semen. There were no significant differences in sperm concentration and testicular size between the young and older men in this study.

 

Necropsies on adult men of different ages have revealed that the testicular volume is lower only in men in the 8th decade of life (637). A recent study examined testicular germ cells obtained by orchiectomy from 36 older men with advanced prostate cancer and by testicular biopsy from 21 younger men with obstructive azoospermia, as controls (638). The ratios of primary spermatocytes, round spermatids, and elongated spermatids to Sertoli cells were significantly decreased in the testes of older men, but the ratio of spermatogonia to Sertoli cell number remained unchanged (638,639). Older men are characterized by lower rates of germ cell apoptosis and cell proliferation compared with younger men, suggesting that germ cell proliferation and apoptosis diminish with aging (639).

 

Other studies evaluating the fidelity of the germ cell compartment are cross-sectional and depend on analyses of sperm number and semen quality; large-scale chromosomal analyses in healthy community dwelling men are scarce as most data are derived from fertility clinics.  A review of studies examining semen quality at different ages demonstrated significant age-related decrease in semen volume and sperm morphology. The change in sperm morphology has been hypothesized to be due to an increase in aneuploidy with age. Härkönen et al (628) found that sperm morphology was directly associated with the number of chromosomes in sperm and that men with higher aneuploidy rates for chromosomes 13, 18, 21, X and Y had lower sperm motility and sperm concentrations. Despite the changes in sperm morphology and motility from older men, in vitro fertilizing capacity of the sperm is well preserved (45,634).

 

There are several difficulties in interpreting these data on age-related changes in sperm density and function. The normal range for sperm concentration in men is wide where sperm concentration above 15 million/ml (total sperm per ejaculate > 39 million) is considered normal. Thus, even though average sperm concentrations decline with aging, they are still are in the normal range (45,632,638). Furthermore, normal sperm counts do not always correlate with normal sperm function.

 

Studies in flies demonstrate more germ cells during larval than adult stages suggesting age-related quiescence of the germ line (640). Significant age-related decreases in germ cells and spermatogenesis also have been reported in rodents and primates (641-645). The Brown Norway rat has been studied as a model of aging of the human male reproductive system because in this rodent model, serum testosterone levels decrease with aging, as they do in humans (642-644). Along with changes in hypothalamic-pituitary hormones, alterations in sperm counts, sperm maturation, Sertoli cell number, and progeny outcomes have been observed in this rodent model (Table 6) (626,642-651). Analysis of ribosomal DNA from germ cells of the male brown Norway rat has revealed hypermethylation of ribosomal DNA (645,652). Alterations in ribosomes have been theorized to promote aging of cells by multiplying errors in protein synthesis which initially might elude gross morphological analysis but eventually might lead to germ cell degeneration (652). Further assessment of spermatogonial stem cell populations is needed.  In many animal models of life span extension, there is a trade-off between longer life and fecundity, although there are some exceptions (653).

 

Table 6.  Changes in the Reproductive Axis in the Brown-Norway Rat

Parameter

Change

Reference

GnRH

¯

530, 531

FSH

  

530, 531

LH

®

530, 531

Testosterone

¯

530-532, 534

Germ Cells

¯

535

Sertoli Cells

¯

531, 537

Spermatogenesis

¯

531, 537

Seminiferous Tubules

¯

531, 537

Seminiferous Tubule Function

altered

534, 537

Epididymal function

¯

538

Sperm morphology

altered

538

Sperm motility

¯

538

Sperm Count

¯

532

 

CHANGES IN SUPPORTIVE CELLS AND ACCESSORY GLANDS

 

Since Sertoli and Leydig cells are crucial to spermatogenesis, changes in these cells could affect sperm number and function. Age-related changes in the supporting structures for sperm maturation have been described in the Brown Norway rat.  These changes include reductions in the numbers of Leydig and Sertoli cells (643-645). Changes in the supporting cells and structures for sperm maturation have been invoked to explain the age-related decrease in sperm number and fecundity in rats. In stallions, the numbers of Sertoli cells decreases with aging but individual Sertoli cells display a remarkable capacity to accommodate greater numbers of developing germ cells (654).  

 

In men, Sertoli cell number has been reported to be lower in men aged 50 to 85 years than in men aged 20 to 48 years (655). The apoptotic rate of primary spermatocytes in aged men was also significantly elevated compared with that of younger men, resulting in a decrease of the number of primary spermatocytes per Sertoli cell (639), leading the authors to suggest that there might be a failure of the Sertoli cells to support spermatogenesis in older men.

 

Sertoli cells produce inhibin, which regulates gonadotropin expression from the pituitary. Inhibin B has been identified as the physiologically important form of inhibin in men and as a valuable serum marker of Sertoli cell function and spermatogenesis.  Higher gonadotropins and lower inhibin levels in older men suggest a decline in Sertoli cell function (655); however changes in circulating inhibin B levels with advancing age have been inconsistent (70,655-657). Overall, these data suggest a possible decline in Sertoli cell number and function in older.

 

Aging is accompanied by a progressive, albeit variable, decline of Leydig cell function with a decrease of mean serum free (or bioavailable) testosterone levels in the population between age 25 and 75 years (658). Total Leydig cell volume and the absolute number of Leydig cells decline with advancing age, although total testis weight does not change substantially with age (658-662). In one study, age accounted for more than a third of the variation in Leydig cell number, and explained more than half the variation in daily sperm production (661). This might in part be explained by a fusion of Leydig cells resulting in fewer but multinucleated Leydig cells with age (662). The functionality of the multinucleated cells is not known.

 

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Dyslipidemia in Chronic Kidney Disease

ABSTRACT

Chronic kidney disease (CKD) is associated with a dyslipidemia comprising high triglycerides, low HDL-cholesterol and altered lipoprotein composition. Cardiovascular diseases are the leading cause of mortality in CKD, especially in end stage renal disease patients. Thus, therapies to reduce cardiovascular risk are urgently needed in CKD. Robust clinical trial evidence has found that use of statins in pre-end stage CKD patients, as well as in renal transplant recipients, can decrease cardiovascular events; however, providers need to be aware of dose restrictions for statin therapy in CKD subjects. Furthermore, statin therapy does not reduce cardiovascular events in dialysis patients, nor does statin therapy confer any protection against progression of renal disease. Niacin and fibrates are effective in lipid lowering in CKD and appear to have some cardiovascular benefit but further study is needed to clearly define their role. Novel therapies with PCSK 9 inhibitors, bempedoic acid, and inclisiran have all been shown to decrease LDL cholesterol levels but there is currently limited data for reduction of cardiovascular events or mortality in patients with CKD/ESRD. This article reviews the epidemiology of CKD, association of CKD with cardiovascular events, and the effects of CKD on lipid levels and metabolism. The article discusses clinical trial evidence for and against statin and non-statin lipid lowering therapy in CKD patients.

CHRONIC KIDNEY DISEASE (CKD) EPIDEMIOLOGY

Chronic kidney disease (CKD) is defined as renal impairment greater than 3 months duration that results in an estimated glomerular filtration rate (eGFR) < 60ml/min/1.73m2. CKD is classified into 5 stages based on the eGFR (Table 1). CKD is a world-wide health problem with rising incidence and prevalence. CKD, especially in the early stages is often asymptomatic; thus, the actual prevalence may be even higher than estimated. End stage renal disease (ESRD) is defined as needing dialysis or transplant, and the prevalence and incidence of ESRD have doubled over the past 10 years (1). The annual mortality rate of dialysis patients is greater than 20%. The burden of co-morbidities and the cost of caring for CKD patients is high, and thus a major focus is increased screening and early detection of CKD when interventions to delay or prevent progression to ESRD may be effective. There are multiple causes of CKD with the most common causes in Westernized nations being hypertension and diabetes; however, a wide range of etiologies including infectious, auto-immune, genetic, obstructive, and ischemic injury are all prevalent. There are ethnic differences in susceptibility with increased prevalence in Mexican-Americans and non-Hispanic blacks compared to Caucasians (2).

Table 1. Stages of CKD

CKD stage

GFR (ml/min/1.73 m2)

CKD 1

≥ 90 (with renal damage or injury)

CKD 2 (mild)

60-89

CKD 3 (moderate)

30-59

CKD 4 (severe)

15-29

CKD 5 (end stage)

<15, dialysis, or transplant

While the burden of CKD itself is significant, the leading causes of morbidity and mortality in CKD are cardiovascular diseases (CVD), primarily atherosclerotic coronary artery disease. Risk factors for CVD in CKD include the traditional risk factors – hypertension, sex, age, smoking, and family history and CKD patients appear to benefit similar to non-CKD patients from therapies targeting these risk factors. Regardless of the cause of CKD, patients with CKD are at increased risk for CVD, which has led to the National Kidney Foundation classifying all patients with CKD as “highest risk” for CVD regardless of their levels of traditional CVD risk factors. The focus of this chapter is on the dyslipidemia of CKD and the risk of CVD in CKD.

Nephrotic Syndrome

Nephrotic syndrome differs from other types of CKD in its presentation and risks. Nephrotic syndrome is comprised of significant proteinuria (typically > 3g/24h), hypoalbuminemia, peripheral (+/- central) edema, and significant hyperlipidemia and lipiduria may also be seen. It is frequently seen in children, and the etiology includes minimal change disease (up to 85%), focal segmental glomerulosclerosis (up to 15%), and secondary causes (rare) including systemic lupus erythematosus, Henoch Schonlein Purpura, or membrano-proliferative glomerulopathy. In adults, the etiology is more likely to involve a systemic disease such as diabetes, amyloidosis, or lupus. Nephrotic syndrome may be transient or persistent. Most (approximately 80% of children) cases of nephrotic syndrome are successfully treated with glucocorticoids with resolution of all features including hyperlipidemia; however, steroid-resistant nephrotic syndrome patients often have persistent dyslipidemia, which may place them at increased risk for CVD. For example, a small study found increased CVD markers including pulse wave velocity, carotid artery intima-media thickness, and left ventricular mass in patients with steroid-resistant nephrotic syndrome compared to controls (3), implying increased risk for CVD events. Treatment of nephrotic syndrome dyslipidemia includes therapies specifically targeting the renal disease (primarily glucocorticoids, but also renin-angiotensin system antagonists which can help decrease proteinuria) and lipid lowering agents.

 CVD IN CKD

CVD accounts for 40-50% of all deaths in ESRD patients, with CVD mortality rates approximately 15 times that seen in the general population (4). However, CVD is highly prevalent in patients who progress to ESRD implying that earlier stages of CKD increase the development of CVD. A number of factors have been proposed as risk factors for CVD in CKD including proteinuria, inflammation, anemia, malnutrition, oxidative stress, and uremic toxins (5). Ongoing research is investigating whether these (and other) markers may be therapeutic targets. Interestingly, proteinuria correlates with blood pressure, total cholesterol, triglycerides, and inversely correlates with HDL-cholesterol (6). Thus, it remains unclear if proteinuria itself is a risk factor (e.g., a cause of CVD) or a biomarker. Meta-analyses of general population and high-risk population cohorts found that both lower eGFR (<60 ml/min/1.73 m2) and higher albuminuria (>10 mg/g creatinine) are predictors of total mortality and CVD mortality; furthermore, eGFR and albuminuria are independent of each other and of traditional CVD risk factors (7, 8). Estimated GFR > 60 ml/min/1.73 m2 is not a risk factor for CVD or total mortality.

Dyslipidemia in CKD

EFFECT OF CKD ON LIPID LEVELS

CKD is associated with a dyslipidemia comprised of elevated triglycerides and low HDL-cholesterol. Levels of LDL-cholesterol (and thus, total cholesterol) are generally not elevated; however, proteinuria correlates with cholesterol and triglycerides. CKD leads to a down regulation of lipoprotein lipase and the LDL-receptor, and increased triglycerides in CKD are due to delayed catabolism of triglyceride rich lipoproteins, with no differences in production rate (9). CKD is associated with lower levels of apoA-I (due to decreased hepatic expression (10)) and higher apoB/apoA-I. Decreased lecithin-cholesterol acyltransferase (LCAT) activity and increased cholesteryl ester transfer protein (CETP) activity contribute to decreased HDL-cholesterol levels. Beyond decreased HDL cholesterol levels, the HDL in CKD is less effective in its anti-oxidative and anti-inflammatory functions [for review see (11)].

As CKD progresses the dyslipidemia often worsens. In an evaluation of 2001-2010 National Health and Nutrition Examination Survey (NHANES), the prevalence of dyslipidemia increased from 45.5% in CKD stage 1 to 67.8% in CKD stage 4; similarly, the use of lipid lowering agents increased from 18.1% in CKD stage 1 to 44.7% in CKD stage 4 (12).  Of more than 1000 hemodialysis patients studied only 20% had “normal” lipid levels (defined as LDL<130 mg/dl, HDL > 40 and triglycerides < 150); of 317 peritoneal dialysis patients only 15% had “normal” lipid levels (13). A larger study evaluating dyslipidemia in > 21,000 incident dialysis patients found 82% prevalence of dyslipidemia and suggested a threshold of non-HDL cholesterol > 100 mg/dl (2.6mmol/L) to identify dyslipidemia in CKD stage 5 subjects (14). Peritoneal dialysis is associated with higher cholesterol levels than hemodialysis, although the reasons aren’t fully understood. In subjects who switched from peritoneal dialysis to hemodialysis there was a drop in cholesterol levels of almost 20% following transition (15). The National Kidney Foundation recommends routine screening of all adults and adolescents with CKD using a standard fasting lipid profile (total cholesterol, LDL-cholesterol, HDL-cholesterol and triglycerides), and follows the classification of the National Cholesterol Education Panel for levels (desirable, borderline or high). Although some studies have found associations between Lp(a) and dialysis patients, this is not well defined and there is no current indication for routine screening of Lp(a).

EFFECT OF CKD ON LIPOPROTEIN COMPOSITION

Beyond simply measuring lipid levels, emerging evidence implies that lipoprotein particle size and composition is altered in CKD, with increased small dense LDL and decreased larger LDL particles in CKD subjects compared to controls (16). Small dense LDL is thought to be more atherogenic than larger LDL particles. An emerging theory is that beyond lipid levels or lipoprotein size, lipoprotein particle “cargo” can affect atherosclerosis development and progression. Lipoprotein particles transport numerous bioactive lipids, microRNAs, other small RNAs, proteins, hormones, etc. For example, a recent study compared LDL particle composition between subjects with stage 4/5 CKD and non-CKD controls, and found similar total lipid and cholesterol content, but altered content of various lipid subclasses; for example decreased phosphatidylcholines, sulfatides, and ceramides and increased N-acyltaurines (17). Many of these lipid species are known to have either pro- or anti-atherogenic properties and thus could directly affect atherogenesis.

EFFECT OF RENAL TRANSPLANTATION ON LIPID LEVELS

Dyslipidemia is frequently seen in renal transplant recipients, including increased total cholesterol, LDL-cholesterol, and triglycerides, and decreased HDL-cholesterol. The dyslipidemia may have existed pre-transplant or be related to transplantation associated factors. Cyclosporine increases LDL-cholesterol via both increased production and decreased clearance. Corticosteroids increase both cholesterol and triglyceride levels in a dose-dependent manner. The adverse effects of cyclosporine and corticosteroids on lipid levels appear to be additive (18). Tacrolimus and azathioprine appear to have less induction of dyslipidemia than cyclosporine (19). Sirolimus increases both cholesterol and triglycerides, in part due to decreased LDL-clearance (20).

EFFECT OF NEPHROTIC SYNDROME ON LIPID LEVELS

The dyslipidemia in nephrotic syndrome can be striking with significant elevations of cholesterol, LDL-cholesterol, triglycerides, and lipoprotein(a); HDL cholesterol is often low, especially HDL2. The cause of elevated lipid levels is multi-factorial, including reduction in oncotic pressure which stimulates apoB synthesis (although the exact mechanism by which this occurs is not known), decreased metabolism of lipoproteins, and decreased clearance. Patients with nephrotic syndrome have decreased LDL-receptor activity and increased acyl-CoA cholesterol acytransferase (ACAT) and HMG-CoA reductase activity leading to increased LDL-cholesterol levels (21, 22). Low HDL-cholesterol is thought to be due at least in part to LCAT deficiency secondary to accelerated renal loss of LCAT (23). Triglycerides are elevated due to impaired clearance of chylomicrons and triglyceride-rich lipoproteins, as well as increased triglyceride production (24).

EVIDENCE FOR/AGAINST LIPID LOWERING THERAPY IN CKD FOR CVD OUTCOMES

Given the high prevalence of CVD in CKD, and the robust clinical evidence in non-CKD subjects that lipid lowering reduces CVD outcomes, there is great interest in using lipid lowering therapy in CKD subjects. Statins are the most commonly used lipid-lowering medications and thus far have been shown to reduce CVD events and/or mortality in virtually every population studied. However, CKD patients seem to be a unique population in that at present there is no evidence of benefit for CVD outcomes in dialysis patients with statin therapy. The Canadian Journal of Cardiology lists CKD as a statin indicated condition in its newest guidelines published in 2021 (25) while AHA/ACC lists CKD as a risk enhancer but not a high-risk condition based on 2018 guidelines (26).  Despite growing evidence to support CKD as a CVD risk equivalent, the use of statin therapy in CKD does not appear to be rising more than in the non-CKD population based on data from Mefford et al looking at trends in statin use amongst US adults with CKD from 1999-2014 (27). As discussed below it appears that statins can reduce CVD events in pre-end stage CKD subjects, and in post-renal transplant subjects, but not in dialysis patients (Table 2).

Use of Statins in Pre-ESRD CKD Patients

Although many of the initial statin CVD studies did not include many CKD patients, evidence from sub-group analyses of large statin studies suggested that CKD subjects had similar benefits to non-CKD individuals. For example, the Heart Protection Study (HPS) which assessed >20,000 subjects at high risk of CVD included a subgroup of 1329 subjects with impaired kidney function. In this subgroup those that received simvastatin had a 28% proportional risk reduction and an 11% absolute risk reduction of a major cardiovascular event compared to those randomized to placebo; similar to the effect on the overall cohort (28). Further, in the Pravastatin Pooling Project, 4,991 subjects with CKD3 were examined and a 23% reduction in cardiovascular events was seen in the pravastatin group (29).  In a retrospective study with 47,200 subjects followed through the Department of Veterans Affairs, starting statin therapy 12 months prior to transitioning to ESRD conferred a reduction in 12-month all-cause mortality (HR 0.79), cardiovascular events (HR 0.83) and hospitalization rate (HR 0.89) (30). Several other studies or meta-analyses similarly predicted that CKD subjects would have reduction in CVD with statin therapy. For example, a meta-analysis of 38 studies with >37,000 participants with CKD but not yet on dialysis found a consistent reduction in major cardiovascular events, all-cause mortality, cardiovascular death, and myocardial infarction in statin users compared to placebo groups. There was no clear effect of statin on stroke, nor was there any effect of statin use on progression of the renal disease (31). Thus, for CKD patients with pre-end stage renal disease statins effectively lower total cholesterol and LDL-cholesterol levels and decrease CVD risk. The different statins have different degrees of renal involvement in their metabolism, and providers should be aware of dose restrictions in CKD (Table 3).

Unclear Whether to Use Statins in Subjects with Nephrotic Syndrome

Several small clinical studies have investigated the use of lipid lowering therapies in nephrotic syndrome, but data is only available for statins and fibrates, and no CVD outcomes data is available. Several small studies using statins have found efficacy in lowering LDL-cholesterol, and that statins were safe and well tolerated (32, 33). Thus, the use of statins in nephrotic syndrome appears to be safe and efficacious in terms of lipid lowering; however, it is not clear if there is any corresponding benefit on either CVD or renal outcomes.

No Benefit to Statins in Subjects with only Microalbuminuria

The Prevention of Renal and Vascular Endstage Disease Intervention Trial (PREVEND IT) randomized 864 subjects with persistent microalbuminuria (urinary albumin of 15-300mg/24h x 2 samples) to fosinopril (an angiotensin converting enzyme inhibitor) or placebo and to pravastatin 20 mg or placebo. Inclusion criteria for the study included blood pressure <160/100 mm Hg and no use of antihypertensive medications and total cholesterol < 300 mg/dl (8 mmol/L) or < 192 mg/dl (5 mmol/L) if patient had known CVD and no use of lipid lowering medications. Although diabetes was not an exclusion criteria, <3% of the subjects had diabetes (34). The use of statin did not affect either urinary albumin excretion or cardiovascular events; however, the use of fosinopril significantly decreased albuminuria and had a trend to reduction in cardiovascular events. Thus, in the absence of other indications for statin therapy, there appears to be no benefit in subjects that solely have microalbuminuria. However, a subsequent analysis found that the subjects with isolated microalbuminuria had an increased risk for CVD events and mortality compared to those without risk factors (35); thus, isolated microalbuminuria appears to indicate high risk and further study is needed to determine effective therapies to reduce risk.

No Benefit to Statins in Dialysis Patients

Studies specifically examining the role of statins in ESRD subjects have not found a benefit. The Deutsche Diabetes Dialyse Studie (4D) randomized 1255 type 2 diabetic subjects on maintenance hemodialysis to either 20 mg atorvastatin or placebo daily. The cholesterol and LDL-cholesterol reduction was similar to that seen in non-dialysis patients; however, unlike non-CKD subjects there was no significant reduction in cardiovascular death, nonfatal myocardial infarction or stroke with atorvastatin compared to placebo (36). A long-term follow-up of the 4D study population found similar effects after 11.5 years as were found at the end of the original study: no CVD benefit, but also no evidence of harm (37). Similarly, A Study to Evaluate the Use of Rosuvastatin in Subjects on Regular Hemodialysis (AURORA) randomized 2776 subjects on maintenance hemodialysis to rosuvastatin 10 mg or placebo. Again, the LDL-cholesterol lowering in dialysis patients was similar to that seen in other studies in non-dialysis patients, but there was no significant effect on the primary endpoint of cardiovascular death, nonfatal myocardial infarction or stroke (38). The Study of Heart and Renal Protection (SHARP) randomized 9270 CKD patients (3023 on dialysis) to simvastatin plus ezetimibe versus placebo. Unlike 4D and AURORA, the SHARP study did report a significant reduction in major atherosclerotic events in the simvastatin plus ezetimibe group, including the dialysis subgroup (39). However, a meta-analysis of 25 studies involving 8289 dialysis patients found no benefit of statin therapy on major cardiovascular events, cardiovascular mortality, all-cause mortality or myocardial infarction, despite efficacious lipid lowering (40). Nevertheless, a post-hoc analysis of the 4D study did demonstrate a benefit of statin therapy in the subgroup that had LDL cholesterol > 145 mg/dl (3.76mmol/l) (41). Although the use of statins in dialysis patients does not clearly cause harm, at present there is no indication for use in dialysis patients, with the exception of a possible benefit in those with significant elevation in LDL-cholesterol.

WHY IS STATIN THERAPY INEFFECTIVE IN DIALYSIS SUBJECTS?

Given the robust data demonstrating statin efficacy in CVD risk reduction in virtually all other populations studied, the lack of efficacy in ESRD subjects in perplexing. However, it may be due to different mechanisms of disease progression in ESRD populations compared to other populations. In ESRD subjects there is increased inflammation and oxidative stress as well as increased non-lipid-associated pro-atherogenic factors, which may be the major cause of atherosclerosis development or progression in CKD subjects [for review see (42)]. Therefore, the relative impact of dyslipidemia on CVD development and progression in ESRD subjects may be less than in other CKD and non-CKD subjects, and thus the potential benefit of lipid lowering therapy is reduced. In ESRD subjects with significant hyperlipidemia (such as genetic hyperlipidemias) there may still be a role for statins, or other lipid lowering therapies. Furthermore, while no benefit has been found for statins in dialysis subjects, there is no evidence of increased harm, and thus consideration of lipid lowering medications in particular individuals with ESRD is warranted.

