Figure 1. Thyroid proteins that serve as autoantigens. Thyroglobulin (Tg) function as storage protein in thyroid cells, playing a critical role in the synthesis and release of thyroid hormones. Thyroid peroxidase (TPO) catalyzes iodination of tyrosines in thyroglobulin, which attaches one or two iodine molecules to form monoiodotyrosine (MIT) or diiodotyrosine (DIT), respectively. In addition, thyroid peroxidase catalyzes the coupling of iodotyrosine residues to form triiodothyronine (T3) and thyroxine (T4) attached to thyroglobulin. Thyrotropin receptor (TSHR) is a transmembrane G-protein coupled receptor that upon stimulation by circulating TSH activates the expression of downstream effector genes to regulate thyroid growth, thyrocyte differentiation, and thyroid hormone synthesis. Sodium/iodide symporter (NIS) is a membrane glycoprotein, which actively cotransports two sodium cations per each iodide anion, using the electrochemical sodium gradient generated by the Na+/K+-ATPase. Pendrin is involved in the apical iodide efflux in thyroid cells. It can also exchange chloride and bicarbonate. [Modified by Boguslawska et al (7)].