Thyrotropin (TSH) secretion is regulated primarily by thyroid hormones and thyrotropin-releasing hormone (TRH). Normally, TSH secretion is exquisitely sensitive to small increases and decreases in serum thyroid hormone concentrations when they occur as a result of alterations in thyroid secretion. Serum TSH responses to TRH are altered by even smaller decreases and increases in serum thyroid hormone concentrations. This sensitivity explains the value of measurements of basal serum TSH concentrations and serum TSH responses to TRH in the diagnosis of hypothyroidism and hyperthyroidism, respectively. How TRH secretion is regulated is unknown, but the direct inhibitory effect of thyroid hormones on the thyrotrophs minimizes the stimulatory effect of any chronic changes in TRH secretion that may occur. In patients with nonthyroid illness, however, the normal relationships between serum thyroxine and triiodothyronine concentrations and TSH secretion are altered. Slightly or moderately ill patients have decreases in extrathyroidal triiodothyronine production that are not followed by an increase in TSH secretion, although the sensitivity of the thyrotrophs to further reduction or to an increase in triiodothyronine concentration is maintained. More severe illness may result in impaired TSH secretion and thus in decreased thyroidal as well as decreased extrathyroidal thyroid hormone production. These alterations in thyrotroph sensitivity and secretion, so that TSH secretion is not increased when extrathyroidal triiodothyronine production is decreased and thyroid secretion is decreased in more severe illness, suggest that decreased thyroid hormone production is a beneficial adaptation to nonthyroid illness.