Use of Statins in Renal Transplant Recipients

The Assessment of Lescol in Renal Transplant (ALERT) study randomized 2102 renal transplant recipients to fluvastatin or placebo. There was a non-significant 17% reduction in the combined primary endpoint (cardiac mortality, nonfatal myocardial infarction or coronary intervention procedures) but a significant reduction in cardiac death or myocardial infarction (43, 44). Furthermore, a post hoc analysis suggested that earlier initiation of statins post-transplant was associated with greater benefit (45). However, a recent small study found no benefit of statin therapy on coronary calcification in renal transplant patients (46). Furthermore, as with pre-end stage CKD patients there did not appear to be any benefit from statin therapy on progression of renal disease or graft loss in statin treated transplant recipients (47). Thus, following renal transplant patients should be considered for statin therapy for CVD risk reduction, but not for graft preservation. Several of the statins have drug interactions, particularly with cyclosporine, thus providers must be aware of dose and drug restrictions (Table 3).

Table 2. Use of Statins in Various CKD Subgroups

Patient population

Statin indicated? Yes/no

Microalbuminuria*

No

CKD 1-4

Yes

Nephrotic syndrome

Unclear

Dialysis patients

No

Renal transplant recipients

Yes

* In the absence of any other indication

EVIDENCE FOR/AGAINST LIPID LOWERING THERAPY IN CKD FOR RENAL OUTCOMES

Given the evidence that renal lipid deposition is associated with progression of renal disease itself, there has been an ongoing interest in whether targeting dyslipidemia in CKD can help delay the progression of the renal disease. The dyslipidemia in CKD is associated not only with increased CVD but also with adverse renal prognosis (48, 49). Biopsy studies have found that the amount of renal apoB/apoE is correlated with increased progression of the renal disease itself (50). Animal studies have supported this concept. A meta-analysis of several small, older studies suggested that the rate of decline in GFR was decreased in subjects receiving a lipid-lowering agent (the included studies mainly used statins but the meta-analysis also included a study using gemfibrozil and another using probucol) (51). However, the relationship between lipid levels and renal disease is unclear, as prospective cohort studies have not found any relationship of lipid levels to progression of kidney disease (52). Furthermore, the SHARP study, which included subjects with earlier stages of CKD (stages 3-5 were included) found no benefit of lipid lowering therapy on the progression of renal disease. A meta-analysis of statins in pre-end stage CKD patients found no overall effect of statins on renal disease progression (31) and the ALERT study found no benefit of statin use on renal graft or renal disease parameters (47). Thus, there does not appear to be any use for statins to improve renal function or CKD itself.

SAFETY OF STATINS IN CKD

Statin Safety– Renal Outcomes

An observational study using administrative databases containing information on > 2 million patients suggested that the use of high potency statins was associated with acute kidney injury, especially within the first 120 days of statin use (53). However, a subsequent analysis of 24 placebo-controlled statin studies and 2 high versus low-dose statin studies found no evidence of renal injury from statin use (54). These discrepant results can be explained by the quality of the data: in randomized controlled trials, albeit not designed or powered to look at renal injury, data quality tends to be higher than that in administrative data sets, which often contain bias for selection, ascertainment, and classification. Furthermore, statins appear to have a nephron-protective role in the prevention of contrast induced acute kidney injury. A meta-analysis of 15 trials examining the effect of statin pre-treatment before coronary angiography found a significant reduction in acute kidney injury in those treated with high dose statin compared to controls treated with either placebo or low dose statin (55). One study specifically examined the use of statins in subjects with diabetes and existing CKD undergoing angiography, and found a benefit to statin pre-treatment in reducing the risk of contrast induced acute renal injury (56). As discussed above, use of statins in pre-end stage CKD or post-renal transplant patients demonstrates neither benefit nor harm on renal outcomes. Thus, based on available evidence there does not seem to be any renal harm from statin use, and the presence of CKD should not be a contra-indication to statin use, although some statins require dose restrictions in CKD (Table 3).

Statin Safety – Diabetes Outcomes

As a class, emerging evidence demonstrates that statins increase new diagnoses of diabetes (57). As diabetes can lead to or exacerbate renal injury, this is another potential harm of statins. However, there is no evidence that statin therapy acutely raises normal fasting glucose into the diabetic range and rather the evidence from clinical trials suggests that statin therapy instead leads individuals at high risk of diabetes to progress to diabetes diagnosis sooner than may have happened without statin therapy. A subsequent meta-analysis of 5 statin trials with >32,000 patients without diabetes at baseline found that high dose statin was associated with increased risk for new diabetes diagnosis compared to low or moderate dose statin therapy (58). However, the number needed to harm (induce diabetes) is 498 whereas the number needed to treat (prevent cardiovascular events) is 155 for intensive statin therapy; implying that despite the increased risk of new onset diabetes, statin therapy’s benefits outweigh the risks.

Which Statins to Use in CKD?

The various statins have different degrees of renal clearance; thus, with CKD patients it is important to be aware of the metabolism of the agent of interest and understand if/when dose adjustments are needed. Most statins are primarily metabolized through hepatic pathways, and dose adjustment in early CKD is typically not needed (eGFR> 30 ml/min). However, with more advanced CKD, eGFR< 30 ml/min (or ESRD, although statins are not indicated in this population) most agents have maximum dose restrictions (Table 3). 

Table 3. Statin Dosing in CKD

Statin

Usual dose range (mg/d)

Clearance route

Dose range for CKD stages1-3

Dose range for CKD stages4-5

Use with cyclosporine

Atorvastatin

10-80

Liver

10-80

10-80

Avoid use with cyclosporine

Fluvastatin

20-80

Liver

20-80

20-40

Max dose 20 mg/d with cyclosporine

Lovastatin

10-80

Liver

10-80

10-20

Avoid use with cyclosporine

Pitavastatin

1-4

Liver/Kidney

1-2

1-2

Avoid use with cyclosporine

Pravastatin

10-80

Liver/Kidney

10-80

10-20

Max dose 20 mg/d when used with cyclosporine

Rosuvastatin

10-40

Liver/Kidney

5-40

5-10

Max dose 5 mg/d with cyclosporine

Simvastatin

5-40

Liver

5-40

5-40

Avoid use with cyclosporine

BEYOND STATINS

There has been relatively little research into the use of non-statin lipid lowering agents in CKD. There is an emerging interest in niacin in CKD patients for its phosphorus-lowering effects, and niacin has similar lipid-altering efficacy in CKD as opposed to non-CKD subjects. Fibrates are metabolized via the kidney and thus generally contraindicated in CKD. Ezetimibe has been shown to be safe and effective in reducing LDL for patients with CKD; however, studies have typically compared treatment with ezetimibe added to statin therapy vs control and few studies compare ezetimibe monotherapy vs control.  PCSK9-inhibitors have been shown to be safe in CKD and efficacious in lowering LDL but there remains limited data regarding morbidity and mortality outcomes with this therapy.  Newer therapies include bempedoic acid and inclisiran which both remain relatively unstudied in CKD/ESRD. The following sections summarize the available data on the use of other lipid lowering agents in CKD (Table 4).

Niacin

As niacin is not cleared via the kidney it is theoretically safe in CKD; however, its use is limited due to side effects (predominantly flushing) and a lack of data. Several short-term studies have evaluated niacin in CKD patients and it is efficacious in lipid lowering. There is an emerging interest in use of niacin or its analog niacinimide in CKD and ESRD patients for their effects to decrease phosphate levels. A meta-analysis of randomized controlled trials of niacin and niacinamide in dialysis patients found that niacin reduced serum phosphorus but did not change serum calcium levels; furthermore niacin increased HDL levels but had no significant effect on LDL-cholesterol, triglycerides, or total cholesterol levels; no CVD outcomes data were provided (59). Animal studies have suggested a beneficial effect of niacin on renal outcomes (60), and clinical literature is suggestive that this may occur in humans (61). Kang et al treated patients with CKD stages 2-4 with niacin 500mg/d x 6 months; niacin led to increased HDL-cholesterol and decreased triglyceride levels, and improved GFR compared to baseline levels (62). Laropiprant has been developed as an inhibitor of prostaglandin-medicated niacin-induced flushing. In a sub-study examining the use of niacin with laropiprant in dyslipidemic subjects with impaired renal function, the use of niacin resulted in a mean decrease in serum phosphorus of 11% with similar effects between those with eGFR above or below 60 ml/min/1.73 m2 (63); the parent study reported significant reduction in lipid parameters including a decrease in LDL-cholesterol of 18%, decrease in triglycerides of 25%, and an increase in HDL of 20% (64). Thus, there may be an indication for use of niacin in CKD subjects beyond lipid lowering considerations. However, cardiovascular outcome studies evaluating the combination of statin plus niacin have not found any additional benefit compared to statin alone (65, 66); thus, at this time further research is needed in CKD subjects to determine if niacin may be more beneficial than statins, or if the addition of niacin to statin may confer non-CVD benefit, for example, from phosphorus lowering.

Fibrates

Fibric acid derivatives are used primarily to raise HDL-cholesterol and lower triglycerides; thus, they target two major components of CKD associated dyslipidemia. However, fibrates are known to decrease renal blood flow and glomerular filtration and they are cleared renally (67); therefore, there is significant concern regarding their use in CKD. Furthermore, the fibric acid derivatives raise serum creatinine levels and may thus trigger medical investigations into renal disease progression. Thus, there is concern regarding their use in CKD. However, there is a potential for fibric acid derivatives to improve both CVD and CKD outcomes. The acute changes in serum creatinine do not necessarily indicate adverse renal effects. A meta-analysis (68)  examined the use of fibrates in CKD subjects and reported beneficial effects to reduce total cholesterol and triglyceride levels and raise HDL-cholesterol levels with no effect on LDL-cholesterol levels. In addition, 3 trials reporting on > 14,000 patients reported that fibrates reduced risk of albuminuria progression in diabetic subjects, with 2 trials (>2,000 patients) reporting albuminuria regression (69-71). This was associated with a reduction in major cardiovascular events, CVD death, stroke, and all-cause mortality in subjects with moderate renal dysfunction, but not in those with eGFR > 60 ml/min/1.73m2.  Thus, despite the elevations in serum creatinine seen with fibrates, there is the potential for both cardiac and renal benefit, and further studies specifically designed to evaluate these outcomes in CKD subjects are urgently needed. At this point, providers are encouraged to consider fibrate therapy for appropriate subjects, especially if statins are not tolerated or are contra-indicated.

Ezetimibe

Ezetimibe is presently the only member of the class of cholesterol absorption inhibitors. As monotherapy it can lower LDL approximately 15%; however, the majority of research has examined ezetimibe in combination with a statin (primarily simvastatin) where the addition of ezetimibe can induce a further 25% lowering of LDL cholesterol. Ezetimibe is metabolized through intestinal and hepatic metabolism, and does not require any dose adjustment in CKD or ESRD, making it potentially attractive therapy in CKD. The Improved Reduction of Outcomes: Vytorin Efficacy International Trial (IMPROVE IT) study demonstrated that the combination of statin + ezetimibe led to further LDL lowering and improved CVD outcomes compared to statin alone in high-risk patients (72).  A secondary analysis of this study evaluating outcomes based on eGFR shows that compared to statin alone, the combination of statin + ezetimibe was more effective in reducing risk of CVD outcomes in those with eGFR < 60/ml/min/1.73m2(73). The Study of Heart and Renal Protection (SHARP) compared CVD and renal effects in CKD patients treated with statin + ezetimibe versus placebo. There was a reduction in CVD events (39); however, there was no effect on renal disease progression (74). Note, neither of these studies included an ezetimibe only arm; thus, the effects of ezetimibe monotherapy on outcomes are unknown, although it can be expected to reduce CVD events in proportion to its degree of LDL-cholesterol lowering. A small study evaluating ezetimibe monotherapy in CKD patients found it safe and effective (75). Thus, the use of ezetimibe with or without statin is likely to benefit pre-end stage CKD patients in terms of CVD outcomes (given that the impact of ezetimibe is on lowering LDL-cholesterol we can anticipate lack of CVD benefit in ESRD subjects based on the statin studies and SHARP).

Fish Oil

Omega-3 polyunsaturated fatty acids can lower triglyceride levels, making them a potential therapy in CKD. The role of fish oil/ omega-3 supplements in the general population for prevention of CVD events remains unclear, with some studies suggesting benefit but others finding no CVD protection. A recent meta-analysis found no evidence for CVD protection (76) while a meta-analysis of thirteen randomized control trials involving 127,477 patients demonstrated marine omega-3 supplementation was associated with small but significantly lower risk of MI, CHD death, total CHD, CVD death and total CVD with linear relationship to dose (77).  In CKD patients there is little data to support the use of fish oil and much of the data it is conflicting. A small randomized study evaluated omega-3 fish oil supplements, coenzyme Q10, or both in subjects with CKD stage 3 for 8 weeks. The group that received the omega-3 supplements had decreased heart rate and blood pressure and triglycerides, but there was no effect on renal function (eGFR, or albuminuria) (78). Conversely, a study evaluating dietary omega-3 intake found that higher consumption was associated with reduced likelihood of CKD (79). A randomized controlled trial in patients with CKD and microalbuminuria showed that omega-3 fatty acid supplementation had no effect on urine albumin excretion; however, there was a beneficial effect on serum triglyceride levels and pulse wave velocity (80). Fish oil supplementation has not been found to have any clear benefit on hemodialysis arteriovenous graft function (81, 82)or on cardiovascular events or mortality in hemodialysis patients (83). Thus, there is no clear benefit to the use of fish oil supplements in CKD, but further research is needed.

Bile Acid Resins

The bile acid resins tend to be used less commonly than other classes of lipid lowering agents overall, and their use in CKD is limited by a lack of data. Bile acid resins as a class can lower LDL-cholesterol by 10-20% so they are less effective than statins; furthermore, they can raise triglyceride levels and their use is contra-indicated with elevated triglyceride levels, for example > 400-500 mg/dl (>4.5 – 5.6 mmol/L). Thus, overall bile acid resins are rarely used in CKD patients. However, their metabolism is intestinal and thus there are no required modifications for their use in mild-moderate CKD. Although there are no theoretical concerns regarding their use in ESRD there is no data to address safety or efficacy.

PCSK9 Inhibitors

Monoclonal antibodies against proprotein convertase subtilisin/kexin type 9 (PCSK9) have been developed and approved for patients for patients with clinical atherosclerotic CVD not meeting lipid goals despite maximally tolerated statin therapy. This class of drugs lowers LDL-C in addition to statin-mediated lowering and has been shown to decrease CVD events in outcome studies in secondary prevention populations (84). Two PCSK9 monoclonal antibody inhibitors are presently available in the US – evolucumab and alirocumab. PCSK9 plasma levels are not influenced by eGFR in CKD patients (85) but are increased in nephrotic syndrome (86). As monoclonal antibodies the inhibitors are not cleared by the kidney and thus are approved to use in CKD and ESRD with no dose adjustment. The ODYSSEY OUTCOME trial randomized post-acute coronary syndrome patients with LDL > 70mg/dL to maximally tolerated statin with placebo vs alirocumab; the intervention arm with alirocumab had nearly twice the absolute reduction in cardiovascular events (87). Of note patients with eGFR < 30 ml/min/m2 were excluded from the ODYSSEY OUTCOME trial. However, a later subanalysis looked at the effect of alirocumab on major adverse cardiovascular events based on renal function. The subanalysis showed that irrespective of eGFR alirocumab was efficacious in reducing LDL. Further, annualized incidence rates of major adverse cardiovascular events and death increased with decreasing eGFR but rates were lower in the alirocumab group compared to placebo and there was no significant difference in incidence of major adverse cardiovascular events between treatment groups with eGFR < 60 ml/min/m2 (88).  Further, data from a pooled analysis of nine trials comparing alirocumab vs control showed that among patients with ASCVD and LDL > 100 mg/dL those with additional risk factors including CKD had the greatest absolute cardiovascular benefit from alirocumab therapy in addition to maximally tolerated statin compared to placebo (89).  Studies remain ongoing to further look at mortality and morbidity outcome in PCSK-9 inhibitors specifically in at risk patients such as those with CKD. There remains very limited data to use in patients with ESRD and PCSK-9 inhibitor use as monotherapy for dyslipidemia.

Bempedoic Acid

Currently approved for use in combination with maximally tolerated statin therapy, bempedoic acid facilitates further LDL reduction by inhibiting cholesterol synthesis in the liver through blocking adenosine triphosphate-citrate lyase (ACL).  Currently, use in CKD is approved without dosage adjustment for eGFR > 30ml/minute/1.73m2; however, below this eGFR threshold there is insufficient data to guide its use. As bempedoic acid has hepatic metabolism it is presumably safe in CKD.  A 52-week study in very high-risk CVD patients demonstrated that bempedoic acid added to maximally tolerated statin therapy was safe and led to a significant reduction in LDL levels (90).  Further, combination with ezetimibe is safe and can increase the cholesterol-lowering effect more than either agent alone when added to standard therapy (91). A cardiovascular outcome study is presently underway (92) but at this time there are limited data regarding mortality and morbidity benefit and use in ESRD.

Inclisiran

Newest to the market, inclisiran is a small interfering RNA (siRNA) that acts in hepatocytes to break down mRNA for PCSK-9 which increases LDL cholesterol receptor recycling thus increasing LDL cholesterol uptake. It is FDA approved for use in heterozygous familial hypercholesterolemia and in secondary prevention of cardiovascular events as an adjunct to lifestyle and maximally tolerated statin. It is administered by subcutaneous injections at 3 and then 6-month intervals. There are no cardiovascular outcomes studies yet available. There is no recommended dosage adjustment in CKD, but there have been no studies done in patients with ESRD. An analysis of the ORION-1 and ORION-7 studies compared inclisiran in patients with renal impairment and those with normal renal function, and found similar safety and efficacy, suggesting no dose adjustment is needed in CKD (93). However, no patients on dialysis were studied in these trials.

Table 4. Non-Statin Treatments

Agent

Usual dose range (mg/d)

Clearance route

Dose range for CKD stages1-3

Dose range for CKD stages4-5

Use with cyclosporine

Niaspan

500-2000

Hepatic/renal

No data

No data

No data

Gemfibrozil

1200

Renal

Avoid if creatinine > 2.0 mg/dl

Avoid if creatinine > 2.0 mg/dl

Cautious use

Fenofibrate

40-200

renal

40-60

avoid

Cautious use

Ezetimibe

10

Intestinal/hepatic

10

10

Cautious use

Colsevelam

3750 (6 x 625 mg tablets daily)

Intestinal

No change

unknown

May reduce levels of cyclosporine

Fish oil

4000

 

No change

Caution

No data

PCSK9 inhibitors

Alirocumab 75-150mg SC q 2 weeks

Evolocumab 140mg weekly SC - 420mg monthly SC

Unknown

No change

Not defined

No data

Bempedoic acid

180 mg daily

Hepatic

No change

Not defined

No data

Inclisiran

284 mg SC at 0 and 3 months then every 6 months

Nucleases

No change

Not defined

No data

SUMMARY

CVD is the leading cause of mortality in CKD, and as with the non-CKD population dyslipidemia is a significant contributor. The dyslipidemia of CKD comprises primarily high triglyceride levels and low HDL-cholesterol levels; however, emerging data suggests that the composition of the lipoprotein particles is altered by CKD, and that altered composition and/or lipoprotein cargo may be a cause of the increased CVD in CKD. The use of statins has been shown to be safe and efficacious in lipid lowering in CKD, and of benefit in reducing CVD events in individuals with pre-end stage CKD, or post renal transplant, but not in dialysis patients. The various available agents have different clearance routes, and some statins need dose adjustment in CKD. In patients that cannot tolerate or who have contra-indications to statin therapy, there may be some benefit from use of PCSK9 inhibitors, fibrates niacin or newer therapies such as bempedoic acid and inclisiran, but further studies are needed to better investigate their use.

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Immune Checkpoint Inhibitors Related Endocrine Adverse Events

ABSTRACT

 

Immune checkpoint inhibitors (ICIs) are currently used for the treatment of various types of cancers. Despite the important clinical benefits, these medications can lead to a spectrum of side effects called immune-related adverse events (irAEs). Endocrine irAEs are among the most common irAEs that have been reported in clinical trials and post-marketing settings with an overall incidence of around 10% of patients treated with ICIs. These include hypothyroidism, hyperthyroidism, hypophysitis, primary adrenal insufficiency, insulin‐deficient diabetes mellitus, hypogonadism, hypoparathyroidism, hypocalcemia, and other less commonly reported side effects. The symptoms can sometimes be nonspecific but life-threatening. Hence, physicians should be aware of the endocrine irAEs which can occur anytime during treatment or even after discontinuation of the medications. In this chapter, we will be discussing in detail the ICI-related endocrine irAEs and their management. In addition, we will be suggesting an algorithm to be used in the clinical setting for screening and monitoring of the endocrine iRAEs.

 

INTRODUCTION

 

Immune checkpoint inhibitors (ICIs) are currently approved by the US Food and Drug Administration (FDA) for the treatment of various types of cancers and have significantly improved clinical outcomes and survival. Antigen-presenting cells (APCs) process and express antigens (including tumor antigens) on major histocompatibility complexes recognized by receptors on T cells, which then stimulates a cascade either to kill the cell expressing the antigen (via CD8+ effector/cytotoxic T cells) or recruit other components of the immune system (via CD4+ helper cells) (1). Many of the ligands presented by the APCs can bind to multiple receptors and deliver stimulatory or inhibitory signals, the latter being referred to as immune checkpoints. Various ligand-receptor interactions between antigen-presenting cells and T cells regulate the T cell response to the antigen (Figure 1). Agonists of stimulatory receptors or antagonists of inhibitory signals can result in amplification of antigen-specific T-cell responses (2). Cancer cells can develop tolerance to the immune system by upregulating the expression of immune checkpoint molecules like programmed cell death ligand (PD-L1) leading to peripheral T cell exhaustion or lose surface antigen expression leading to immunologic escape. ICIs help overcoming this tolerance by inhibiting the checkpoints and these inhibitory compounds currently used in pharmacologic intervention target three ligands/receptors- CTLA-4, PD-1, and PD-L1 (3).

 

Cytotoxic T-Lymphocyte-Associated Protein 4 (CTLA-4) Inhibitors

 

CTLA-4 was first described by Leach et. al. in 1996 as a receptor on T cells (3), where it acts as a physiologic brake on the T-cell activation. It competes with the CD28 stimulatory receptor present on T cells (1). Both bind CD80 and CD86 ligands (also known as B7.1 and B7.2 respectively, collectively as B7) seen on APCs, but CTLA-4 has a 500-2500 times higher affinity for these ligands than CD28 does. Blocking CTLA-4: B7 interactions favors CD28:B7 interactions, which results in proliferation of T cells, increased T cell survival, activation of T effector cells, and increased diversity of T cell responses on tumors. This is the basis of CTLA-4 inhibitor therapy with ipilimumab (trade name Yervoy) and tremelimumab (4, 5).

