Figure 4. The relationship between serum TSH and free T4 concentration is shown for normal subjects (N) and in the typical abnormalities of thyroid function: A, primary hypothyroidism; B, central or pituitary-dependent hypothyroidism; C, hyperthyroidism due to autonomy or abnormal stimulation of the gland; D, TSH-dependent hyperthyroidism or thyroid hormone resistance. Note that linear changes in the concentration of T4 correspond approximately to logarithmic changes in serum TSH (178).