 

Programmed Death-1 (PD-1) and Programmed Death-Ligand 1 (PD-L1) Inhibitors

 

PD-1 receptors on the T cell interact with PD-L1 (another member of the B7 family) and inhibit T-cell expression and decrease expression of proinflammatory cytokines such as interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and interleukin -2 (IL-2) similar to CTLA-4. PD-L1 is found on leukocytes, nonlymphoid tissue, and tumor cells and modulates CD8+ T cell function (1). PD-L1 is aberrantly expressed on many cancers, including lung, ovary, colon, head and neck, and breast (6) and results in tumor cells evading the immune system (7).  Inhibition of PD-1: PD-L1 interaction increases the number of T cells and inflammatory markers at tumor sites, creating an environment more conducive to tumor suppression. Drugs that target PD-1 include pembrolizumab (Keytruda), nivolumab (Opdivo), and dostarlimab (Jemperli) while PD-L1 inhibitors include atezolizumab (Tecentriq), avelumab (Bevancio), and Durvalumab (Imfinzi). PDL-2 is expressed on dendritic cells, monocytes, and mast cells and modulates CD4+ function.

Figure 1. Interactions between antigen-presenting cells (APCs) and T cells that regulate T-cell responses. From DM Pardoll (2)

 

Immune checkpoints normally inhibit the function of T cells, which helps prevent autoimmunity but can also benefit cancer cells. ICIs prevent the apoptosis and downregulation of T cells, which allows the immune system to naturally fight malignant cells. Despite the important clinical benefits, this unique mechanism of action itself can lead to a spectrum of side effects called immune-related adverse events (irAEs). Endocrine irAEs are among the most common irAEs that have been reported in clinical trials and post-marketing settings with a meta-analysis of 38 randomized trials showing an overall incidence of endocrinopathies among 10% of patients treated with ICIs (8). These include hypothyroidism, hyperthyroidism, hypophysitis, primary adrenal insufficiency (PAI), and insulin‐deficient diabetes mellitus. Median time to onset of moderate to severe endocrinopathy is 1.75-5 months with ipilimumab and 1.4-4.9 months for any endocrinopathy with PD-1 inhibitors (9, 10). Patients with pre-existing autoimmune disorders are at higher risk of exacerbation of the autoimmune condition as well as development of an unrelated irAEs (11). Multiple large prospective studies and meta-analyses showed that irAEs are associated with improved treatment outcomes suggesting the activated immune system is also concurrently targeting the cancer (12-14).  Hence, the general principle of management of irAEs is to control symptoms with minimum amount of immunosuppression. In this article, we will be discussing in detail the ICI-related endocrine irAEs and its management. We will be suggesting algorithm for screening, monitoring and treatment of the patients and we will be listing a summary of the side effects grading system and incidence in different ICI. (Figure 2-4, Table 3-4).

 

Immune Checkpoint Inhibitor Induced Thyroid Diseases

 

ICI-mediated thyroid disease is one of the common endocrine irAEs. It can manifest as primary hypothyroidism secondary to destructive thyroiditis or as hyperthyroidism due to Graves' disease.

 

HYPOTHYROIDISM

 

ICI-mediated hypothyroidism can present as primary or secondary hypothyroidism (secondary to hypophysitis, which is discussed below). Primary hypothyroidism usually ensues after an occurrence of ICI-induced thyrotoxicosis. In a study by Abdel-Rahman et. al., authors found a higher risk of all-grade hypothyroidism compared to hyperthyroidism associated with ICIs therapy (15).

 

Incidence 

 

The incidence of hypothyroidism with the use of immune checkpoint inhibitors varies based on the type of immune checkpoint inhibitors used and monotherapy vs combination therapy. In the largest meta-analysis of 38 randomized control trials comprising 7551 patients, the overall incidence of hypothyroidism was found to be 6.6%. The incidence of hypothyroidism ranged from 3.8% with ipilimumab to 13.2% (95% CI, 6.9%-23.8%) with combination therapy (8). Various other studies have also found similar findings of higher incidence of hypothyroidism with the use of PD-1 inhibitors (7-21%) compared to CTLA-4 inhibitor (0-6%) ipilimumab (16).

 

Pathophysiology 

 

Anti-thyroid antibodies are often absent in ICI-associated hypothyroidism, suggesting a role of cell-mediated rather than humoral autoimmunity (17). In addition, some studies have suggested an increased risk of ICI-induced thyroid dysfunction among patients with pre-existing anti-thyroid antibodies compared to those without these antibodies suggesting unmasking of autoimmune destruction with the use of ICIs (18, 19). The complete pathophysiology behind the development of thyroid dysfunction is not completely understood, but increased cytokine levels following anti-PD1 therapy have been found to correlate with thyroid dysfunction (20). Fine-needle aspiration biopsy obtained during active ICI-induced thyroiditis showed lymphocytic infiltrate along with CD163+ histiocytes (21).

 

Clinical Characteristics

 

The median time to thyroid dysfunction following initiation of ICIs is 6 weeks and most of the patients develop biochemical hypothyroidism (22). Nonetheless, thyroid dysfunction can happen at any time during therapy. Most of the patient are asymptomatic or have very few symptoms. Common presenting symptoms include fatigue, depressed mood, mild weight gain, and constipation however with severe hypothyroidism, the patient can present with altered mental status (23).

 

Screening and Monitoring

 

Thyroid function tests should be performed in all the patients receiving ICIs, by measuring TSH (thyroid stimulating hormone) and free T4 (free thyroxine). In the setting of abnormal thyroid function tests, routine monitoring is recommended at 4-6 weeks or more frequently if clinically indicated. However, in presence of normal thyroid function tests, the frequency could be increased to every 12-18 weeks. ICI-induced hypothyroidism is diagnosed by the presence of elevated TSH and decreased free T4. However, TSH is the more sensitive and preferred test. Currently, anti-thyroid antibodies have not been proven to be helpful in the screening and treatment of these patients. For patients who have subclinical hypothyroidism (elevated TSH and normal Free T4), routine monitoring is recommended while continuing treatment with immunotherapy.

 

Treatment

 

The diagnosis of primary hypothyroidism is based on elevated TSH (>10 mIU/L) and low free T4 along with clinical symptoms. Once the diagnosis is established, treatment is recommended with levothyroxine supplementation. For young patients with TSH >10 and low free T4, a full replacement dose at 1.6 mcg/kg should be considered. However, in elderly patients or among patients with cardiovascular comorbidities, a lower starting dose of 50 mcg is recommended. The dose should be changed by ~10% every 4-6 weeks to achieve reference range or age-appropriate range TSH and free T4. ICIs are usually continued while treating hypothyroidism with mild to moderate symptoms (24, 25). Although the guidelines to diagnose and treat ICI–associated primary hypothyroidism is well established, the recommendations for the management of patients with subclinical hypothyroidism (mildly elevated TSH with normal free T4) is not well established and should be based on the patient’s symptoms, age, and co-morbid conditions (26, 27).

 

THYROTOXICOSIS

 

ICI-mediated thyrotoxicosis can present as transient thyrotoxicosis or persistent hyperthyroidism. Transient thyrotoxicosis is far more common among patients treated with ICIs and is often followed by primary hypothyroidism; persistent hyperthyroidism is less frequent. Hyperthyroidism is more commonly reported with combination therapy and is rare with PD-L1 inhibitors. Patients with hyperthyroidism can be symptomatic and need supportive care with beta-blockers and anti-thyroid medications in some cases.

 

Incidence 

 

The prevalence of ICI-associated transient thyrotoxicosis has varied significantly among the studies and can range from 3.0-9.0% (23, 28) and is followed by primary hypothyroidism (8). The incidence of transient thyrotoxicosis is higher among patients treated with combination therapy compared to monotherapy with anti-PD1 or anti-PD-L1 therapy (23). In the largest to date meta-analysis, the overall incidence of hyperthyroidism was estimated to be 2.9%. The incidence of hyperthyroidism ranged from 0.6% with the PD-L1 inhibitor to 8.0% with combination therapy. Combination therapy was found to have an increased risk of higher-grade hyperthyroidism compared to monotherapy. Moreover, the risk of hyperthyroidism was greater with PD-1 inhibitors compared to PD-L1 inhibitors (8). ICI-induced Graves’ disease is extremely rare, with only a few reported cases in the literature (29).

 

Pathophysiology 

 

The pathophysiology of ICI-thyrotoxicosis remains poorly understood. Autoimmunity is believed to play a critical role in leading to thyroiditis among patients treated with ICIs. In one study, combination ICI therapy (ipilimumab and nivolumab) resulted in a more robust antibody response compared to monotherapy with nivolumab, leading to faster destruction of the thyroid (30). Moreover, patients with elevated anti-TPO or antithyroglobulin antibodies required a higher dose of levothyroxine compared to those who did not have elevated antibodies. In addition to antibody-mediated thyroid destruction, circulating CD56, CD16, and natural killer cells have been implicated in the development of pembrolizumab-induced thyroiditis in one study (17). Another study found an association between PD-L1 and PD-L2 expression on the thyroid gland and destructive thyroiditis (31). Worsening of pre-existing autoimmune thyroid disease and subclinical hypothyroidism in patients treated with ICIs have also been reported, suggesting synergistic roles of autoimmunity and inflammatory mechanisms (30).

 

Clinical Characteristics

 

Most patients with thyrotoxicosis are asymptomatic or present with symptoms such as palpitation, agitation, anxiety, and insomnia (23). Although uncommon, ICI-induced Graves' disease following use of CTLA-4-inhibitor and PD1-inhibitors have also been reported and can be associated with graves orbitopathy (23). Graves’ orbitopathy can occur with and without TRAb antibodies among patients treated with ICIs (29). ICI-induced thyroid storm is extremely rare and has only been reported a few times in the literature (32, 33). Toxic autonomous nodules or toxic multinodular goiter is not associated with ICIs and if seen among patients treated with ICIs, should be considered a co-incidental finding (23).

 

Screening and Monitoring

 

Screening of ICI-induced thyrotoxicosis is performed by TSH and free T4. Thyrotoxicosis is defined as suppressed TSH and it can either be (i) clinical when free T4 is elevated or (ii) subclinical when free T4 is normal. The most common cause of ICI-induced thyrotoxicosis is thyroiditis, which is due to the destruction of thyroid follicular cells with the release of preformed thyroid hormone. This is often associated with transient thyrotoxicosis and eventually progresses to hypothyroidism in the majority (50 to 90%) of the cases (22, 28). Hence, monitoring of TFTs with TSH and free T4 every 4-6 weeks is recommended. The usual duration of thyrotoxicosis with ICIs is about 4-6 weeks (30, 34) and if thyrotoxicosis persists beyond this period, evaluation for Graves’ disease should be considered by checking thyroid-stimulating hormone receptor antibody (TRAb) or thyroid-stimulating immunoglobulin (TSI) or a thyroid uptake scan (28)( Figure 2).

 

Treatment

 

For patients with minimal symptoms of thyroiditis-associated thyrotoxicosis, and presence of suppressed TSH and elevated free T4, supportive treatment with non-selective beta-blockers such as propranolol should be considered (24). When propranolol is used, the recommended dose is 10-20 mg every 4 to 6 hours for symptomatic management and until thyrotoxicosis resolves. As most of the time, patients with ICI-induced thyrotoxicosis progress to develop primary hypothyroidism (defined by elevated TSH levels), further treatment with thyroid hormone replacement should be considered. However, in the minority of cases (such as prominent initial symptoms, significantly elevated free T4 levels, signs of Graves’ orbitopathy, or persistent thyrotoxicosis), further evaluation and treatment for Graves’ disease should be considered (30, 34). Graves’ disease should be treated with anti-thyroid medications, radioactive iodine, or surgery depending on the clinical setting and patient preference (35). Rarely, patients can develop thyroid storm and high-dose steroids should be used in conjunction with standard management among these patients (34). If asymptomatic or only mildly symptomatic, continuation of ICIs is recommended (24, 25).

Figure 2. Algorithm suggested to diagnose and treat ICI thyroid disease.

Hypophysitis

 

Hypophysitis is one of the more common endocrine side effects reported with the use of ICIs particularly with CTLA-4 antibodies and combination therapy including both CTLA-4 and PD1 or PD-L1 inhibitors. It is less likely with PD-1 or PD-L1 inhibitor monotherapy. Hypophysitis is characterized by infiltration and inflammation of the pituitary gland. It can occur in the first few weeks of treatment with frequent hormonal deficiencies at the time of diagnosis. Pituitary enlargement is considered both a highly sensitive and specific indicator of hypophysitis after ruling out metastatic disease. Moreover, the symptoms of hypophysitis can sometimes be non-specific, hence the importance of close monitoring of these patients for early diagnosis and prompt treatment (36, 37).

 

INCIDENCE

 

Hypophysitis estimated incidence was one in nine million people per year (38). ICI-induced hypophysitis has been reported in 0-17% of ICI-treated patients. Some studies showed the incidence increased up to 25% while using higher doses of ipilimumab of 10 mg/kg (36, 37, 39). There have been some variations in the observed incidence rate of ICI-induced hypophysitis which has been attributed to not only the dose of the medication but also to the difference in the use, the intensity, and the frequency of hormonal monitoring, in addition to clinical awareness of and suspicion for the condition (37, 40).

 

Adrenocorticotropic hormone (ACTH) is one of the most common hormone deficiencies in hypophysitis. In a study of ipilimumab-induced hypophysitis, 80% had central adrenal insufficiency. Lu et. al. found hypophysitis occurred in 3.25% of patients using ICIs. Of these, it was more common with combination therapy at 7.68% followed by anti-CTLA-4 at 4.53% then anti-PD-1 and anti-PD-L1 at less than 1% of cases (41). Chang et. al. found the combination of ipilimumab (anti-CTLA-4) and nivolumab (anti-PD-1) caused hypophysitis in 6.4% of patients. Incidence of anti-CTLA-4 alone was 3.2%, anti-PD-1 alone is 0.4%, and anti-PD-L1 alone was 0.1% (42). Overall, the evidence suggests that combination therapy and anti-CTLA-4 have the highest incidence of hypophysitis, while anti-PD-1 and anti-PD-L1 are less common causes of hypophysitis.

 

PATHOPHYSIOLOGY

 

The actual pathogenesis is not well defined. Since many patients had no previous immune-related disease before the development of ICI-associated hypophysitis, it was suggested that this condition is not triggered by a pre-existing immune condition. Hypophysitis was initially considered a specific irAE of ipilimumab considering the presence of pituitary expression of CTLA-4 antigens in the TSH, follicle stimulating hormone (FSH), ACTH, and prolactin-secreting cells. Now more recent data suggested that it can occur with any ICI target (CTLA-4, PD-1, or PD-L1) (43, 44). Garon Czmin et. al. reported that the time to develop hypophysitis following initiation of ICIs was significantly shorter with ipilimumab alone or combined with nivolumab (83 days) compared to nivolumab or pembrolizumab alone (165 days). Moreover, ICI-associated hypophysitis is more common in men while autoimmune lymphocytic hypophysitis has a higher prevalence in the female population (43, 45). In one study, hypophysitis with anti-CTLA was four times more common in males compared to females. This may be related to more men having melanoma, but studies controlling for this factor have found similar results (42). Corticotrophs and thyrotrophs are the most common cell types affected while gonadotroph deficiency was more common in male patients. The somatotroph axis and prolactin levels were rarely involved (36).

 

CLINICAL CHARACTERISTICS

 

Hypophysitis can occur weeks to months after the initiation of ICIs. In the study by Albarel et. al., mean hypophysitis occurred at 9-9.5 weeks ± 6 weeks after the treatment initiation with a mean age at diagnosis of 55.2 years (36). The initial symptoms are usually related to tumor mass or hormone deficiencies, and rarely visual disturbance or diabetes insipidus. Symptoms may be acute or subacute, but they are usually nonspecific, including headaches, anorexia, dizziness, nausea, weight loss, and/ or fatigue. More serious signs are hypotension, lethargy, confusion, and electrolyte abnormalities including hyponatremia. Hyponatremia occurs due to increased antidiuretic hormone (ADH) stimulated by hypothalamic secretion of CRH. The most common hormone deficiencies include TSH, ACTH, FSH, luteinizing hormone (LH). Panhypopituitarism is less likely to happen. Although hypogonadotropic hypogonadism and central hypothyroidism may resolve, central adrenal insufficiency is permanent requiring lifelong treatment (36, 42). Although, hypo enhancing lesions in the anterior pituitary are characteristic of ICI-associated hypophysitis, a few cases of PD-1/PD-L1 induced-hypophysitis have been reported in the absence of any radiographic abnormalities and just on clinical grounds (46). This suggests that PD-1/PD-L1 may not always show classic pituitary enlargement or enhancement on MRI (47).

 

SCREENING AND MONITORING

 

The National Comprehensive Cancer Network (NCCN) guidelines recommend initial serum pituitary hormonal evaluation including morning cortisol, ACTH, TSH, FT3, LH, FSH, testosterone in men, estrogen in premenopausal women, prolactin, growth hormone, and IGF1. The sodium and potassium levels should also be checked. Cosyntropin stimulation test can be normal in acute secondary adrenal insufficiency. Diagnostics radiology reports of brain MRIs in patients receiving ICIs should routinely include comparisons of pituitary size with prior studies. In case of suspected hypophysitis, a dedicated pituitary MRI is recommended.  MRI usually showed a pituitary enlargement with or without mass effect however some cases showed pituitary adenoma, empty sella syndrome, or a normal pituitary gland on the imaging studies.

 

TREATMENT

 

Once high suspicion for ICI-induced hypophysitis, an endocrinology consult is recommended. High-dose glucocorticoids should be initiated for patients with ipilimumab-induced hypophysitis who have serious mass-effect-related symptoms, such as severe headache, visual-field disturbance, or simultaneously the presence of other irAEs. Patients should be started on methylprednisolone/prednisone at 1-2 mg/kg /day until symptoms resolve, typically 1-2 weeks then taper the steroids rapidly to a physiological dose. In patients without mass effect, studies have suggested that high dose glucocorticoid therapy was not associated with improved outcomes in patients nor change in the natural history of hypophysitis, thus physiological replacement doses can be considered in these patients (28, 36, 48).ICIs should be held until acute symptoms or symptoms related to mass effect have resolved and hormone replacement is initiated (42). One study compared discontinuing ipilimumab to restarting ipilimumab and found no effect on the resolution of hypophysitis (42). In the case of central hypothyroidism, replacement should be started after steroids are initiated. Testosterone and estrogen replacement should be considered in patients with central hypogonadism after discussing the risks and benefits of the medications (28).

 

Adrenalitis

 

Primary adrenal insufficiency (PAI), although being a rare endocrine irAEs is a potentially serious condition with significant morbidity and mortality if not identified early. Metastasis to the adrenal gland should be excluded. Other causes of adrenal insufficiency include sudden withdrawal of glucocorticoids and central adrenal insufficiency related to hypophysitis (49).

 

INCIDENCE

 

PAI is a rare side effect of ICIs, but early identification is essential given the risk of severe outcomes including death. Early evidence of adrenal insufficiency from ICIs came from case reports, but increasingly more evidence is available from larger studies and meta-analyses (50).

A review and meta-analysis by Barroso-Sousa et. al. that included 62 studies with 5831 patients, found the incidence of PAI was 0.7% for single ICI and 4.2% for combinations of ICIs (8). Another review and meta-analysis by Lu et. al. that included 160 studies and 40,432 patients, examined the rate of pituitary-adrenal dysfunction but did not distinguish the cause of adrenal insufficiency. One complicating factor in the study of PAI is that similar symptoms could occur from hypophysitis or discontinuation of steroids (41). Lu et. al. found adrenal insufficiency occurred in patients on ICIs in 2.43% of cases (ranging from 0-6.4% in studies) with serious grade adrenal effects in 0.15% of cases (ranging from 0-3.3%). Anti-CTLA-4 was associated with higher rates of adrenal insufficiency at 5.32% and serious grade events at 0.42%. Combination therapy also resulted in higher rates of adrenal insufficiency at 4.05%. Anti-PD-1 and anti-PD-L1 accounted for a smaller proportion of events at 0.49% and 0.43% respectively (41).

 

Grouthier et. al. used the World Health Organization global database, VigiBase, to examine individual safety reports for PAI and ICIs (4). The study found 451 cases of PAI, of which 45 were definite PAI and 406 possible PAI. In the study, 90% of cases involved significant morbidity including prolonged hospitalization, life-threatening illness, and disability. The mortality rate was 7.3%. Importantly the mortality rate appeared to be similar across immunotherapy treatments and combination treatments (4). This suggests that despite a relatively low incidence rate of PAI from ICIs, providers need to be able to identify these cases to prevent the significant risk of morbidity and mortality. 

 

PATHOPHYSIOLOGY

 

PAI is most frequently caused by autoimmune adrenal insufficiency (AI). Autoimmune AI is seen predominantly in women who make up between 54% to 83% of cases. In contrast, males accounted for the majority of ICI-related PAI cases at 58%, while females accounted for 36% of cases. In the remaining 6%, sex was unspecified. Autoimmune AI generally occurs between 30 to 50 years of age. In contrast, the age of onset with PAI caused by ICIs was 66 years on average with a range of 30-95 years old (1). In autoimmune AI, antibodies to the adrenal cortex including anti-21-hydroxylase are found in 83% to 88% of cases (4). The same antibodies have been found in case reports of ICI-related PAI (42). Adrenal metastasis should be excluded during the workup of adrenal insufficiency.

 

CLINICAL CHARACTERISTICS

 

Symptoms of PAI related to ICIs are similar to PAI from other causes. Symptoms include fatigue, postural dizziness, orthostatic hypotension, anorexia, weight loss, and abdominal pain. Adrenal crisis is suggested by altered mental status, weakness, syncope, nausea, and vomiting (42). In 52% of cases, other irAEs were also present. Other endocrinopathies made up 14.9% of these irAEs. The median time to onset was 120 days (ranging from 6-576 days) from starting the ICIs (4). Lab findings include hyponatremia, hyperkalemia, hypoglycemia, hypercalcemia, low aldosterone, elevated renin, elevated ACTH, and low to low normal cortisol. Imaging may reveal adrenalitis with enlarged adrenal glands. Interestingly, one case report found imaging evidence of adrenalitis present on a positron emission tomography (PET) scan after starting ipilimumab, but no symptoms or biochemical evidence of adrenal insufficiency. Repeat imaging revealed normal adrenal glands months later. This case suggests adrenalitis may occur without adrenal insufficiency (42).

 

SCREENING AND MONITORING

 

The NCCN guidelines recommend checking morning cortisol before each treatment or every four weeks during treatment. Additionally, follow-up testing is recommended for an additional six to twelve weeks. If cortisol is low or subnormal, ACTH monitoring is recommended. To monitor for pituitary and thyroid dysfunction, TSH and T4 monitoring at similar intervals are also recommended (28). In a review by Chang et. al., monitoring was recommended only in symptomatic patients, but a low index for suspicion was recommended as symptoms are nonspecific. When a patient has suspicious symptoms for adrenal dysfunction, ACTH and cortisol should be obtained before corticosteroid treatment only if this can be done safely. Additionally, measuring renin and aldosterone is helpful to determine if mineralocorticoid deficiency is present. This can be particularly helpful as case reports of central and PAI coexisting have been reported. The utility of adrenal autoantibodies, including 21-hydroxylase, is not well-established (42).

 

TREATMENT

 

PAI caused by ICIs is treated the same as other causes of PAI. If adrenal crisis or other critical illness is present, stress dose steroids with 100mg IV then 50mg IV every six hours is initiated. In stable patients, 15-25mg hydrocortisone is started in divided doses. Fludrocortisone is used to treat mineralocorticoid deficiency in PAI starting at 0.5-1mg daily. Additionally, patients will need to be educated on sick day rules and be provided with medical alert bracelets, and have high-dose corticosteroids for emergency purposes (28). The other important aspect of treatment is the decision to continue the ICIs. Holding the ICIs is recommended upon identification of adrenal insufficiency. Restarting immunotherapy can be considered after stabilization on hydrocortisone and fludrocortisone replacement.

 

Type 1 Diabetes

 

Rapid onset of autoimmune diabetes has been reported with ICIs use.  It is a rare but life-threatening side effect as it can present with diabetic ketoacidosis (DKA). The diabetes is permanent and requires lifelong treatment with insulin therapy (51). Notably, ICI-induced type 1 diabetes (T1D) has been reported with all clinically available PD-1 (nivolumab, pembrolizumab) and PD-L1 inhibitors (avelumab, durvalumab, atezolizumab) but rarely with the CTLA-4 inhibitor (ipilimumab).

 

INCIDENCE

 

The incidence of ICI-induced T1D comes from large case series at academic medical centers reporting 27 cases out of 2960 patients receiving ICI therapy (0.9%) (52) and 1/1163 (1.8%) (53).  Additionally, the prescription label for nivolumab reports that 17/1994 (0.9%) cases developed T1D (54). However, when examining the clinical trials evaluating the efficacy of PD-1 and PD-L1 inhibitors, there is a wide range of reported hyperglycemia and diabetes (55-64) (Table 1). From this analysis, hyperglycemia or diabetes was reported in approximately 2.5% of treated individuals.

 

Table 1. Clinical Trials Reporting Hyperglycemia/Diabetes with ICIs Use

Authors, Journal and Publication Year

Cases (n)

Study

Participants

(n)

Sideeffect (%)

Side effect

Drug

Cancer Type

Hamid et. al. NEJM, 2013 (55)

4

135

2.96

Hyperglycemia

Lambrolizumab

Melanoma

Borghaei et. al. NEJM, 2015 (56)

13

287

4.52

Hyperglycemia

Nivolumab

Lung

Motzer et. al. NEJM, 2015 (57)

9

406

2.21

Hyperglycemia

Nivolumab

Renal cell

Robert et. al. NEJM, 2016 (58)

1

206

0.48

Diabetes

Nivolumab

Melanoma

Nghiem et. al. NEJM, 2016 (59)

1

26

3.84

Hyperglycemia

Pembrolizumab

Merkel-cell

Kaufman et. al. Lancet, 2016 (60)

1

88

1.13

Type 1 Diabetes

Avelumab

Merkel-cell

Reck et. al. NEJM, 2016 (61)

1

154

0.64

Type 1 Diabetes

Pembrolizumab

Lung

Heery et. al. Lancet, 2017 (62)

3

53

5.66

Hyperglycemia

Avelumab

Solid tumors

Weber et. al. NEJM, 2017 (63)

2

452

0.44

Diabetes

Nivolumab

Melanoma

Choueiri et. al. Lancet, 2018 (64)

7

55

12.7

Hyperglycemia

Avelumab

Renal cell

 

Of note, most of the clinical trials in Table 1 excluded patients with a preexisting autoimmune condition, and some even excluded patients with a family history of autoimmunity. As these therapies are now being more widely used in clinical practice, there is an increased reporting of ICIs-induced diabetes (65). This is likely due to the increasing use of ICIs therapy and differences in patient populations between phase 2/3 clinical trials and clinical practice. Although T1D is a relatively rare occurrence with ICIs therapy, the events are clinically significant.

 

PATHOPHYSIOLOGY

 

The first case series reporting ICIs-induced autoimmune diabetes was described in 2015 (66). In this series of five patients, both humoral and cellular diabetes-associated autoimmunity were described. Some patients had positive T1D associated autoantibodies and diabetes-specific CD8+ T cells in the peripheral blood, consistent with findings from childhood-onset T1D (66).

The role of the PD-1/PD-L1 pathway in preclinical animal models of T1D has been appreciated for over a decade. Non-obese-diabetic (NOD) mice develop spontaneous autoimmune diabetes so it has been used extensively as an animal model to understand the mechanisms of T1D development (67). NOD mice with a knockout of either PD-1 or PD-L1 (but not PD-L2) have accelerated onset of diabetes with lymphocytic infiltration of the pancreatic islets (e.g., insulitis) compared to mice with these immune regulatory molecules (68, 69). Furthermore, administration of anti-PD-1or PD-L1 monoclonal antibodies to NOD mice also accelerated the onset of T1D (70). When examining the islets in NOD mice, insulin-producing beta-cells express PD-L1 during the progression of autoimmune diabetes (71). Similar to NOD mice, human islets from T1D organ donors exhibit upregulation of PD-L1, which was strongly associated with insulitis (72). This likely represents a protective mechanism for beta-cells to lessen their autoimmune destruction. These studies may explain why anti-PD-1/PD-L1 therapies induce T1D, while there is an absence of diabetes with anti-CTLA-4 therapy, whose ligands are CD80 and CD86 on antigen-presenting cells such as B cells, dendritic cells, and macrophages.

 

CLINICAL CHARACTERISTICS

 

Over the last 4 years, cases have described rapid-onset insulin-dependent diabetes with undetectable C-peptide levels (a measure of residual beta-cell function) and both positive and negative T1D associated autoantibodies at presentation (73, 74). Cases of ICIs-induced T1D have remained insulin-dependent even upon stopping therapy. Steroid treatment has not been able to reverse T1D, and as expected, blood glucose worsens with steroid administration (75, 76).

 

ICIs-induced T1D is mostly reported in older patients (50-70 years old) due to the nature of end-stage cancers developing later in life. More cases have been reported with anti-PD-1 therapies (nivolumab and pembrolizumab) as these agents were approved before monoclonal antibodies targeting anti-PD-L1 (51, 66, 73, 74, 77). Melanoma is the most common cancer in patients that present with ICIs-induced T1D, likely due to this being the first approved indication for ICIs therapy, and more patients with melanoma have been exposed to ICIs therapy compared to other cancer types. However, with the expanding indications and recent approval of ICIs therapy for use in pediatric cancers, ICIs-induced T1D may increase and present in younger individuals (78).

 

METABOLIC FEATURES

 

ICIs-induced T1D presents within days to a year after the initiation of PD-1 or PD-L1 therapy. HbA1c, which is a measure of the average blood glucose over the preceding three months, is generally lower than 10% at presentation with most patients presenting between 7 to 8%. As these values are mildly elevated, this suggests significant hyperglycemia over a short period rather than a gradual increase in hyperglycemia over a longer period. Most of the patients present with severe DKA that can be life-threatening. In most cases, C-peptide levels were inappropriately low for the presenting blood glucose or undetectable; ‘honeymoon’ periods tend to be absent after diagnosis. These observations suggest a destruction of beta-cell mass. In some patients, increased amylase and/or lipase has beenreported, suggesting more generalized pancreatic inflammation (52, 79).

 

IMMUNOLOGIC FEATURES

 

At least one T1D associated autoantibody, directed against insulin, glutamic acid decarboxylase (GAD), islet antigen-2 (IA-2), and zinc transporter 8 (ZnT8), was reported in 40-50% of the cases (52, 79). Almost all antibody-positive cases had GADA antibodies; however, not all four major autoantibodies were reported or measured in these case series. It is speculated that there is an association between antibody presence and earlier onset of ICIs-induced T1D in a subset of patients. In one case, positive conversion of antibodies after ICIs therapy was reported (52). Polyclonal and predominantly IgG1 subclass for GADA was shown at the presentation of another case that developed T1D five days after the initiation of PD-1 inhibitor therapy. Since IgG antibodies are involved in memory immune response and the short time interval from the initiation of anti-PD-1 treatment to the onset of T1D, these antibodies were likely present before the start of therapy (51). Based on these findings, a subset of patients developing ICIs-induced T1D likely have preexisting T1D associated antibodies which may be an early form of latent autoimmune diabetes of adulthood (LADA); however, prospective studies measuring T1D associated antibodies before the start of ICIs therapy are needed to evaluate this hypothesis.

 

GENETIC RISKS

 

Human leukocyte antigen (HLA) genes on chromosome 6 confer genetic risk for many autoimmune disorders including childhood-onset T1D (80). The polymorphic class II HLA genes (DQ, DR, and DP) confer this risk, especially the DR4-DQ8 and DR3-DQ2 haplotypes (81, 82). Only a small number of cases with ICIs-induced T1D have reported HLA genes with some having T1D risk alleles. In one case series, the frequency of HLA-DR4 was found to be enriched in those with ICIs-induced T1D compared to rates among Caucasians in the US population (52, 79). Further research is necessary to identify HLA and other genetic variants that may confer risk for ICIs-induced T1D.

 

COMPARISON TO CHILDHOOD-ONSET TYPE 1 DIABETES

 

We believe it is useful to compare the current knowledge of ICIs-induced T1D to prototypical childhood-onset T1D (Table 2). The age of onset is distinctly different between the two types of diabetes. Presentation with DKA is more common and the onset of diabetes more rapid than traditional T1D. T1D associated autoantibodies are present in ~90% of children and adolescents with T1D compared to half of the reported cases in ICIs-induced T1D. There is a predominance of GAD autoantibodies at the presentation of ICIs-induced T1D; however, more research is needed to measure all four major T1D associated autoantibodies in these patients and those directed against post-translationally modified antigens may also reveal insights into the pathogenesis of the disorder. C-peptide levels are low or undetected in those treated with ICIs therapy that develops T1D compared to C-peptide levels that vary and gradually go down after the diagnosis of childhood T1D. As a corollary, the honeymoon phase is generally absent in ICIs-induced T1D (80-83).

 

Table 2. Comparison Between Prototypical and Immune Checkpoint Inhibitor-Induced Type 1 Diabetes

Characteristics

Prototypical Type 1 Diabetes

Immune Checkpoint Inhibitor-Induced Type 1 Diabetes

Age of Onset

Peak in early childhood & adolescence

Later adulthood, 60’s

Diabetic ketoacidosis at Onset

Common

Very common

Pathophysiology

Autoimmune (years)

Autoimmune (days to months)

Autoantibodies

Present in 90-95%

Present in ~50%*

HLA Risk Genes

~90%

75-80%+

C-peptide at presentation

Varies

Low/absent

Honeymoon phase

Present

Absent

*Predominantly GADA antibodies;

+Small sample size, as not all cases report HLA alleles; there is an association with HLA-DR4

 

SCREENING AND MONITORING

 

The most updated recommendation on screening for diabetes in patient receiving ICIss comes from 2018 American Society of Clinical Oncology (ASCO) clinical practice guidelines, which recommends monitoring blood glucose at baseline, with each treatment cycle for 12 weeks and then every 3-6 weeks thereafter (24). In cases with suspected T1DM such as new onset hyperglycemia >200 mg/dl, random blood sugar >250 or history of T2DM with glucose levels >250 mg/dl, further testing for ketosis and anion gap is recommended (24). Discussing the risk of developing T1D with patients and educating them about the signs and symptoms of diabetes and DKA are recommended. Based on the current evidence, patients who have positive T1D associated antibodies and certain HLA alleles may have an increased risk to develop diabetes, so screening antibodies and reporting HLA alleles before the initiation of treatment may identify these patients with greater risk.

 

A retrospective study evaluated fasting blood glucose levels of patients receiving ICIs treatment during patient visits and showed no detectable upward drift of glycemia before DKA presentation (83). This is likely due to the rapid onset and progression of ICI-induced T1D. However, we believe monitoring blood glucose and HbA1c levels during patient visits are still necessary. Considering the rapid onset of diabetes, this approach alone may miss a significant amount of hyperglycemia and DKA. We recommend routine self-monitoring of blood glucose by patients and/or using continuous glucose monitoring to recognize hyperglycemia before DKA presentation. Close monitoring of patients with preexisting autoimmunity may also be useful (51). Our suggested screening and monitoring algorithm is depicted in Figure 3.

 

Figure 3. Proposed algorithm to screen and manage patients for ICI-induced T1D. (DKA = diabetic ketoacidosis; HbA1c = Hemoglobin A1c, T1D= Type 1 diabetes, HLA = human leukocyte antigen, CGM = continuous glucose monitor)

Hypogonadism

 

The effects of ICIs on sexual function are not very well known. ICI-induced primary hypogonadism is rare but a life-changing side effect, as it can potentially lead to infertility. Notably, gonadal dysfunction has been reported for ipilimumab monotherapy or in combination with PD-1/PD-L1 inhibitors (84). The long-term effects are still largely unknown. ICI-induced male hypogonadism is characterized by a deficiency in testosterone, which can be due to testicular, hypothalamic, or pituitary abnormalities. ICI-associated hypophysitis is discussed separately, and this section will primarily focus on ICI-induced primary hypogonadism.

 

INCIDENCE

 

Although, ICI-associated hypogonadism can be seen in patients who develop panhypopituitarism secondary to ICI-associated hypophysitis, the true occurrence of primary hypogonadism is uncommon and is based on a few case reports and ongoing studies (84-86). A recent analysis of VigiBase, the WHO global database of individual case safety reports between 2011 and 2019, found only 1 case of primary hypogonadism (87). This surprisingly low incidence may in fact be due to lack of proper evaluation looking for primary hypogonadism. For example, many studies reporting occurrence of secondary hypogonadism lacked data on the levels of pituitary gonadotropins, FSH and LH, which is necessary to differentiate between primary and secondary hypogonadism (43). Moreover, the majority of the pivotal trials leading to FDA approval of ICIs lacked information regarding fertility, menopause status, sex hormone levels, or sexual health-related quality of life. Additionally, not much is known about ICI-associated infertility. In a study of patients with malignant melanoma treated with ICIs, 6 of 7 men (86%) with testicular autopsy tissue samples had impaired spermatogenesis (88). This may suggest higher prevalence of infertility among men receiving ICIs. No data on potential effects on female fertility are currently available.

 

PATHOPHYSIOLOGY

 

ICIs may cause irAEs affecting any organ in the body by blocking regulators of self-tolerance. The understanding of pathophysiologic mechanism of ICI-induced primary hypogonadism comes from limited number of cases reports (85, 86). In the first case, the patient developed bilateral orchitis two weeks following administration of nivolumab and laboratory workup confirmed diagnosis of primary hypogonadism (decreased testosterone with elevated LH) (85). However, it self-resolved within one week without use of steroids or any other therapy, and there was no recurrence.  The intensity and timing of the orchitis suggests an intense immune stimulation leading to orchitis and primary hypogonadism (85). In another case, the patient developed bilateral epididymo-orchitis following administration of the third dose of pembrolizumab and needed high-dose steroids resulting in complete resolution (86). The testis is considered an immune-privileged organ due to its ability to tolerate autoantigens. The use of experimental autoimmune orchitis (EAO) in rats has allowed analysis of the autoimmune inflammatory response to spermatic antigens, providing a steppingstone towards understanding the ICI-induced primary hypogonadism. The main mechanisms responsible for preventing autoimmune disease of testes are: (a) secretion of immunosuppressive factors by macrophages, Sertoli cells, and Leydig cells, (b) presence of blood-testis barrier (BTB), and (c) presence of regulatory T cells. There is a fine equilibrium between dendritic cells, macrophages, T cells, and cytokines in maintaining immunosuppression in testes. While there have been no studies to date specifically evaluating the mechanism of ICI-induced orchitis, the examination of the normal and altered autoimmune immunobiology elucidates the possible mechanisms involved (89). This is briefly described below:

 

Secretion of Immunosuppressive Factors

 

In the normal testis macrophages, Sertoli and Leydig cells create an immunosuppressor microenvironment by secreting factors and cytokines that inhibit immune reactions. These include transforming growth factor-beta, granulocyte-macrophage colony stimulating factor, alpha-endorphin, and insulin-growth factor-1 (89, 90). In the setting of EAO, there is increased recruitment and activation of immune cells to the interstitium which bring along with them secretion of pro-inflammatory cytokines (IL-6, IFN-gamma, TNF-alpha, IL-17, IL-23). This brings about a cascade of events leading to germ cell apoptosis, primarily via the section of TNF-alpha (89, 91)

 

Blood-Testis Barrier (BTB)

 

In the normal testis, the BTB limits the interaction between germ cell antigens and interstitial immune cells. Secretion of pro-inflammatory cytokines mentioned above act on adherens and tight junctions, altering the BTB permeability (92). After crossing the BTB, these cytokines enter the seminiferous tubules inducing apoptosis of germ cells and facilitating the release of spermatic antigens, which then go on to interact with interstitial immune cells (92).

 

Presence of Regulatory T Cells (Tregs)

 

In the normal testis, there are several subsets of T cells present, regulating immune responses. Tregs specifically, mediate tolerance to self-antigens and their suppression sets the stage for autoimmunity. While there are increased Tregs seen in chronic inflamed testis, these are overwhelmed by the inhibitory effects of effector T cells, affecting the ability of Tregs to control autoimmunity (93). CTLA-4 inhibits effector T cells and PD-1/PDL-1 binding promotes the conversion of Teff to Treg. Therefore, it is plausible that the use of the combination of ipilimumab with an anti-PD-1/PDL-1 antibody, tips the balance between Tregs and effector cells toward the effector T cells. Consequently, creating a pro-inflammatory state resulting in orchitis.         

 

LONG-TERM OUTCOMES AND TREATMENT

 

It is well established that inflammation and infection of the male reproductive tract may lead to infertility in males (94). Therefore, it is reasonable to postulate that ICI-induced orchitis may also lead to male infertility, a consequence that should be addressed by providers. The long-term outcomes of ICIs are just beginning to be explored. One retrospective review assessed patients who became infertile after ICI therapy and subsequently died. Retrospective cohort cadaver study analyzing tissue specimens of the testes showed 86% of men who received ICI therapy had impaired spermatogenesis (88).  Notably, there was no increased peritubular hyalinization or fibrosis in the treated group, and no changes in Leydig cells (88). These findings support the previously mentioned pathophysiology of ICI-induced orchitis and address the possibility of infertility as a long-term consequence. Given the limited information on the effects of ICIs in spermatogenesis, providers should provide patients with their options, such as sperm banking and cryopreservation (95).

 

Other Uncommon Endocrine Side Effects  

 

ACQUIRED GENERALIZED LIPODYSTROPHY

 

Lipodystrophy is characterized by absent of visceral or subcutaneous adipose tissue in the settings of normal non-starvation nutritional state. It is a known common side effect from certain medications such as older HIV protease inhibitor, which is a reversible side effect. While the mechanism of lipodystrophy from ICIs is currently unclear, it is believed that the medication may induced an autoimmune process that leads to fat destruction by forming anti-adipocyte antibodies. In ICI-induced lipodystrophy, the more common form appears to be acquired generalized lipodystrophy (AGL) in which all fat tissues are affected but may spare the neck and face region. Onset of AGL, can be as early as 2-4 months which is roughly after 4-5 doses of ICIs. Currently, most of the cases of ICI-induced AGL are associated with nivolumab therapy (96, 97).

 

HYPOPARATHYROIDISM AND HYPOCALCEMIA  

 

Another rare but crucial endocrine irAEs is hypocalcemia secondary to hypoparathyroidism. While the exact mechanism is unclear, the proposed etiology is due to calcium-sensing receptor (CaSR) activating autoantibodies. This antibody is also present in patients with autoimmune polyendocrine syndrome type 1 (APS1) or idiopathic hypoparathyroidism. The clinical presentation can be as abrupt as an acute symptomatic hypocalcemia episode which includes paresthesia, tetany, and potential arrhythmias requiring hospitalization but may also present as very mild to asymptomatic hypocalcemia. For both circumstances, calcium and vitamin D replacement are adequate therapy but patients should be closely monitored for severe symptoms (98).

 

CENTRAL DIABETES INSIPIDUS  

 

Posterior pituitary hormone secretion can also be affected with ICIs, mainly antidiuretic hormones (ADH), which can subsequently lead to sodium and water dysregulation. To our knowledge only 3 cases of central diabetes insipidus (CDI) has been reported with the use of nivolumab (PD-1 inhibitor) and Azelumab (PD-L1 inhibitor) (99-101).The patients presented with classic polyuria/polydipsia symptoms along with hypernatremia which responded well to desmopressin (99-101). In the case report described by Fosci et. al., the authors described coexistence of metastatic localization and infundibulo-neurohypophysitis on MRI (100) while in the case report by Deligiori et. al., there was no signs of hypophysitis on imaging (99). Thus, further investigation is needed to fully understand the possible mechanisms for CDI.

 

SYNDROME OF INAPPROPRIATE ANTIDIURETIC HORMONE SECRETION (SIADH)

 

SIADH is the opposite scenario in which patients present with euvolemic hyponatremia. It is somewhat difficult to distinguish for certain that SIADH is truly from ICIs since SIADH is quite common in patients with underlying malignancies. Additionally, pain in patients with cancer itself can be the underlying cause of SIADH. Additionally, there are some reports of hyponatremia as a manifestation of adrenal insufficiency in patients on ICIs and hence it is crucial to rule out adrenal insufficiency for any patient with hyponatremia, as immediate recognition and treatment can be lifesaving (102, 103).

 

VITILIGO

 

Depigmentation of skin or vitiligo is thought to be from inducing an immune response to normal melanocyte antigens leading to the destructive process. While vitiligo itself may not be directly endocrine-related, its presence has been strongly associated with common endocrinopathies such as thyroid and adrenal disease as well as autoimmune diabetes. Interestingly, when vitiligo is present as one of the side effects from ICIs, this may represent a better prognosis in melanoma cases (104).

Figure 4. Proposed algorithm to screen and manage patients with endocrine irAEs

 

Table 3. Summary of the Common Terminology Criteria for Adverse Events (28)

Grade

Severity of Adverse events

Management

1

Mild (asymptomatic or mild symptoms)

Clinical or diagnostic observation

2

Moderate

Minimal, local or noninvasive intervention indicated

3

Severe or medically significant but not immediate life threatening

Intervention is required

4

Life threatening

Urgent intervention indication

5

Death

 

 

Table 4. Summary of the Incidence of Endocrine iRAEs (8,16,23, 41, 55-64, 105)

irAEs

PD-1/L1 inhibitors

CTLA-4 inhibitors

Combination

Hypophysitis

Less than 1 %

0-17%

 

 

 

More common than single drug use. 

Hypothyroidism

7-21%

0-6%

hyperthyroidism

Higher in PD1 inhibitors compared to PDL1 inhibitors

Less common than PD-1/PDL1 inhibitors

Primary adrenal insufficiency

Less common than CTLA-4 inhibitors

More common than PD-1/PDL1 inhibitors

Diabetes

Around 2.5%

None reported

 

CONCLUSION

 

Considering the increasing use of immune checkpoint inhibitors in clinical practice, health care providers and patients should be aware of endocrine irAEs. Educating patients receiving and providers using these state-of-the-art therapies about the signs and symptoms of different endocrinopathies is critical for an early diagnosis to prevent life-threatening complications. Developing screening and monitoring guidelines are essential to identify at-risk patients for close monitoring of these unwanted side effect.

 

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Medical Interventions for Transgender Youth

ABSTRACT

 

Up to 1.8% of youth and 0.6% of adults in the United States identify as transgender, meaning their gender identity differs from or is opposite their sex designated at birth. This chapter provides an overview including epidemiology and gender development. Then, it aims to summarize medical interventions for transgender youth as outlined in the Endocrine Society Clinical Practice Guidelines and World Professional Association for Transgender Health standards of care. The chapter concludes with research on mental health in this population and future directions.

 

INTRODUCTION

 

Throughout history and across cultures there have been people who live with, what we would now term, gender incongruence (definitions in Table 1). Prior to identification of sex steroids in the 1930s (1-5), and the development of exogenous sex steroids and surgical techniques, there were no options to change one’s secondary sex characteristics. The first modern orchiectomy for gender reassignment was performed in 1930 (6), and the first feminizing genital surgeries in the 1940s and 50s in Germany and Denmark, respectively (7,8). Harry Benjamin, known for his 1966 book, The Transsexual Phenomenon (9), treated Christine Jorgensen, the first widely published case of a transgender female in the United States (U.S.), treated with feminizing hormones and surgery. In 1979, the Harry Benjamin International Gender Dysphoria Association was formed, now the World Professional Association for Transgender Health (WPATH). The first standards of care were published in 1979, with the 7th edition released in 2012, and the 8th edition coming soon. The Endocrine Society first published a clinical practice guideline regarding the care of transgender persons, including support for pubertal suppression and gender affirming hormone therapy (GAHT) in 2009, with an updated guideline released in 2017 (10,11). In the over 40 years since the first edition of the WPATH Standards of Care, transgender rights, access to care, bathroom use, and sports participation, among other topics, are often featured and debated in mainstream media, politics, and healthcare (12). Furthermore, as care becomes increasingly politicized, numerous bills to both expand or limit the rights of transgender people and their access to medical care are being introduced in the U.S.(13). Medical care that respects the gender identity of the patient is recommended by numerous medical organizations, including the American Academy of Pediatrics (14),Endocrine Society (11), and the American Psychological Association (15).

 

Table 1. Definitions

Agender

A person with very little or no connection to the traditional systems of gender; existing without gender

Cisgender

Gender identity aligns with biologic sex

Gender affirming hormone therapy

Hormones, including testosterone and /or estradiol, that are prescribed to eligible individuals to induce development of secondary sex characteristics that align with gender identity

Gender affirming surgery (sometimes referred to as gender-confirming or gender-reassignment surgery)

Surgery or surgeries to align one’s body with one’s gender identity

Gender diverse

Individuals with a variety of gender identities across the gender spectrum, including those who identify as transgender

Gender dysphoria

Distress experienced when gender identity and body are not congruent. Defined in the DSM-5, which replaced “gender identity disorder” in the DSM-IV

Gender expression

External manifestations of gender, expressed through name, pronouns, clothing, haircut, behavior, voice, or other characteristics

Gender identity/experienced gender

One’s internal, deeply held sense of gender; not visible to others

Gender incongruence

Umbrella term used when gender identity and/or expression differ from what is typically/societally associated with their sex designated at birth

Gender role

Behaviors, attitudes, and personality trait that a society (in a given culture and historical period) designates as masculine or feminine and/or that society associates with the typical social role of men or women

Non-binary

A person whose gender identity is neither male nor female, both male and female or some combination of genders

Sex designated at birth

Sex at birth, typically based on external appearance of genitalia

Sex

Attributes that characterize biologic maleness or femaleness; factors that influence sex include sex chromosomes, gonads, sex steroids, internal reproductive structures, external genitalia, secondary sex characteristics

Sexual orientation

Physical and emotional attraction to others. Gender identity and sexual orientation are not the same

Transgender

Gender identity differs from sex designated at birth

Transgender male (also transgender man, female-to-male)

Individuals designed female at birth who identify and live as men

Transgender female (also transgender woman, male-to-female)

Individuals designated male at birth who identify and live as women

Transition

Process during which persons change their physical, social, and/or legal characteristics consistent with their affirmed gender identity

Adapted from Table 1 in the 2017 Endocrine Society Guidelines (11)

 

Centers around the world are seeing a rise in the number of transgender and gender diverse (TGD, Table 1) people seeking care to align their bodies with their identities (16). Despite the rise in the number of TGD people seeking care, there remains a lack of education and knowledge among providers as to how best serve this group (17). This article will define terminology, briefly review gender development, review current guidelines regarding medical treatment of pediatric TGD individuals, and mental health considerations.

 

EPIDEMIOLOGY  

 

Recent population-based studies in the U.S. report that 1.8% of youth and 0.6% of adults identify as transgender (18,19). Gender diverse describes individuals with a variety of gender identities across the gender spectrum, including those who identify as transgender (Table 1). Gender dysphoria, which describes the distress associated with a conflict between gender identity and anatomy or sex, is a listed diagnosis in the Diagnostic and Statistical Manual of Mental Disorders 5th edition (DSM-5, Table 2) (20).

 

Table 2. DSM-5 Criteria for Gender Dysphoria (20)

A.    A marked incongruence between one’s experienced/expressed gender and natal gender of at least 6 months in duration, as manifested by at least two of the following:

1.     A) A marked incongruence between one’s experienced/expressed gender and primary and/or secondary sex characteristics (or in young adolescents, the anticipated secondary sex characteristics)

2.     B) A strong desire to be rid of one’s primary and/or secondary sex characteristics because of a marked incongruence with one’s experienced/expressed gender (or in young adolescents, a desire to prevent the development of the anticipated secondary sex characteristics)

3.     C) A strong desire for the primary and/or secondary sex characteristics of the other gender

4.     D) A strong desire to be of the other gender (or some alternative gender different from one’s designated gender)

5.     E) A strong desire to be treated as the other gender (or some alternative gender different from one’s designated gender)

6.     F) A strong conviction that one has the typical feelings and reactions of the other gender (or some alternative gender different from one’s designated gender)

B.    The condition is associated with clinically significant distress or impairment in social, occupational, or other important areas of functioning. Specify if:

1.     A) The condition exists with a disorder of sex development.

2.     B) The condition is post-transitional, in that the individual has transitioned to full-time living in the desired gender (with or without legalization of gender change) and has undergone (or is preparing to have) at least one sex-related medical procedure or treatment regimen—namely, regular sex hormone treatment or gender reassignment surgery confirming the desired gender (e.g., penectomy, vaginoplasty in natal males; mastectomy or phalloplasty in natal females).

 

One’s sex refers to the physical attributes that characterize biologic maleness or femaleness and is typically assigned or designated at birth based on the appearance of the external genitalia (or prior to birth based on sex chromosome complement and/or the appearance of the genitalia on the prenatal anatomy ultrasound).

 

Note that terminology in this field is constantly evolving, and for clinicians, it is important to ask individuals what terms they use to describe their gender identity, and what that term means to them. Table 3 includes suggestions on how to ask these questions.

 

Table 3. Suggested Ways of Asking About Name and Gender Identity

Name

“Is there a name you go by other than your legal name?”

“What name do you go by?”

“What would you like me to call you?”

Pronouns

“What pronouns do you use?”

“I’d like to use the pronouns that feel best to you. What pronouns would you like me to use?”

Model pronoun use: “Hello, my name is Dr. ____ I use ___ pronouns.”

Gender identity

“How do you identify your gender?”

“What does [gender identity term] mean to you?”

Suggestion for children: “Some kids tell me think of themselves as girls, some as boys, some as part girl and boy, or something entirely different. How do you think about yourself?”

Suggestion for adolescents: “There are lots of ways people think about their gender identity, how do you think of yours?”

 

GENDER IDENTITY DEVELOPMENT AND NATURAL HISTORY OF GENDER INCONGRUENCE  

 

Gender identity is multidimensional with biological, cultural, and environmental contributions (21,22). Studies of gender identity among individuals with differences/disorders of sex development underscore the influence of the hormonal milieu, and prenatal androgen exposure in particular, in gender development (23-26). There are also genetic influences, as some studies show concordance rates of gender dysphoria up to 39% among identical twins (27). Although studies have sought to identify genes associated with transgender identity, results have largely been inconsistent or inconclusive, with a possible role for genes related to sex steroids and their receptors (28-32).

 

In childhood, learning about gender starts early, and progress through many stages (33). In their review of gender development in childhood, Perry, Pauletti, and Cooper describe eight dimensions of gender identity: (1) gender self-categorization, (2) felt same-gender typicality, (3) felt other-gender typicality, (4) gender contentedness, (5) felt pressure for gender differentiation, (6) intergroup bias, (7) gender centrality, and (8) gender frustration. By age 18-24 months most children can categorize their own and others’ gender (34), and by age 6, have a developed gender identity (35). More individuals with a female sex designated at birth express dissatisfaction with their gender (36,37). This reflects the current sex ratio of individuals being referred to gender clinics, with more individuals with a female sex designated at birth currently referred, but the opposite being true prior to the 2000s (38-40).

 

Despite adolescence being a period of identity formation (41,42), there is a surprising lack of research on adolescent gender identity development. Development of identity is an individual and social process and shaped by external surroundings (42). However, there are also numerous psychological and biological factors that influence gender identity, as outlined in a review by Steensma and colleagues (43).

 

Overall, there is still much to learn about gender development among gender variant or nonconforming individuals. Prospective studies of children referred to gender clinics, primarily in Europe, show that less than a quarter of children will remain or meet criteria for gender identity disorder (the DSM-IV diagnosis prior to the DSM-5 gender dysphoria) after adolescence (44-47). In follow-up studies, the period of early adolescence/puberty, age 10-13 is critically important. There are three possible factors that contribute to an increase or decrease in gender discomfort and cross-gender identification: (1) physical puberty, (2) changing environment and being treated as their sex assigned at birth, and (3) the discovery of sexuality (47). More recent studies that reflect the rise in referral rates, and in various places around the world, will be critically important.

 

MEDICAL MANAGEMENT  

 

The WPATH Standards of Care (48) outline three categories of physical interventions for adolescents, indulging (1) fully reversible interventions, such as the use of gonadotropin releasing hormone (GnRH) agonists, medications to suppress menses (such as progestins), and medications to decrease the effects of androgens (such as spironolactone); (2) partially reversible interventions, including testosterone or estradiol; and (3) irreversible interventions, such as surgical procedures. Many individuals also seek care, including behavioral health consultation, for reversible interventions such as name, pronoun and gender marker change, discussing gender identity with friends, family and school, voice therapy, or wearables (including binders and packers to flatten the chest or give the appearance of male genitalia, respectively) (48).

 

It is important to note, as outlined in the WPATH Standards of Care (48), individuals who have gender variance or incongruence, but whom do not experience distress may not require clinical attention or intervention.  Furthermore, there is an increasing recognition that interventions should align with one’s individuals gender goals and gender embodiment (49).

 

Pubertal Blockade

 

The onset of puberty (gonadarche) is characterized by breast budding in people designated female at birth and by testicular enlargement to 4mL or greater in people designated male at birth, characterized as Tanner or Sexual Maturity Rating stage 2 (50,51). The average age of pubertal onset is age 10-11 years in someone designated female at birth (range 8-13 years, can be younger in African Americans), and 11-12 years in individuals designated male at birth (range 9-14 years). For individuals designated male at birth, external virilization typically starts around a testicular volume of 10 mL (11), voice drop at >8-10mL (52), and spermarche at 11-12 mL (53). In individuals designated female at birth, breast developmental progresses from stage 2 to 5 (fully developed) within 4-5 years and menarche typically occurs about 2-2.5 years after breast budding (54). Pubic hair and/or axillary hair and/or body odor reflect the onset of adrenarche or adrenal androgen production, which, by themselves are not indicative of central puberty (50,51). Height velocity increases during puberty and peaks about 2.5 years after the start of pubertal growth acceleration (55). An understanding of typical pubertal development and timing of external secondary sex characteristics is useful in counseling families about the timeliness and risk/benefit of GnRH agonist therapy to halt further pubertal progression. For example, towards the end of puberty or in post-pubertal individuals, GnRH agonist therapy may be used in certain circumstances for sex steroid suppression but would not block any pubertal changes, as these are complete.

 

GnRH agonists were first used in youth for the treatment of central precocious puberty in the 1980s (56). In 1998, Drs. Cohen-Kettenis and van Goozen in the Netherlands published the first report of a transgender patient treated with triptorelin, a GnRH agonist (57). The “Dutch model” of using pubertal suppression followed by gender affirming hormones (testosterone or estradiol) subsequently became incorporated into the WPATH and Endocrine Society standards of care (10,48). Their use became more widespread in the U.S. after publication of the 2009 Endocrine Society guidelines (10). The 2017 Endocrine Society guidelines suggest that “adolescents who meet diagnostic criteria for gender dysphoria/gender incongruence, fulfill criteria for treatment, and are requesting treatment should initially undergo treatment to suppress pubertal development”(11). The guidelines suggest beginning pubertal hormone suppression after the onset of the physical changes of puberty (Tanner Stage or Sexual Maturity Rating 2) for individuals who meet criteria, including being diagnosed with gender dysphoria, experienced worsening dysphoria with the onset of puberty, existing psychological, medical and/or social problems are addressed and the adolescent has sufficient mental capacity to consent to treatment (11). Treatment with a GnRH agonists suppresses gonadotropins (after an initial increase of gonadotropins) (58). There are also gonadotropin releasing hormone antagonists that immediately suppress gonadotropins, but are not available in children. GnRH agonists are typically administered as either an injection (IM or SQ) or as an implant (preparations listed in Table 4). Insurance coverage for this off-label, and costly therapy, varies (59). GnRH agonist treatment will pause or halt pubertal changes and may cause slight regression of breast tissue or testicular volume (11). On their own, these are reversible interventions, and if the individual decided that they wanted to progress through their endogenous puberty, these medications can be discontinued. During GnRH agonist treatment, the Endocrine Society recommends measurement of height, weight, sitting height, blood pressure, and Tanner stages every 3-6 months, measurement of LH, FSH, estradiol or testosterone, and 25OH vitamin D every 6-12 months, and bone density using dual-energy X-ray absorptiometry (DXA) and bone age x-ray of the left hand every 1-2 years (11).

 

Table 4. Hormonal Interventions for Transgender Adolescents

Pubertal blockade/inhibition of sex steroid secretion

GnRH agonist: inhibition of the hypothalamic-pituitary-gonadal access

Leuprolide acetate IM (1-, 3-, 4- or 6-mo preparations) or SQ (1-, 3-, 4- or 6-mo preparation)

Triptorelin IM (4-, 12- or 24-week preparation)

Histrelin acetate SQ implant (one-yearly dosing, although reports of longer effectiveness)

Medroxyprogesterone acetate: inhibition of the hypothalamic-pituitary-gonadal access and direct inhibition of gonadal steroidogenesis

Orally (up to 40 mg/day) or IM (150 mg every 3 mo, may be given more frequently for suppression of sex steroids)

Inhibition of testosterone secretion or action

Spironolactone: inhibition of testosterone synthesis and action

Titrate up to 10-300 mg/day orally (typically in divided doses)

Cyproterone acetate: inhibition of testosterone synthesis and action (not available in US)

25-50 mg/day orally

Finasteride: inhibition of type II 5 α-reductase, blocks conversion of testosterone to 5 α-dihydrotestosterone

2.5-5 mg/day orally

Bicalutamide: androgen receptor blockade

50 mg/day orally

Sex steroids

Estrogen/17β-estradiol

Oral/sublingual: start with lower doses for pubertal induction, titrate up to adult doses 2-6 mg/day

Transdermal: start with lower doses for pubertal induction, titrate up to adult doses 0.025-0.2 mg/day (patches are typically once or twice weekly)

Parenteral: estradiol valerate (5-30 mg every 2 weeks) or cypionate (2-10 mg IM every week)

Testosterone

Parenteral IM or SQ testosterone cypionate or enanthate (start at 12.5 mg/week or 25 mg q2 week with gradual increases to 50-100 mg/week or 100-200 mg every 2 weeks)

Transdermal (typically after full adult dose has been achieved parenterally): patch (2.5-7.5 mg/day or 1% or 1.6% gel

Note that all medications are currently off-label for gender non-conforming/transgender youth. Note that certain GnRH preparations are approved in children for central precocious puberty and other formulations are approved for adults only, with off-label use in children. Different formulations are available in different countries. This table was adapted from the following references (11,178). Note that some centers/providers also use GnRH agonists for testosterone blockade in older adolescents and/or adults. GnRH: gonadotropin releasing hormone, IM: intramuscular, SQ: subcutaneous

 

Recent reviews of puberty blockade have been published (60-62). Small studies have demonstrated effectiveness of GnRH agonist treatment for suppression of the hypothalamic-pituitary-gonadal axis in transgender youth (63). Studies, primarily in Europe, have demonstrated improvements in psychological functioning, behavioral/emotional problems, and depressive symptoms during GnRH agonist treatment in transgender youth (64,65). A recent systemic review found that GnRH agonist therapy is associated with decreased suicidality in adulthood, improved affect and psychological functioning, and improved social life (61).

 

Potential risks of GnRH agonist therapy include impacts on growth, bone health, body composition, fertility, and neurodevelopment, as well as difficulties accessing treatment due to cost and insurance coverage (61,62,66). GnRH agonist use in TGD youth is associated with increased body fat and decrease in lean mass after initiation (67,68), and compared to age- and BMI-matched control youth (69), and may also have an adverse effect on insulin sensitivity (69). If GnRH agonists are started prior to skeletal maturity, they will decrease skeletal advancement during monotherapy due to suppression of sex steroids, which are necessary for growth plate closure (70,71). There is a dearth of research on growth trajectories during treatment with a GnRH agonist in this population. One multicenter study in the U.S. showed that transgender youth treated with GnRH therapy have growth velocity similar to prepubertal children, but those who start GnRH agonist treatment later in puberty have growth velocity below the prepubertal range (72). The growth spurt and skeletal advancement will progress either when exogenous testosterone or estradiol are started, or if the GnRH agonists are discontinued and the individual progresses through their endogenous puberty. There is a growing body of research of bone health in transgender individuals, as well as the impact of GnRH agonists and later gender affirming hormones on bone health. Studies in the Netherlands have demonstrated decreased bone turnover, and a decrease in bone mineral apparent density Z-scores of the lumbar spine in transwomen after initiation of GnRH agonist therapy (73). However, studies in the U.S. (74), United Kingdom (75) and Netherlands (73,76) have also shown decreased bone mineral density Z-scores determined by DXA are low prior to treatment with GnRH agonist, and some studies showing Z-scores did not completely normalize with sex hormone treatment (73,76). In the U.S., the individuals with lower baseline bone mineral density Z-scores also reported less physical activity, an area warrants further research (74).

 

Overall, there is a paucity of research on neurodevelopment and a recent consensus parameter was published with recommended research methodologies to evaluate the neurodevelopmental effects of puberty suppression in this population (77). There is also very little research on sexual function and future surgical options among individuals who received early puberty blockade. Recently, there has been a call for more information to better inform the impacts on future sexual function(78,79) (as GnRH agonists limit penile and testicular growth/size (80)) and on implications for future surgical intervention for those individuals pursuing vaginoplasty (as the scrotal tissue is used to construct the vagina) (80). Finally, treatment with GnRH agonists will impair spermatogenesis and oocyte maturation temporarily, and the Endocrine Society recommends fertility counseling (11). Treatment may be delayed to preserve fertility, but many individuals do not choose this, as delay will also cause further progression of unwanted secondary sex characteristics (11). There are limited options available for early tissue cryopreservation, an area that is gaining more attention (81).

 

If GnRH analogues are not available or are cost prohibitive, medroxyprogesterone may be used as an alternative agent for pubertal suppression (Table 4) (11,48). At high doses, medroxyprogesterone inhibits the pituitary-gonadal axis and suppresses testosterone (82-84).  Medroxyprogesterone was used for treatment of precocious puberty in the 1960s and 70s (85-87). It is typically safe, although may have some side effects, including due to the estrogenic effects (bloating, nausea/vomiting, breast fullness, breakthrough bleeding for those menstruating, irritability, headache, hypertension), progestational effects (headache, breast pain/tenderness, hypertension), and androgenic effects (acne, oily skin, weight gain, hirsutism, fatigue, depression) (88). At extremely high doses (100 mg four times a day), it may cause Cushing’s syndrome, adrenal insufficiency, and diabetes (89). There is one small study of medroxyprogesterone in transgender youth demonstrating effective sex steroid suppression with doses of oral medroxyprogesterone 10-30 mg BID or 150 mg IM every 2-3 months (90).

 

Gender Affirming Hormone Therapy

 

Gender affirming hormone therapy (GAHT) refer to hormones that induce secondary sex characteristics to align the body with one’s gender identity. The Endocrine Society recommends treatment with sex steroids (testosterone or estradiol) “using a gradually increasing dose schedule after a multidisciplinary team of medical and mental health professionals has confirmed the persistence of gender dysphoria/gender incongruence and sufficient mental capacity to give informed consent, which most adolescents have by age 16 years” (full criteria in guidelines) (11). However, they also state that “there may be compelling reasons to initiate sex hormone treatment prior to age 16 years in some adolescents” (11). The WPATH Standards of Care, 8th version are forthcoming, but in the current 7th version, the criteria for hormone therapy are: “(1) persistent, well-documented gender dysphoria; (2) capacity to make a fully informed decision and to consent for treatment; (3) age of majority in a given country; (4) if significant medical or mental concerns are present, they must be reasonably well-controlled” (48). It is recommended, that for adolescents who have not reached the age of majority in their country, that consent from all parents or medical decision-makers is obtained prior to starting this partially irreversible therapy.

 

Feminizing Hormone Therapy  

 

ESTRADIOL THERAPY  

 

For eligible adolescents, the Endocrine Society recommends a gradually increasing dose schedule of oral or transdermal 17β-estradiol (11). This will cause feminization of the body, with expected effects including body fat redistribution, decreased muscle mass/strength, softening of the skin/decreased oiliness, decreased libido/erections, breast growth, decreased testicular volume, decreased sperm production, and thinning and slowed growth of body and facial hair occurring one to several months after treatment with maximum effects generally about 2-3 years or more into treatment (11,48). For younger individuals, the Endocrine Society recommends starting oral estradiol at a dose of 5 µg/kg/day and increasing doses every 6 months up to a dose of 2-6 mg/day for an adult (11). In post-pubertal individuals, the starting dose may be higher and titrated more quickly (start at 1 mg/day for 6 months and increase to 2 mg/day orally) (11). For transdermal estradiol, it is recommended to start at a dose of 6.25-12.5 µg/24 hours and increase the dose every 6 months to an adult dose of 50-200 µg/24 hours. During induction of puberty, it is recommended to measure height, weight, sitting height, blood pressure, and Tanner stages every 3-6 months, and measure prolactin, estradiol and 25OH vitamin D every 6-12 months (11). Additionally, DXA and bone age (if clinically indicated or a growing patient) is recommended every 1-2 years (11).

 

Potential Adverse Effects of Estradiol Therapy

 

Risks with estradiol therapy as outlined in the Endocrine Society guidelines include thromboembolic disease, macroprolactinoma, breast cancer, coronary artery disease, cerebrovascular disease, cholelithiasis, and hypertriglyceridemia (11).

 

In adults, studies using three large cohorts have shown an increased risk of myocardial infarction and venous thromboembolism. In Europe, transgender women on estradiol therapy have a higher risk of stroke and venous thromboembolism than both cisgender reference women and men, and a higher risk of myocardial infarction than cisgender women (but not men) (91). In the U.S., two large cohorts have been used to examine outcomes, the Kaiser STRONG cohort and self-report data from the Behavioral Risk Factor Surveillance System (BRFSS). Data on hormone treatment is not collected in BRFSS. Transgender women in BRFSS were more likely (>2-fold increase risk) to have a history of myocardial infarction than cisgender women (but not men) (92,93). In the Kaiser STRONG cohort (94), both prevalent and incident type 2 diabetes was more common in the transfeminine cohort compared to cisgender females (95). In a meta-analysis commissioned by the Endocrine Society to accompany the 2017 updated guidelines, transgender women on estradiol therapy had increased triglycerides, but no changes in other lipid parameters (96). There were few reports of myocardial infarction, stroke, venous thromboembolism or death (96). It is well-known that transgender women on estradiol therapy have increases in body weight and fat and decreases in lean body mass (97). Estradiol therapy is associated with increases in lumbar spine bone mineral density compared to baseline (98).

 

In youth, there is a growing body of literature on the effects of GAHT, particularly on cardiometabolic health. TGD youth on estradiol have changes in HDL, aspartate aminotransferase, potassium, prolactin, and hemoglobin after about two years (99). One study found that transgender females on estradiol therapy were more insulin resistant than matched cisgender males (100). The presence of obesity attenuates the beneficial effect of estradiol on HDL (101). There are also studies investigating baseline differences between TGD youth and cisgender controls prior to hormone therapy, with recent studies showing TGD youth have lower HDL and low bone mineral density (72,102).

 

Testosterone Blockade/Suppression

 

There are many options for blockade and/or suppression of testosterone (all off-label use, Table 4). When available and affordable, some centers utilize GnRH agonists for suppression of testosterone. For example, in the United Kingdom, GnRH analogues are heavily subsidized (103). There are also many antiandrogens available, and a systemic review of options has recently been published (104). Spironolactone is widely available, inexpensive, and commonly used in the U.S. and Australia. Spironolactone is a weak androgen receptor antagonist (105,106), weak progesterone receptor agonist, and weak estrogen receptor agonist (104). It also partially inhibits 17α-hydroxylase/17,20 lyase, which are involved in testosterone synthesis (107). Even at high doses, spironolactone does not cause a significant reduction in serum total testosterone concentration (108). Although the combination of spironolactone with estradiol does appear to suppress testosterone in transgender women (109). Side effects include irregular menses (only for people who are menstruating, not a consideration for transgender women), hypotension, polyuria, and hyperkalemia (110,111).

 

Cyproterone acetate is available in Europe and Australia, but not in the U.S. and is a moderate androgen receptor antagonist, strong progesterone receptor agonist, and does not have any estrogen receptor activity but does suppress the hypothalamic pituitary gonadal axis (104). Cyproterone acetate has been associated with increased risk of meningiomas (112) and prolactinomas (113). Other side effects include weight gain, headache, gastrointestinal disorders, mood effects, and edema (114).

 

Nonsteroidal antiandrogens, such as bicalutamide have also been used at some centers. Bicalutamide has strong androgen receptor antagonist activity and does not have any estrogen or progesterone agonist activity (104). It does not cause a reduction in testosterone concentrations. There is some feminization, thought to be due to increased aromatization of testosterone to estradiol (115). There is currently one published study of the use of bicalutamide in transgender adolescents as an alternative to GnRH agonists (115). In that study, hepatic enzymes remained normal and there were no adverse effects, however, effectiveness and the potential risk of liver toxicity needs to be examined in larger studies.

 

Finally, 5-alpha reductase inhibitors, such as finasteride, block conversion of testosterone to dihydrotestosterone. These are not recommended by the Endocrine Society due to adverse effects (11), but the WPATH guidelines state, “these medications have beneficial effects on scalp hair loss, body hair growth, sebaceous glands, and skin consistency” (48). Side effects include sexual dysfunction and decreased muscle (which may be perceived as a risk or benefit in this population), anhedonia, and trouble concentrating (116).

 

Overall, the selection of which agent alone or in combination with estradiol depends on many factors including patient age, country, insurance coverage, cost, goals of care, and tolerability of side effects (e.g. severe and fatal hepatotoxicity has been reported with cyproterone acetate and bicalutamide (117)). Further studies are needed to determine superiority for relevant patient outcomes including body composition, breast development, facial and body hair (104).

 

Masculinizing Hormone Therapy

 

TESTOSTERONE  

 

For eligible adolescents, the Endocrine Society recommends a gradually increasing dose schedule of testosterone (typically injectable IM or SQ) (11). This will cause masculinization of the body, with expected effects including skin oiliness/acne, facial/body hair growth, scalp hair loss, increased muscle mass/strength, body fat redistribution, cessation of menses, clitoral enlargement, vaginal atrophy, and deepened voice with onset occurring one to several months after treatment with maximum effects generally about 2-5 years or more into treatment (11,48). For younger individuals, the Endocrine Society recommends starting injectable testosterone esters at a dose of 25 mg/m2 IM or SQ every 2 weeks and increasing every 6 months up to an adult dose of 100-200 mg every 2 weeks (11). In post-pubertal individuals, the starting dose may be higher and titrated more quickly (start at 75 mg every 2 weeks for 6 months and increase to 125 mg every 2 weeks) (11). Subcutaneous testosterone is gaining in popularity, and has shown to be effective and preferred by patients (118-120). Pharmacokinetic studies of weekly subcutaneous testosterone injections show that steady state is approached after the third dose, and that serum concentrations stay relatively constant throughout the week between doses (121). Finally, SQ testosterone doses may be lower than those delivered IM, with two studies reporting doses of 50-80 mg/week to achieve target testosterone concentrations in adults or older adolescents (118,119). During induction of puberty, it is recommended to measure height, weight, sitting height, blood pressure, and Tanner stages every 3-6 months, and measure hemoglobin/hematocrit, lipids, testosterone, and 25OH vitamin D every 6-12 months (11). Additionally, DXA and bone age (if clinically indicated or a growing patient) is recommended every 1-2 years (11).

 

The most common adverse effect of testosterone is erythrocytosis/polycythemia (hematocrit >50%) (11). Other risks as outlined in the Endocrine Society guidelines include liver dysfunction, coronary artery disease, cerebrovascular disease, hypertension, and breast or uterine cancer (11).

 

In adults, studies using three large cohorts have shown conflicting results. In Europe, transgender men on testosterone therapy have a higher risk of myocardial infarction than cisgender women (but not men) and no increased risk of stroke or venous thromboembolism compared to reference populations (91). Transgender men in the 2015 U.S. BRFSS survey had no increased risk of hypertension, myocardial infarction, stroke, angina/coronary heart disease compared to cisgender men or women (92). However, another analysis of BRFSS data (years 2014-2017) reported a >2-fold increase risk of myocardial infarction compared to cisgender men and 4-fold increase compared to cisgender women (93). In the Kaiser STRONG cohort, there was no increased risk of type 2 diabetes among transgender men compared to cisgender men (95). In a meta-analysis, testosterone therapy in transgender men was associated with increases in serum triglycerides and low-density lipoprotein cholesterol (LDL-C) concentrations and decreases in high-density lipoprotein cholesterol (HDL) (96). Testosterone therapy in transgender men is known to result in increased body weight and lean mass and decreased body fat (97). In meta-analyses, testosterone therapy is not associated with significant changes in bone mineral density (98,122). There is a recent position statement from the International Society for Clinical Densitometry on bone densitometry in TGD individuals (123).

 

Among TGD youth starting testosterone therapy, there is an increase in BMI and decrease in HDL (124). The decrease in HDL is exacerbated by obesity (101). Other studies have found that testosterone treatment in TGD youth is associated with statistical, but not clinically significant increases in triglycerides, alanine aminotransferase, potassium, and hemoglobin (99).

 

Non-Binary Care

 

Non-binary or gender non-conforming individuals represent a growing proportion of patients presenting to gender clinics and may have additional challenges accessing healthcare (125). Limited studies have reported worse mental and physical health among individuals who identify as gender non-conforming compared to matched controls (92). An individualized approach to understand the individual’s gender identity, sources of dysphoria (if any), and gender goals are important. Some individuals may desire reversible interventions such as menstrual suppression, others may request certain hormones and/or surgical interventions as a part of their gender goals. The 8th version of the WPATH standards of care will include a chapter on non-binary care.

 

Menstrual Management  

 

Many transmasculine and non-binary individuals who are designated female at birth seek medical attention or desire interventions for menstrual management (126). Some also utilize these methods for contraception. It is important to ask individuals about their individual goals, as well as their sexual orientation, partners (including sex assigned at birth and what body parts they currently have), and types of sex they are engaging in. These factors can guide choice of intervention for menstrual management and/or contraception. An overview of options is in Table 5.  Progestin-only methods, including norethindrone or depo medroxyprogesterone are particularly popular choices among this population (126). Review of options for menstrual management and contraceptive options for transgender individuals was recently published (127). For those patients wishing to and eligible for testosterone therapy, menses suppression typically is achieved within 6-12 months of the start of testosterone therapy (128).

 

Table 5. Options for Menstrual Suppression/Management

Combined hormonal contraceptives (pills, patch, ring)

Progestins

   Norethindrone acetate (5-15 mg/day orally)

   Medroxyprogesterone acetate (150 mg IM every 3 months)

   Etonogestrel implant

   Levonorgestrel IUD

IUD: intrauterine device

 

SURGICAL MANAGEMENT

 

Surgeries that impact fertility are generally not available until the individual has reached the age of majority in their country. There are a wide variety of surgical options for transgender adults (and some options, primarily chest surgery, for adolescents), and this has recently been reviewed (129). Physicians (including surgeons and non-surgeons) and behavioral health providers should be aware of the criteria needed for each surgical procedure, including whether social transition is recommended, whether hormonal therapy is needed (and length), and how many referral letters are needed and by whom (48,130). In general, a documentation of persistent gender dysphoria by a qualified mental health provider is a requirement for surgery (48). Additionally, guidelines may change over time (the 8th version of the WPATH standards of care are coming soon) and may vary by location (country/state) and insurance coverage. Table 6 summarizes the various gender affirming surgical options. Genital surgery or removal of the gonads is generally not performed until the individual is the age of majority in a given country (age 18 years or older in the U.S.). Individuals younger than 18 may be eligible for chest/breast surgery, with consent from medical decision-makers. The WPATH Standards of Care state for mastectomy/chest masculinizing surgery or for breast augmentation surgery, the individual must have “(1) persistent, well-documented gender dysphoria, (2) capacity to make s fully informed decision and to consent for treatment, (3) age of majority in a given country, (4) if significant medical or mental health concerns are present, they must be reasonably well controlled.” Although masculinizing hormone therapy (testosterone) is not a prerequisite for chest masculinizing surgery in the WPATH guidelines, it is recommended (although not an explicit criterion) that individuals on feminizing hormone therapy be on for a minimum of 12 months prior to breast augmentation surgery for better aesthetic results (48). However, the Endocrine Society Guidelines recommend 2 years of testosterone therapy prior to mastectomy/chest masculinizing surgery. Neither the WPATH or Endocrine Society guidelines recommend a specific age cutoff, but “suggest that clinicians determine the timing of breast surgery for transgender males based upon the physical and mental health status of the individual” (11).

 

Table 6. Gender Affirming Surgical Options

Feminizing surgeries

Breast augmentation

Increasing the size of the breasts

Facial feminization surgery

May include: forehead feminization, rhinoplasty, periorbital rejuvenation, rhytidectomy (face lift), cheek augmentation, rhinoplasty, lip feminization, gonial angle shave, genioplasty

Genital surgery/vaginoplasty

May include penectomy, orchiectomy, surgical creation of a vagina (penile inversion, intestinal conduit), clitoroplasty, labiaplasty

Orchiectomy

Removal of testes

Tracheal shave

Thyroid cartilage shave

Masculinizing surgeries

Chest masculinizing surgery (mastectomy)

Removal of breast tissue

Facial masculinization surgery

Rhinoplasty, gonial implants, genioplasty

Hysterectomy, salpingectomy, oophorectomy

Removal of uterus and/or fallopian tubes, and/or ovaries

Metoidioplasty

Creation of a phallus using existing genital tissue 

Phalloplasty

Construction of phallus, glansplasty, urethroplasty, erectile prosthesis, scrotoplasty, testicular implants

 

MENTAL HEALTH

 

Recently studies have demonstrated a strikingly high prevalence of behavioral health diagnoses among youth diagnosed with gender dysphoria (up to 60%) (131,132). Studies evaluating behavioral health outcomes among TGD youth have most frequently demonstrated disproportionate anxiety (132-134), depression (133-136), suicidality (19,132-134,137), self-harm (132-135), and substance use problems (19). Large surveys of TGD individuals in the U.S. have shown that 40% of adults (138) and 35% of youth (19) have attempted suicide. Poor behavioral health outcomes may be conceptualized as the result of complex and layered socio-cultural and political factors that impact TGD youth (15,139). Risk factors that are likely to impact overall mental health for TGD individuals include minority stress (e.g., victimization, discrimination) (19,140), gender dysphoria and appearance congruence (141), feelings of isolation, inadequate family support (142), emotional/social isolation (143), lack of autonomy over decision making (143), barriers to accessing gender affirming care (143-145), employment discrimination (143), and limited financial resources (143). In the Youth Risk Behavior Survey, TGD youth were two to six times more likely to be victimized, including experiencing sexual dating violence, experiencing physical dating violence, being bullied at school, being electronically bullied, feeling unsafe during travel to or from school, and being forced to have sexual intercourse (19).

 

The co-occurrence of autism spectrum disorders (ASD) and gender dysphoria is a growing area of interest (132,146-150). A recent meta-analysis found that the prevalence of ASD among those with GD has ranged from 6% to 68% depending on the methodology of the study (151). In a large 2020 study, TGD individuals were 3.0 to 6.4 times more likely to be diagnosed with ASD than their cisgender counterparts (136). Other samples have shown that youth with gender dysphoria are about 2-3x as likely to have a diagnosis of ASD than their matched cisgender counterparts (131,132). The exact link between GD and ASD is not known, but factors contributing may include: symptom overlap between the two diagnoses, misclassification due to symptom overlap, children with ASD may be more likely to express their gender identity and dysphoria, or they may be more likely to be referred to care to be diagnosed with either GD or ASD (152).

 

Protective factors including social support (153,154), parental support/affirmation of gender identity (155,156), higher self-esteem (153), resiliency (153,157), and access to affirming care (144,158,159) have improved well-being and decreased mental health distress. Access to gender-affirming interventions, including hormone therapy and surgery, has been shown to improve gender dysphoria, psychological symptoms and quality of life in small samples and meta-analyses (28). Recent studies have shown that those who were older at presentation have worse mental health than those who presented to care at a younger age (160) and those who had access to GnRH agonists had lower lifetime odds of suicidal ideation than those who did not have access (144).

 

Finally, there are many other important topics that impact the care of transgender individuals that are beyond the scope of this chapter including dermatologic considerations and hair loss (161-163), chest binding (164), sexual health (79), HIV prevention and treatment (165-167), fertility (81), sleep (168), athletic performance and sports participation (169), eating disorders (170,171), homelessness, the impact of family support, and the underpinnings of links between gender diversity and neurodiversity (172).

 

CONCLUSION

 

An improved understanding of the variety of individual gender trajectories is needed, as well as how best to individualize care, how to improve mental health, and minimize risks of medical intervention. Large, multi-center, prospective cohorts, as are currently established in the U.S. (94,173) and Europe (174), will help answer some of these important questions. And community-based participatory research and hearing the voices of individuals from the community about their own research priorities and dissemination of results is of utmost importance. There is also much to be learned about the impact of early GnRH agonist therapy on growth, bone health, physical development, long-term health, mental health, cognitive development, and overall wellbeing. Finally, an improved understanding on the impact of other stressors including minority stress and depression on overall health (175,176) for TGD persons is needed. The American Heart Association published a scientific statement with recommendations to assess and address cardiovascular health among TGD people (177).

 

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Thyroid Hormones in Brain Development and Function

ABSTRACT

 

Thyroid hormones are essential for normal brain development. They influence neurogenesis, neuronal and glial cell differentiation and migration, synaptogenesis, and myelination. Thyroid hormone deficiency may severely affect the brain during fetal and postnatal development, causing retarded maturation, intellectual deficits, and neurological impairment. Neural cells express the thyroid hormone nuclear receptors THRA and THRB, which mediate most actions of T3, the active hormone. Brain T3 derives in part from the circulation, and part from type-2 deiodinase-mediated 5’-deiodination of T4 in glial cells. Type 3 deiodinase inactivates T4 and T3 by 5-deiodination in neurons. Membrane transporters facilitate the passage of T4 and T3 across the brain barriers. The main transporters are the monocarboxylate transporter 8 (MCT8) and the organic anion transporter polypeptide 1C1 (OATP1C1). MCT8 facilitates T4 and T3 transport whereas OATP1C1 transports T4 but not T3. T3 regulates the expression of a large number of genes in the brain, mostly during developmental stages, but also in the adult. Rodent models of disease have provided most of our knowledge on thyroid hormone action in the brain. However, species-specific differences in brain maturation and organization make it difficult sometimes to extrapolate the data obtained in rodent models to the human. This review will present a summary of the main concepts developed from rodent studies, with a focus on the human brain.

 

INTRODUCTION

 

Thyroid hormones are crucial for brain development, and influence brain function throughout life. In adults, hypothyroidism causes lethargy, hyporeflexia, and poor motor coordination (1,2), is associated with bipolar affective disorders, depression, or loss of cognitive functions (3,4). Subclinical hypothyroidism is often associated with memory impairment. Conversely, hyperthyroidism causes hyperreflexia, irritability, and anxiety among other symptoms (5). Hypo- or hyperthyroidism can lead to mood disorders, dementia, confusion, and personality changes. Most of these disorders are usually reversible with proper treatment, indicating that adult-onset thyroid hormone alterations affect neural function but do not leave permanent structural defects.

 

The actions of thyroid hormones during mammalian brain maturation are qualitatively different. They influence many developmental processes, usually during limited time windows.  They are required for the timely synchronization of independent events and facilitate the transition between fetal and postnatal stages (6,7), similarly to promoting amphibian metamorphosis (8,9). Thyroid hormone deficiency during critical transition periods may lead to irreversible brain damage, the consequences of which depend on the severity and duration of the deficiency, and most importantly its time of onset (10-14).

 

Until recently, the rat was the most widely used animal model in the study of thyroid physiology and the actions of thyroid hormones in the brain. However, for more than 20 years, the mouse is the preferred animal model, and the use of knockout and knockin mice has facilitated our understanding. However, it is important to be aware of species specificities which make it difficult to extrapolate the results to the human situation. The timing of development in relation to birth among mammals presents substantial differences, even if the sequence of events might be similar (15-17). The web resource www.translatingtime.org (18) provides tools to compare neurodevelopmental time across species. As an approximation, the newborn rat may be compared with a second-trimester human fetus, and the maturation of a newborn human cerebral cortex to that of a 12-13-day old rat pup (19). For integrative reviews on molecular and evolutionary aspects of the cerebral cortex and cerebellar development and the effects of thyroid hormones see (20-23).

 

Whenever possible, this chapter focuses on observations in humans. Gene and protein notations follow the HUGO nomenclature (http://www.genenames.org): for human genes, names are in italics and in capital letters, protein names are written in non-italic capital letters irrespective of species.

 

STRUCTURAL DEFECTS CAUSED BY THYROID HORMONE DEFICIENCY

 

As an approach to understanding how thyroid hormones influence brain development, the changes caused by thyroid hormone deprivation are informative. In rats, perinatal hypothyroidism causes reduced myelination and diverse structural defects (for a classical review on this topic see (24). A reduction of the neuropil causes increased cell density in the cerebral cortex (25,26). Reduction in total cell numbers of regions with significant neurogenesis during the postnatal period, such as the olfactory bulb and the granular layers of the hippocampus and cerebellum (26-28). Transient structures show retarded disappearance. An important example is the subplate, a transient structure of the cortex involved in the organization of thalamic afferents to the cortex (29). In the cerebellum, regression of the external granular, or germinal layer, is retarded by a few days (30). The Cajal-Retzius cells, formed early during cortical development and involved in the regulation of the inside-out migration of neurons have delayed appearance (Fig. 1 left panel) (31). The GABAergic interneurons have altered distribution and connectivity, and the parvalbumin subclass is reduced in number (32-35). The maturation of several types of neurons is compromised, with stunted dendritic and/or axonal growth and maturation, for example cholinergic cells (36), cerebellar Purkinje cells (Fig. 1 central panel) (37,38), and cortex layer V pyramidal cells (39,40). Changes in dendritic spine number are also observed in the cortex and hippocampus after adult-onset hypothyroidism and are reversible with thyroxine treatment (41,42). Hypothyroidism also causes delayed and poor deposition of myelin (43-46) whereas hyperthyroidism accelerates myelination (47).  After prolonged neonatal hypothyroidism, the number of myelinated axons in adult rats is abnormally low in hypothyroid animals although most of the myelinated axons appear to have a normal thickness of the myelin sheath (Fig. 1 right panel).

Fig. 1. Examples of the effects of hypothyroidism on developmental timing, cell differentiation, and cell migration. Left panel: In the cortical plate, hypothyroidism of fetal onset reduced the number of Cajal-Retzius cells, Rln mRNA (reelin), and reelin protein at P0. Normal amounts of reelin were present in the cortex at P5, indicating that thyroid hormones control the timing of Rln expression (from (31)). Central panel: Neonatal hypothyroidism causes arrested Purkinje cell differentiation (upper panels), and delayed disappearance of the external (germinal) granular layer due to delayed migration of granular cells. The right panel shows a permanent myelin deficit in hypothyroidism. Myelin staining of the anterior commissure of adult rats with fetal/neonatal-onset hypothyroidism and their respective euthyroid controls shows a thinner and paler commissure in the hypothyroid rats.  Electron microscope imaging reveals that a higher proportion of axons of hypothyroid rats were of lower diameter and unmyelinated (from (48)).

 

Less information is available on how hypothyroidism affects the structure of the human brain. Post mortem examination in two cases of deficient thyroid hormone transport to the brain caused by MCT8 mutations revealed anatomical changes compatible with thyroid hormone deficiency (49) (Fig. 2): delayed maturation of the neocortex and cerebellum, delayed myelination, altered neuronal differentiation with lower expression of neurofilaments, and reduced synaptogenesis with reduced synaptophysin expression. Specific cellular changes included a reduced number of Cajal-Retzius cells and parvalbumin interneurons in the cortex. Cerebellar Purkinje cells had a normal morphology in contrast to the usual finding in hypothyroid rodents. These findings indicate that it is possible to extrapolate, but with caution, the brain lesions observed in hypothyroid rodents to humans.

Fig. 2. MCT8 mutations cause anatomical changes compatible with cerebral hypothyroidism already during fetal stages. The left upper panels show staining of the cortex with neurofilament (brown color) in a control fetus (a) and a fetus with a MCT8 mutation (b). The normal cortex shows staining of Cajal-Retzius cells, which were absent in the MCT8 fetus. The left lower panels show staining of the cerebellum with anti-myelin basic protein. The normal fetus (c) shows immunoreactivity, which is not present in the MCT8 fetus (d). The right panels correspond to an 11-years old boy with MCT8 mutation (panels f, h, k l, n) and control of the same age (panels e, g, i, j, m). Panels e, f: cross-section of the pyramidal track stained with neurofilament; in the MCT8-mutated patient the axonal diameter is smaller than in the control. Panels g, h: staining of the cortex with anti-parvalbumin antibody shows that parvalbumin interneurons are present in the normal cortex but not in the MCT8 cortex. Panels i-l: cerebellar staining for myelin basic protein (i, j) and myelin (k, l). Panels m. n: cerebellar staining for synaptophysin showing the presence of synaptic boutons around the body of a Purkinje cell in the control (m) but not in the patient (n). Data from (49).

 

THYROID HORMONES IN THE DEVELOPING BRAIN

 

Maternal and Fetal Thyroid Hormones

 

The relative roles of the maternal and fetal thyroid hormones on brain development is a most important question. Fetal brain thyroid hormones depend on the transplacental passage of the maternal hormones and the onset of fetal thyroid function. In rodents, the onset of active fetal thyroid secretion occurs at E17.5, but thyroid hormones are present earlier in the embryos, a few days after implantation. This hormone is of maternal origin, as T4 and T3 cross the placenta (50-55). In normal rats at term, the maternal T4 accounts for about 17.5% of the total extra-thyroidal fetal thyroxine pool (56). However, in the fetal brain the concentrations of T4 and T3 are low before the onset of fetal thyroid function, and rapidly increase several-fold from E18 to E21 (57).

 

In humans, Vulsma et al. (58) demonstrated the transplacental passage of T4 by showing that in neonates with thyroid agenesis or a total organification defect, T4 was present in cord serum at 30-50% of the normal concentration. The only possible origin of this T4 was maternal. Transfer of T4 from the mother to the fetus protects the fetal brain in congenital hypothyroidism, preventing neurological damage before birth, and making it possible that early postnatal treatment is effective (59). The importance of maternal hormones becomes apparent in situations where protection does not take place. This occurs in the combined maternal and fetal thyroid failure, as in feto-maternal PIT-1 deficiency (60), or presence of high titers of thyroid stimulation blocking antibodies (61).  In these cases, mothers and neonates had extremely low thyroid hormones. The infants suffered profound developmental delays, permanent sensorineural deafness, and irreversible neuromotor impairment.

 

STUDIES IN RODENTS

 

If thyroid hormones from the mother offer protection to the brain in cases of fetal thyroid failure, one could then ask about the relevance of maternal thyroid hormones for brain development in the presence of a normal fetal thyroid gland, or before the gland becomes functional. If maternal thyroid hormones contribute to fetal brain development, then alterations of maternal thyroid physiology might have consequences on the fetal brain. A relevant question is whether the isolated failure of the mother’s thyroid gland causes thyroid hormone deficiency in the fetal brain. Thyroidectomy in pregnant rats, a situation of maternal hypothyroidism with intact fetal thyroid, causes a reduction in the total extra-thyroidal T4 and T3 of the fetuses and developmental delays, but at E21 there was no reduction of T4 and T3 in the brain compared to fetuses from control dams (62). In these experiments, fetal thyroid activity compensated for any possible reduction of brain thyroid hormones caused by maternal thyroidectomy. More recent studies (63), confirmed that thyroidectomy of the dams did not alter the concentrations of T4 and T3 in the fetal cerebral cortex at E21, whereas dams treated with methyl-mercapto-imidazole (methimazole), which causes maternal and fetal hypothyroidism, had greatly reduced brain T4 and T3 concentrations. Microarray analysis of the fetal cerebral cortex successfully identified many genes with altered expression in the combined maternal and fetal hypothyroidism methimazole-induced model but with unchanged fetal expression in the offspring of thyroidectomized mothers.  The conclusion emanating from the comparison of these two models is that fetal brain gene expression in the rat at term is under predominant control of the fetal thyroid gland.

 

In contrast to these studies, the offspring from rat dams thyroidectomized at day 16 of pregnancy showed altered cortical lamination and other structural defects when analyzed at P40 (29). The bulk microarray studies of the cortex might have failed to identify subtle, layer-specific changes of gene expression requiring time to transduce into structural alterations. Other studies support the effect of maternal thyroid hormones before the onset of fetal thyroid function. Transient maternal hypothyroidism of pregnant rats, from E12 to E15, caused the displacement of cells in the neocortex and hippocampus of the offspring, associated with audiogenic seizures when analyzed at 40 days of age, (64). Moderate thyroid hormone deficiency during pregnancy caused neuronal ectopias in the corpus callosum of the progeny (65). Thus, although the fetal thyroid gland exerts the main control on fetal brain development, there is experimental support for subtle actions of maternal thyroid hormone before the onset of fetal thyroid function. These actions do not have yet a correlate in terms of defined molecular events, and the possibility remains that they reflect indirect effects of systemic hypothyroidism rather than direct actions of the hormones on neural targets (66).

 

STUDIES IN HUMANS  

 

Early studies on the human fetal thyroid gland development showed that colloid formation, iodide concentration, and synthesis of thyroglobulin and T4 could be demonstrated by the 11th week of gestation (67,68). Recent gene expression studies showed that the genes encoding thyroid transcription factors, thyroglobulin, thyroid peroxidase, and the TSH receptor are expressed already by week 7, and that the sodium-iodide symporter is strongly upregulated by the 10th week (69).  From the 11th week, serum total and free T4 and T3 increase with time (Fig 3) (70,71), and T4 reaches maternal concentrations by the 36th week (72). The increased T4 and T3 observed in Fig. 3 are clearly due to fetal thyroid gland activity, and it is uncertain to what extent the maternal hormones contribute to the fetal hormone pool in the presence of a normally active fetal thyroid gland. Supporting the contribution of maternal hormones in a situation of normality, its interruption might explain the relative hypothyroxinemia of premature babies (73). Early in gestation, T4 is present in low amounts in the coelomic fluid from the 5th-6thgestational weeks, and the amniotic fluid contains T4 and T3 from the 10th-12th week (74,75).

Fig. 3. Developmental events in the human fetal cortex and changes of thyroid hormone concentrations and nuclear receptor. The lower part of the figure shows the approximate timing of developmental events, the formation of the neural tube; the start of neurogenesis on GW5-5; the formation of the preplate, i.e. the formation of the first neurons, the Cajal-Retzius cells in the marginal zone, and the subplate cells; the preplate split, when the first projection neurons migrate and start filling the space between the Cajal-Retzius and the subplate cells, leading to the formation of the cortical plate; the appearance of tight junctions between vascular endothelial cells, and the blood-brain barrier (BBB), neuronal migration, and gliogenesis with the appearance of astroglial cells and oligodendrocytes. The vertical gray band marks the onset of thyroid gland function. Data on thyroid hormone concentrations in serum are from (71). Data on thyroid hormone concentrations in the cortex are from (76). Data on T3 nuclear receptor are from (77).

 

The recognition of the transplacental passage of thyroid hormones, described above, raised the question of the role of maternal T4 on fetal brain development (78). The implication is that maternal T4 exerts actions on the fetal brain before onset of the fetal thyroid gland, and even complements the fetal hormones after midgestation (79). Reports are indicating adverse effects of hypothyroxinemia on cognition and behavior of the progeny (80-82), or no effects (83). The issue is unsettled, mainly due to disparities in methodological approaches and the presence of compounding factors. Among these, the definition of hypothyroxinemia, the presence or not of hypothyroidism, iodine deficiency as the main cause of hypothyroxinemia (84), environmental pollutants, and indirect effects of hypothyroxinemia through complications of pregnancy (85-88). Clinical trials involving treatment of hypothyroxinemic pregnant women with T4 gave no clear answers yet (89-91).

 

Transport of Thyroid Hormones into the Brain

 

Other chapters of this book describe in detail the issue of thyroid hormone transport (92). This section contains only a few specificities concerning transport in the brain. The crucial role of transporters in the brain is to facilitate that T4 and T3 from the circulation cross the brain barriers (93-97). There are two main barriers in the mature brain: the blood-brain barrier (BBB) and the blood-cerebrospinal-spinal fluid barrier (98,99). The presence of tight junctions between endothelial vascular cells, or choroid plexus epithelial cells form barriers that hamper paracellular transport. Crossing these barriers requires transporters at the opposite sides of the plasma membrane facilitating the influx and efflux of the transported solutes. The gradient concentration of the free solutes determines the flux direction. The BBB surface is 5000-fold larger than that of the blood-CSF barrier (100). The larger exchange surface and the short distance of individual neurons from microvessels (8-20 µm (98)), makes the BBB the most relevant site of transport from the blood to the brain parenchyma.

Tight junctions between vascular endothelial cells, i.e., the BBB proper, are present in rodents by E15-E16 (101,102), and in humans by GW12 (99) (Fig. 3).

Fig. 4. Thyroid hormone transport and metabolism in the rodent brain. MCT8 in the blood-brain barrier facilitates the transport of circulating T4 and T3 to the brain parenchyma and OATP1C1 the transfer of T4. T4 in the astrocytes is converted to T3 by DIO2, providing additional T3 to nearby neural cells. DIO3 in neurons degrades T3 to T2 and T4 to rT3. Modified with permission from (103).

 

Further maturation and consolidation require the presence of pericytes and astrocytes in the vascular unit (104). Lopez-Espindola et al. (96) found immunoreactive MCT8 at GW12 in the human fetal brain endothelial cells, possibly highlighting the importance of MCT8-mediated transport at a time when the BBB starts limiting the entry of solutes to the brain. MCT8 was also present in other barriers such as the blood-cerebrospinal fluid barrier, the transient outer cerebrospinal fluid-brain barrier, and the ependymocytes. These findings suggest the presence of alternative routes by which T4 and T3 could reach the brain parenchyma in the fetus.

 

There is a fundamental difference between rodents and humans in the transport of thyroid hormones through the BBB. The rodent BBB contains MCT8 and OATP1C1 (Fig. 4). MCT8 transports T4 and T3 whereas OATP1C1 transports T4, reverse T3 (rT3), and T4 sulfate. In humans, as in other primates, the BBB contains MCT8 but lacks OATP1C1 (93,105). One explanation why MCT8 mutations cause profound neurological impairment in patients but not in mice is that T4 transport through OATP1C1 compensates similar mutations in mice by making T4 available as a DIO2 substrate. There is experimental support for this mechanism as follows: i. MCT8-deficient mice lack neurological impairment (106), with only slight alterations of brain gene expression (107), contrary to what would be expected; ii. DIO2 activity and T3 generation increase in the brain of MCT8-deficient mice (108,109); and iii. To achieve cerebral hypothyroidism in mice, a compound deficiency of MCT8 and DIO2, or MCT8 and OATP1C1 is needed, i.e., a knockout of Slc16a2 and Dio2 or Slc16a2 and Slco1c1 (66,107,110,111).

 

Role of Deiodinases

 

The deiodinases present in the cerebrum are DIO2 and DIO3 (112). The adult mouse cerebellum also contains significant DIO1 activity (113). Different cells express DIO2 and DIO3 in the brain. Glial cells and some interneurons express DIO2, and excitatory neurons express DIO3 (114-118). The glial cells expressing DIO2 include radial glial cells, astrocytes, and tanycytes (Fig. 5) (114,116,118). Dio2 expression occurs in most brain areas, and increase in hypothyroidism (Fig. 5). Dio3 shows a restricted high expression at P0 in discrete nuclei, related to sexual differentiation of the brain, but the physiological relevance of this expression has not been studied (Fig. 5 (119)).

Fig. 5. Dio2 and Dio3 expression by in situ hybridization. Panels A, E, Dio2: in euthyroid rats (panel A) Dio2is distributed all over the brain especially in the upper layers of the cortex, the hippocampus, the thalamic ventromedial nucleus, with increased expression in hypothyroid rats (panel B). The median eminence, infundibulum, and walls of the 3rd ventricle show high expression in tanycytes, a specialized type of glial cells (panel C). Dio2 increases in hypothyroidism also in this region (panel D). In the rest of the brain, Dio2is expressed in the astrocytes as shown in panel E after GFAP counterstaining of the in situ hybridization. Panels F-H; Dio3 expression in the mouse at P0: Selective expression in the accumbens nucleus and the anterior pole of the bed nucleus of stria terminalis (Acb/BST, panel F), BST, median and medial preoptic nuclei (MnPO and MPO, panel G) and amygdala (panel H).  Panels A, B, and E (114,115). Panels C and D are adapted from (116). Panels F,G,H are from (119).

 

Astrocytes, which exceed by ten-fold the number of neurons supply T3 to other neural cells by DIO2 deiodination of T4 (Fig. 3) (114,115,120). In rodents, the majority of astrocytes arise during the first postnatal week, and DIO2 expression and activity increase accordingly. Tanycytes are specialized glial cells lining the walls of the 3rd ventricle and the cause for the high hypothalamic DIO2 activity (121). Tanycytes in the median eminence form a blood-hypothalamus barrier, modulated in response to metabolic factors, which control the access of blood-borne substances to the arcuate nucleus (122). These cells are linked to the central control of feeding, body weight, and energy balance, and may act as stem cells to produce hypothalamic neurons (123). The hypothalamic paraventricular nucleus does not express DIO2, and T3 generated in astrocytes or tanycytes might control TRH production in this nucleus (114).

 

It is estimated that in DIO2-expressing tissues, such as the brain, brown adipose tissue, and pituitary, 50% or more of T3 derives from local T4 deiodination (124-127). In the adult rat brain, as much as 80% of nuclear-bound T3 is formed locally from T4 (128). Through DIO2 and DIO3 expression, the concentrations of T3 meet the local requirements of the particular developmental stage independently of fluctuations of circulating T3. Some discrete examples are the roles of Dio3 (129) and Dio2 (130) in the mouse cochlea with a peak of DIO2 activity on P7 before the onset of hearing, and the sequential expression of Dio3 and Dio2 in the control of retinal cone specification (131).

 

The balance between DIO2 and DIO3 activity during development is critical to ensure adequate amounts of T3 in neural tissue. Extremely high levels of Dio3 expression and activity are present in the uterine implantation site, and later on in the uterine epithelium, preventing that inadequate amounts of T4 and T3 reach the embryos at early stages (132). By the end of pregnancy in the rat, DIO2 activity increases markedly in the fetal brain, in parallel with a 10-fold increase of T4 and 28-fold increase of T3 (133,134). The increase in DIO2 activity occurs after the start of gliogenesis and generation of astrocytes by E17 (135).

 

The fetal brain produces most T3 locally from circulating T4. Circulating T3 has poor access to the fetal brain. In a classical experiment, Morreale de Escobar and coworkers (136) showed that generation of T3 from T4 provides a means to regulate the concentrations of T3 within narrow limits (Fig. 6).

Fig. 6. T3 concentrations in the fetal brain after infusion of pregnant rats with increasing doses of T4 or T3. MMI-treated pregnant rats were infused with T4 or T3 using osmotic pumps from gestational day 15 through 21. The doses shown correspond to the starting dose on E15 and remained constant during the infusion period. Infusion of T4, but not T3, increased T3 in the brain. Data from (136).

 

In this experiment, they infused increasing doses of T4 or T3 to hypothyroid pregnant rats and then measured the resulting concentrations of T3 in the hypothyroid fetal brain. After T4 infusion to the mother, the fetal brain T3 concentration increased, reaching a normal concentration with a relatively low dose of T4. In contrast, it was difficult to increase T3 in the fetal brain after T3 infusion only over a wide dose range. DIO2-dependent generation of T3 in the fetal brain from maternal T4 also occurs in euthyroid pregnant mice. Administration of T3 to the mothers increased T3 in the fetal brain, not by direct transfer, but by stimulating Dio2 gene expression and DIO2 activity (137).

 

The reason why the fetal rodent brain is not permeable to T3 is unknown. One possibility would be lack of expression of the T4 and T3 transporter MCT8, but the fetal brain expresses high concentrations of MCT8, even larger than in the postnatal period (138). One possible mechanism would be selective T3 degradation by DIO3 after crossing the brain barriers. Different cellular transport routes for T4 and T3 could be involved in the selectivity of this mechanism. Knocking out Dio3 increases the sensitivity of the fetal and postnatal mouse brain to thyroid hormones (139-141).

 

STUDIES IN HUMANS

 

In the developing human cerebral cortex, DIO2 mRNA and activity are present already by GW7-GW8 (142). The relative expression of DIO2 and DIO3 is an important determinant of the T3 concentrations as development proceeds and has a strong regional component, as shown by Kester et al. (76) (Fig. 7).

Fig. 7. T3 concentrations in the fetal cortex and cerebellum respectively at different gestation weeks. Data extracted from (76).

 

These authors measured T4 and T3 concentrations and deiodinase activities in several brain regions from GW12-GW20 fetuses. The cerebral cortex had a progressive increase in T3 concentrations (Fig. 3), which correlated with an increased concentration of T4 and DIO2 activity. The parallel increase of T4 and DIO2 activity indicated an increased DIO2 transcription since T4 inhibits DIO2 activity. DIO3 activity was very low, near the limit of detection. The cerebellum showed a completely different picture: T3 was very low, and D3 activity was the highest among all regions. The difference between the cortex and the cerebellum (Fig. 7) supports the notion that the relative expression of DIO2 and DIO3 regulate the tissue response to thyroid hormones during development.

 

These changes occur in parallel to the accumulation of the nuclear receptor (77), indicating an increased sensitivity of the cortex to thyroid hormones from GW12 onwards (Fig. 3). This occurs after the preplate split, i.e., when the first migrating neurons start the formation of the cortical plate overlapping the period of neurogenesis and cell migration. Gliogenesis and the generation of astrocytes, the main cells expressing DIO2, occur around GW25 in the cortex (143).

 

The cells expressing DIO2 before astrocytes are formed are the radial glial cells (96,118), especially the outer radial glial cells (Fig. 8). This is most surprising because the outer radial glial cells, the universal stem cells of the cortex, are especially abundant in the cortex of primates, including humans. They are responsible for the enlargement of the subventricular zone and cortex expansion in these species (144). Furthermore, there is tightly correlated coexpression of DIO2 and the T4 transporter SLCO1C1 (OATP1C1) (Fig. 9) (118), indicating that the outer radial glia is the origin of the T3 formed locally in the human cortex at midgestation.

Fig. 8. A scheme representing a cross-section of the human fetal cerebral cortex at 18 -19 weeks of gestation. MZ, marginal zone; CP, cortical plate; IZ, intermediate zone; OSVZ, outer subventricular zone; ISVZ, inner subventricular zone; VZ, ventricular zone. In red, the ventricular or apical radial glia, with their bodies located in the ventricular zone, extending an apical process to the ventricular Surface and a basal process to the Surface of the brain. In green, the outer radial glia, lacking the apical process. Migrating neurons arriving at the cortical plate along the radial glial processes. In yellow, intermediate progenitors in the internal subventricular zone. Drawing reproduced with permission from (144).

 

The locally produced T3 could be involved in neurogenesis or could act on the newly formed neurons, which express THRA (145), or interneurons expressing THRB (118) to regulate their migration or terminal differentiation. T4 could reach the radial glia from the circulation. Alternatively, T4 could reach the apical processes of the ventricular radial glia by crossing the neuroepithelium from the inner CSF, or their basal processes from the outer CSF in the subarachnoid space (146). After the onset of gliogenesis, astrocytes express DIO2 (96,118). In contrast to the specific expression of SLCO1C1/OATP1C1, SLC16A2/MCT8 is widely expressed in most cell types (Fig. 9).

Fig. 9. Single-cell transcriptome profiling of DIO2, SLCO1C1, SLC16A2, THRA, and THRB expression (data from reference 118). In color are bulk cell clusters obtained using Uniform Manifold Approximation and Projection plots from the single-cell RNA sequencing datasets of human fetuses at gestational weeks 17–18 by Polioudakis et al. (147). Endo: endothelial cells, DIV: proliferating cells, ExM-U: excitatory neurons upper cortex enriched, InCGE: interneurons from the caudal ganglionic eminence, InMGE: interneurons from the medial ganglionic eminence, IP: intermediate progenitors, Mg: microglia, Neu: excitatory projection neurons, OPC: oligodendrocyte precursor cells, oRG: outer radial glial cells, Per: pericytes, vRG: ventricular radial glial cells. Blue dots represent single cells. DIO2 and SLCO1C1 coexpress in the outer radial glial cells. DIO2 is also expressed in some interneurons.

 

THYROID HORMONE RECEPTORS IN THE BRAIN

 

Flamant and coworkers have reviewed the relative roles of the receptor isoforms on developing and adult brain, using mouse genetic models (148,149). Two different genes THRA and THRB encode two receptor types, alpha (THRA1) and beta (THRB1 and THRB2), respectively. THRA also encodes THRA2, which does not bind T3 and is not a T3 receptor.

 

The cellular and regional complexity of the brain and its asynchronous maturation requires a previous understanding of the ontogeny and patterns of expression of the receptors to understand their physiological role. The low number of receptor molecules per nucleus, and the relative lack of specificity of receptor antibodies have precluded their use to analyze receptor expression by conventional immunohistochemistry. Analysis of the regional and cellular expression requires in situ hybridization histochemistry. This technique provides data on receptor mRNA expression that may not reflect accurately the concentration of the receptor protein (150,151). 

 

Studies on receptor protein concentration used T3 binding assays in nuclei isolated from whole brain or brain regions, with a sensitivity of about 100-200 molecules/nucleus. Binding assays detect indistinctly the THRA1 and THRB subtypes, but discrimination between THRA1 and THRB may be possible to some extent using a combination of T3 binding and immunoprecipitation, or by competitive binding assays using the acetic acid derivative triac. THRA1 has the same affinity for triac as for T3, and THRB up to ten-fold higher affinity for triac than for T3 (152). Receptor expression studies have also used knock-in mice in which the receptor protein was tagged with green fluorescent protein (GFP) (145), or with hemagglutinin epitopes (151).

 

There are discrepancies between the cellular abundance of receptor-encoded mRNAs and the corresponding receptor proteins (150). At the mRNA level, all receptor isoforms are present in the brain. Apart from the highly abundant, non-receptor encoding, Thra2 transcript, the predominant receptor isoform is Thra1, widely distributed in the CNS from E14 to adulthood (151,153-155). From E19 to P0, Thra1 is present in the outer part of the cerebral cortex and hippocampal CA1 field. During the late fetal stage, Thra1 is present in the piriform cortex, superior colliculus, pyramidal cell layer of the hippocampus, and the granular layer of the dentate gyrus. In adult rodents, Thra1 expression is prominent in the cerebral cortex, cerebellum, hippocampus, striatum, and olfactory bulb. Knock-in mice studies with green fluorescent protein-tagged THRA1 (145) showed that most neurons express the THRA1-GFP as they become postmitotic in tanycytes of the third ventricle and very low expression in astrocytes. In the cerebellum, the conjugated protein was present in migrating granule cells of the molecular layer and mature granule cells of the internal granule cell layer but not in the proliferating cells of the external germinal layer (also known as the external granular layer). Juvenile, but not adult Purkinje cells expressed THRA1-GFP. In the study by Minakhina et al. (151) the hemagglutinin-tagged THRA1 protein was the predominant receptor subtype in the mouse brain, and THRA2 concentration was 5-10-fold higher.

 

In cultured cells T3 receptors are present in neurons, astrocytes, oligodendrocytes, sensory neurons, Schwann cells, and microglia (156-160). Primary cultures of mouse astrocytes, which respond transcriptionally to T3, express Thra and Thrb (7). Receptor-specific actions can be demonstrated on some genes such as Dio3, which is specific for THRA1 (161-164). Some cells express specific receptor isoforms, for example, the mouse retina expresses Thra1 widely whereas immature photoreceptor cells selectively express Thrb1 (165,166).

 

Thrb1 expression during rodent development is low during the fetal period and increases during the postnatal period and through adulthood.  Between E17 and E20, only low levels are present in the brain, especially in the hippocampal pyramidal layer. On P0, an increase occurs in the accumbens, striatum, and hippocampus. From around P7 Thrb1 appears in the cerebral cortex. The patterns of expression of Thra1 and Thrb1 overlap (Fig. 10), but in some cells, one of the isoforms is predominant.

Fig. 10. T3 receptor mRNA expression in the mouse brain, by in situ hybridization with Thra1 and Thrb1-specific probes. In the cerebrum (left panels) there is an overlapping distribution of both receptor subtypes, with some differences in the hippocampus, amygdala, and hypothalamus. In the cerebellum (right panels) Thra1 is expressed in the granular layer (left upper panel), whereas Thrb1 is expressed in the Purkinje cell layer.

 

As an example, in the cerebellum, differentiated granular cells express Thra1 while Purkinje cells express Thrb1. Thrb2, which is abundant in the pituitary, is also expressed in the retina and the hypothalamus (166,167). During fetal stages, low levels of Thrb2 are present in the striatum (153). The web resource of the Allan Brain Institute (https://celltypes.brain-map.org/rnaseq/mouse_ctx-hip_smart-seq) provides a tool to explore the specific distribution of Thra1 and Thrb during mouse development.

 

Ontogeny of Thyroid Hormone Receptors in the Developing Brain

 

T3 binding assays detect the T3 receptor for the first time in the rat brain at E13.5-E14.5 and then increase, reaching stable concentrations from E17 to P0 (168). In close agreement, studies in knock-in mice expressing a conjugated TRα1-green fluorescent protein detect the receptor for the first time at E13.5 in the cortical plate (145). This date marks the onset of brain responsiveness to thyroid hormones (141). The receptor increases at birth and the highest concentrations are reached at P6 (169,170), but receptor occupancy is maximal at P15 (171).

 

In the human brain, the receptor protein quantified by nuclear binding assays, and receptor mRNAs by PCR, are detected during the first trimester (77,142,172). The receptor protein, with a binding profile typical of THRB, is present at low levels in the fetus around GW10 and increases in concentration 10-fold up to GW18-GW18 (77) (Fig. 3), during neurogenesis and neuron migration. At these stages, subsets of CALB2 (calretinin) and SST(somatostatin) interneurons selectively express THRB (Fig. 9) (118), but its contribution to the total receptor protein measured by binding assays is unknown.

 

Occupancy of receptors with the ligand in the brain antecedes the occupancy in other tissues. In the human brain, the T3 ligand is present from GW10, at concentrations enough to result in about 25% occupancy of receptor (77,173), and when no T3 is detected in other organs. As stated above, this is due to the early presence of DIO2 activity (76).  A similar situation occurs in fetal lambs at gestational day 100. The brain had a 74% receptor occupancy compared with 10% in the liver and lung (174). In rats total receptor occupancy by the hormone increases in parallel with the postnatal increase in plasma and cytosol total and free T3, and reach a maximum of 50-60% at postnatal day 15 (171).

 

THYROID HORMONE ACTION ON BRAIN GENE EXPRESSION

 

Aporeceptor and Holoreceptor

 

Thyroid hormones regulate the expression of many genes in the rodent brain, mainly during the postnatal period, but the brain is also sensitive in the fetus and in adult animals (6,7,107,175-182). T3 interacting with the nuclear receptors, i.e., the holoreceptor, has a modulatory role on the receptor transcriptional activity, upregulating or downregulating gene expression. In the absence of the hormone, the aporeceptor has intrinsic transcriptional activity, relevant in events such as amphibian metamorphosis (183), PC12 cell differentiation (184) or development of the inner ear in mice (185). To some extent, the hypothyroid phenotype is the consequence of aporeceptor activity (30,186). Similarly, mutations of receptors that abolish T3 binding cause developmental abnormalities (166,187,188) (see the chapter on the syndromes of resistance to thyroid hormones).

 

Non-genomic Actions

 

It is unclear to what extent non-genomic actions might also mediate effects of T4 and T3 in the brain (189-191). Interaction of T4 with integrin αvβ3 might have a role in the expansion of progenitors in the neocortex (192). Proposed non-genomic actions of T3 include the maturation and plasticity of hippocampal pyramidal neurons (193), and the regulation of the onset and duration of the sensitive period of imprinting in chicks (194).

 

For many genes, the role of thyroid hormones during development is to regulate the timing of gene expression with a strong regional specificity, for example on the myelin genes (46) (see also Fig. 1). In the absence of DIO3, developmental expression of some T3 target genes is accelerated (141). The response of many target genes is region-dependent and shows partial overlap in different brain areas (9). Even within the same area, the same gene may be responsive only in a fraction of cells, as shown by Nrgn (RC3/neurogranin (175)). Transcription of thyroid hormone target genes is likely the result of the combinatorial activity of transcription factors, which include the T3 receptor whose relative weight may depend on the specific region and developmental period. It also reflects the diversity existing within apparently homogeneous cell classes.

 

Thyroid Hormone-Responsive Elements

 

Early studies showed the presence of thyroid hormone-responsive elements in the promoter or intronic regions of thyroid hormone-dependent brain genes. To name a few, the myelin basic protein (195); the Purkinje cell-specific gene PCP2 (196); the calmodulin-binding and PKC substrate RC3 (197); the prostaglandin D2 synthetase (198,199); the transcription factor Hairless (200); the neuronal cell adhesion molecule NCAM (201); and the early response gene NGFI-A (202). More recently, Chromatin immunoprecipitation assays identified receptor binding sites in the developing mouse cerebellum (203) or cerebellar cell lines expressing THRA1 or THRB1 (162). Other targets are regulated at the levels of mRNA stability (acetylcholinesterase), protein translation (MAP2 (204)) or mRNA splicing (TAU (205)). Regulation of splicing might be due to a primary action on the transcription of splicing regulators (206).

 

Genes Responsive to Thyroid Hormones in the Fetal and Postnatal Rodent Brain

 

Thyroid hormone regulation of several genes occurs in the rodent fetal brain in vivo (207-209). In the fetal cerebral cortex at the end of gestation, thyroid hormone controls the expression of genes involved in the biogenesis of the cytoskeleton, cell migration, and branching of neurites. In some studies, a large percentage of the thyroid hormone-dependent genes were related to Camk4 signaling pathways (63,210). Camk4 is regulated by T3 in cultured primary neurons (211,212), but is not in the postnatal rat brain in vivo (213). Results of extensive analysis by RNA-Seq of primary embryonal cerebrocortical cultures disclosed many thyroid hormone targets with roles during cortical development (6,20). The responsiveness of the fetal brain to thyroid hormones increases in Dio3 knockout mice (139,141).

 

Many of the thyroid hormone-regulated genes identified during the postnatal period in the rodent brain are sensitive to the hormone only during a time window that spans the first 2-3 weeks after birth. Many of these genes are not dependent on thyroid hormone in the fetal or in the adult brain, possibly due to a DIO3-related mechanism (141). Chatonnet et al. (9), performed an analysis of the published data on thyroid hormone action on gene expression in the brain and cultured cells and arrived at a list of at least 37 genes consistently found in different studies as targets of the T3 receptors. Some of them are candidates for transcriptional regulation because they contain a thyroid hormone-responsive element. These genes include Adamtsl4, Dbp, Fos, Hr, Kcna1, Klf9, Scd1, Stat5a, and Txnip. Additional genes found regulated transcriptionally by T3 in several independent studies are Shh, Hr, Dbp, Gbp3, and Nrgn.

 

In our studies, employing RNA-Seq of primary cerebrocortical cells (6) we found that T3 up-or down-regulated up to 7.7% of expressed genes 24 hours after treatment. T3 was active in the presence of cycloheximide on about 30% of these genes indicating an effect on transcription. No response to T3 occurred in similar cells from Thra and Thrbknockout mice, as expected for receptor-mediated actions. Additionally, a large proportion of the T3-responsive genes in the presence of cycloheximide contained T3-responsive elements (162). The estimation is that T3 regulates around 2.5% of all expressed cellular genes and at least 1% at the transcriptional level through a T3 responsive element.

 

Brain deprivation of thyroid hormones caused by systemic hypothyroidism causes larger changes of gene expression than compound inactivation of the MCT8 and OATP1C1 transporters, or inactivation of MCT8 and DIO2. In the MCT8 plus OATP1C1 deficiency, the transport of T4 and T3 is severely compromised, resulting in isolated brain hypothyroidism with low tissue T4 and T3 (66). In the MCT8 plus DIO2 deficiency, the transport of T3 and the generation of local T3 from T4 are blocked, leading also to brain hypothyroidism buy with normal T4 and low T3. Contrary to what would be expected, the changes in gene expression are not equivalent in these two models of brain hypothyroidism, which are also different from systemic hypothyroidism. The latter causes altered expression of about the double number of genes in the cerebral cortex and the striatum than the cerebral hypothyroidism of the double knockouts. One reason is that systemic hypothyroidism causes many effects on gene expression that are secondary to other actions of T3 elsewhere in the body. When transcriptionally-regulated genes were compared, similar profiles were found in the three conditions. In systemic hypothyroidism, among the transcriptional targets of T3, the most affected downregulated genes in the cortex were Kcnj10, Hr, Ky, Cyp11a1, Nefm, Npt, Stac2, Hcrtr1, Shh, Prlr, Sema7a, and upregulated, Aldh1a3, Adamts18, Ntf3, Mc4r, Dio2, Trhr. In the striatum, the most affected downregulated genes were Prlr, Cyp11a1, Cnr1, Sema7a, Gls2, Shh, Igsf9, Enpp2, Gdf10, Arg2, Nefm, and upregulated, Cyp26b1, Syt10, Trhr, Mc4r, Dio2, Gabra5.

 

Interactions with Glucocorticoids and Retinoids

 

The interactions of thyroid hormones with other hormonal systems (214) might be relevant to their actions on neural cells. In primary cerebrocortical cultures, one of the most significant processes regulated by T3 was the control of G-protein-coupled receptor activity (164), estimated to be important in the transition from fetal type to adult-type gene expression. A detailed analysis of these interactions is beyond the scope of this chapter, and only one example is mentioned related to the crosstalk with glucocorticoids and retinoid signaling pathways. T3 controls the expression of several enzymes involved in retinoic acid (RA) metabolism: the RA synthesizing enzymes ALDH1A1 and ALDH1A3, and the degrading enzyme CYP26B1 (9,163). The final effect on RA concentrations depends on the relative expression of each of the enzymes, which have developmental and regional variations, and on glucocorticoid signaling. Aldh1a1 is upregulated by T3, preferentially through TRα1, and glucocorticoids potentiated the effect of T3. Additionally, T3 controls the expression of Nr3c1, the glucocorticoid receptor. Aldh1a3 is downregulated by T3 in primary cells and increases in expression in hypothyroidism as indicated in the previous paragraph. On the other hand, Cyp26b1 is upregulated by T3 in primary cells and is downregulated in the hypothyroid cortex but upregulated in the hypothyroid striatum. By increasing ALDH1A1, especially in the presence of glucocorticoids, T3 will increment RA concentrations, whereas acting on ALDH1A3 and CYP26B1, T3 will reduce RA concentrations. A recent study on Dio3 knockout mice has found evidence for these interactions (141).

 

Actions on Gene Expression in the Adult Brain

 

In adult subjects, thyroid hormones influence mood and behavior, and thyroid dysfunction affects neurotransmitter systems (215) often leading to psychiatric disorders (216). High doses of T4 improve mood in bipolar depression (217,218). In the adult rat striatum, administration of a large single T3 dose leads to up-regulation of 149 genes and down-regulation of 88 genes (177). Physiological doses of T3 given for several days to hypothyroid animals led to up-regulation of 18 genes, and down-regulation of just one gene. Therefore, acute large doses of thyroid hormone cause large changes in gene expression, with more modest changes with lower doses. Some of the regulated genes are related to circadian rhythms and wakefulness, with one of them (Dbp or D-site binding protein) proposed as a candidate gene in bipolar disorders (219), and likely to be regulated directly by TRα1 (176). Many other genes were involved in striatal physiology as components of several signaling pathways. Fig. 11 shows a putative model of T3 action on the adult striatum.

Fig. 11. Regulation of gene expression by thyroid hormones in the adult striatum. Signaling pathways are schematically represented and the main groups of regulated genes are shown in numerals. 1: G-protein coupled receptor signaling (Cnr1, Rgs9, Rasd2, Rasgrp1). 2: Ca2+/calmodulin pathway (Nrgn); MAPK pathways (Map2k3, Fos). 4: Early genes (Nr4a1, Arc, Dusp1, Egr1, Homer). 5: Ion channels (Scn4b). Abbreviations: VDCC, voltage-dependent sodium channels, NMDA, N-methyl-D-aspartate, D1 and D2, dopamine receptors 1 and 2.  From (177).

 

ACTIONS OF THYROID HORMONES ON SPECIFIC DEVELOPMENTAL EVENTS

 

There are several possible approaches for studying the role of thyroid hormones on brain development. One way would be to analyze the effects of hypothyroidism and thyroid hormone treatment on the development of the cerebral cortex, the striatum, the cerebellum, the hippocampus, and other regions. Another way is to analyze common events occurring in the different brain regions, such as neurogenesis, cell migration, and differentiation, among others. In this review, we have opted for this second approach, with a focus on the T3 regulation of genes involved in these processes. Reviews on the cerebral cortex, and the cerebellum are available (20,21,23).

 

Neurogenesis

 

Generation of neurons, or neurogenesis, starts in humans around the fifth gestational week (GW5, Fig. 3), and in mice around embryonic day 10 (E10). The bulk of neurogenesis takes place until GW25 or E16/17 respectively, partially overlapping gliogenesis. The granular cell layers of the hippocampus, olfactory bulb, and cerebellum continue accumulating neurons postnatally, a reason why they are especially sensitive to thyroid function. At the onset of neurogenesis, the neuroepithelial cells sequentially express Pax6, Neurog1/2, and NeuroD, undergoing glutamatergic identity, and under the influence of FGF10 undergo a fast transition to radial glia (20,220). These cells remain attached by an apical process to the ventricular surface and elongate as the embryonic brain epithelium thickens, adopting a bipolar morphology (Fig. 8). In the cortex, the elongated basal process reaches the pia providing a scaffold for cell migration (221,222). In addition to this structural and supportive function, the radial glia is the universal cortical stem cell that generates all neurons and glia, directly or through intermediate precursors. (220). In primates, enlargement of the cortex is due to the accumulation of a population of radial glial cells that lose the apical process and accumulate in the outer part of an enlarged subventricular zone (Fig. 8). Human-specific changes occur with further enlargement of the cortex over the great apes (223).

 

The role of thyroid hormones on proliferation and differentiation of neural precursors in the embryonic neurogenic areas has been shown during tadpole premetamorphosis (224) and concerning the effects of maternal thyroid hormones (192,225,226). NeuroD, mentioned above, is sensitive to thyroid hormones in the cerebellum (227), and THRA1 mutations cause abnormal proliferation and adhesion of human cortical progenitors (228). Iodine deficiency in rats affects hippocampal radial glial cells (229). The first neurons originating in the cortex, the Cajal-Retzius cells, and the subplate cells, are sensitive to thyroid hormones in rodents and in humans (31,49,210).

 

Limited neurogenesis also occurs in the adult brain, related in humans to neuropsychiatric conditions, cognitive deficits, and depression. Adult neurogenesis takes place in two structures: the subventricular zone, located underneath the surface of the lateral ventricles, and the subgranular zone, adjacent to the granular layer of the hippocampal dentate gyrus. The subventricular zone generates olfactory bulb interneurons in adult rodents, and in humans provides new interneurons to the adjacent striatum (230). The subgranular zone generates dentate gyrus granular neurons in adult rodents and humans (231). Hypothyroidism and thyroid hormones influence neurogenesis in the rodent subventricular zone and subgranular zone (224,225,232-241).

 

Neural Cell Differentiation

 

Thyroid hormone controls the expression of many genes with roles on terminal cell differentiation, such as cell cycle regulators, cytoskeletal proteins, neurotrophins, and neurotrophin receptors, and extracellular matrix proteins. Among the cell cycle regulators, E2F1, p53, cyclins, and cyclin-dependent kinase inhibitors are regulated by thyroid hormone in cell culture (242-244).

 

Neural cell shape is determined by the cytoskeleton, which consists of microtubules (tubulin), microfilaments (actin), and intermediate filaments, specific for neurons (neurofilaments), glia (glial fibrillary acidic protein), or maturing cells (vimentin, nestin). Tubulins α1 and α2 are downregulated by thyroid hormone, and tubulin β4 is upregulated (245,246). Microtubule-associated proteins (MAPs) are also under thyroid hormone control at a posttranscriptional level. For example, thyroid hormone regulates Map2 protein distribution in the Purkinje cell dendritic tree (204), and conversion of immature forms of the microtubule-associated protein TAU (MAPT) to mature forms by alternative splicing of the MAPT mRNA (205). The neurofilament genes Nefh, and Nefm are also under thyroid hormone control in the fetal and postnatal cerebral cortex (63,107).

 

Astrocytes (247,248) cerebellar Golgi epithelial cells (249), and microglia (160) are thyroid hormone targets. Thyroid hormones influence the in vivo expression of astroglial genes encoding tenascin C, laminin, and L1 adhesion molecule, with effects on neuronal migration and differentiation, and axonal fasciculation (250-252) . In vitro, β-adrenergic receptor antagonists block the effect of T3 on astrocyte differentiation (253). In another studies T3 upregulates Arrb1 (arrestin beta1), facilitating endocytosis of β2 adrenergic receptor (ADRB2) and ERK activation (254).

 

The control of neurotrophin expression might mediate some of the effects of thyroid hormone on differentiation and survival. Interactions between thyroid hormone and NGF are important for the growth and maintenance of cholinergic neurons in the basal forebrain (36). Unliganded THRA1 in PC12 cells expressing Thra1 blocks NGF-induced differentiation and the blockade is released by T3 (184), suggesting a possible mechanism for the control of differentiation timing. Changes in NGF, NTRK, P75NTR, BDNF, and NTF3 after hypothyroidism have been described (255-257).

 

Thyroid hormones influence myelination directly through effects on oligodendrocyte differentiation (258,259), and may also have indirect effects by stimulating axonal maturation, which is impaired in hypothyroidism (Fig. 1 right panel (260,261)). A lower axon diameter prevents reaching the critical size to become myelinated (262). Low axon diameter also occurred in a patient with MCT8 mutation (Fig. 3 (49)), which might contribute to the hypomyelination of these patients.

 

As thyroid hormones are required for terminal differentiation of oligodendroglial cells, they influence the expression of practically all myelin protein genes, but only during the myelination period. In the rat, this period extends from about the end of the first postnatal week up to the end of the first month, with a strong regional component in parallel with the wave of myelination (46,263). The control of myelin gene expression is transient, and in hypothyroidism, there is a developmental delay, but the effect on the myelin content is permanent (Fig. 1).

 

Early studies showed that T3 inhibits proliferation and promotes differentiation of oligodendrocyte precursor cells (OPC) (264,265) through repression of the E2F1 transcription factor (266). Transcriptomic analysis identified the universal T3-target gene Klf9 (Krüppel-like factor 9) as a mediator of the effect of T3 on OPC differentiation (267). The transcriptional repressors NCoR and HDAC repress the oligodendrocyte differentiation pathway. T3 relieves this repression and induces Sox10, needed for the maintenance of the differentiated state (268). Oligodendrocyte precursor cells express Thra1 and Thrb (269-271), but Thra1 is the predominant receptor gene expressed in the newly formed and myelinating oligodendrocytes.

 

Neural Cell Migration

 

Thyroid hormone also influences neuronal migration in the cerebral cortex, hippocampus, and cerebellum. Thyroid hormone deficiency during cortical development leads to less than the normal definition of cortical layers (260,272,273). Thyroid hormone influences the maturation of the radial glia, the path along which radial migration occurs in the cerebral cortex and the hippocampus (229).

 

An important cellular target of thyroid hormone is the Cajal-Retzius cell. These cells are located in the marginal zone, the future cortex layer 1, and are required for proper migration of neurons in the cerebral cortex and the hippocampus, cortical lamination, and establishment of synaptic connections (274-276).  These cells secrete reelin (RLN), an extracellular matrix protein under thyroid hormone control (31). RLN function is essential for the inside-out pattern of cerebral cortex development. The protein disabled (DAB), a component of the RLN signaling pathway is also under thyroid hormone control. The Rln and Dab1 genes are not regulated by T3 at the transcriptional level, but other genes expressed in Cajal-Retzius cells are transcriptional targets of T3 in primary neurons: these include Rgs4, Npnt, Ephb6, Clstn2, and Dnmt3a (6,20). Cajal-Retzius cells, therefore, appear to be important and selective cellular targets of thyroid hormones during development. Their number is low in perinatal hypothyroidism in rat pups (277), and in a human embryo with mutated MCT8 (49).

 

There are many extracellular matrix proteins regulated by thyroid hormones, with actions on cell migration and neuronal differentiation, synaptic plasticity, etc. The extracellular matrix proteins are heterogeneous, comprising laminin, fibronectin, collagen, neurotrophic factors, adhesion molecules, hyaluronan proteins, proteoglycans, and other components. As already mentioned, thyroid hormones regulate negatively tenascin C, laminin, and L1 (250-252), and the adhesion molecule NCAM (201,278). In addition to these proteins, regulated by T3 in vivo, T3 regulates many other genes of this heterogeneous group in cultured primary cerebrocortical cells (6) twenty-five of them at the transcriptional level. Among these, seven have thyroid hormone receptor responsive elements: Adamts2, Lingo3, Mfap3l, Bmp1, Megf10, Nav2, and Crim1. The function of these genes and the significance of their regulation has been discussed (20).

 

In the rodent cerebellum (279,280) thyroid hormones are involved in the late phase of granular cell migration from the external germinal layer (EGL) to the internal granular layer (IGL). This process takes place postnatally in rodents ending by P20 with the complete disappearance of the EGL. A characteristic feature of the hypothyroid cerebellum is a delay in the migration of granule cells and the persistence of the EGL beyond P20 (Fig. 1) (281). It is unclear how the absence of thyroid hormone interferes with granule cell migration. Cell migration in Thra1 knock-out mice proceeds normally, and there is no effect of hypothyroidism (30). This suggests that the migration defect in hypothyroid mice represents a non-physiological action of the unliganded receptor. On the other hand, these cells express Thra1 during the migration period (P7-P19) and not before (145), which might indicate an action of the receptor on granule cell maturation during migration. Several groups have described T3-regulated genes involved in different processes (9,163,282).

 

CONCLUSIONS

 

Thyroid hormone actions in the brain are extremely complex with a continuously changing landscape as development proceeds. Brain maturation involves continuous changes in cell composition, i.e., the target organ of thyroid hormone is under constant change. This is reflected in the changing regulated gene network. Genes that are transcriptional targets of T3 at a certain developmental time may be refractory at another time. The importance of thyroid hormone for the brain requires tightly controlled mechanisms of thyroid hormone delivery to the brain and cellular interactions in the metabolism of thyroid hormones, with crucial roles of DIO2 and DIO3 regulating the cellular concentration of T3.

 

 Disruption of these mechanisms results in syndromes of profound neurological impairment. The challenge is to understand in detail the mechanisms of action of thyroid hormones at different stages of development in the human brain, and not merely extrapolating from rodent models, for a better understanding of thyroid hormone action defects.

 

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