Absence of receptors for thyrotropin (TSH)-releasing hormone in human TSH-secreting pituitary adenomas associated with hyperthyroidism.

Abstract

In patients with TSH-secreting pituitary adenomas associated with hyperthyroidism, TSH secretion usually does not respond to exogenous TRH stimulation. To determine the basis for this unresponsiveness, we studied TRH binding to the membranes of two such TSH-secreting pituitary adenomas. The patients, a 28-yr-old man and a 60-yr-old woman, had clinical and biochemical hyperthyroidism with increased serum TSH levels (15.7 and 14 mU/L, respectively; normal range, 1.1-7.2) and alpha-subunit to TSH molar ratios greater than 1 (2.4 and 1.7, respectively). Neither patients had an increase in serum TSH in response to TRH (200 micrograms, iv). Immunocytochemistry of the two adenomas, removed by transsphenoidal surgery showed a pure population of thyrotropic cells. Binding studies were performed by incubation of tumor cell membrane suspensions with increasing amounts of [3H]TRH. Two PRL-secreting adenomas and one normal human pituitary were used as controls. The characteristics of the TRH-binding sites from the control tissues were similar to those previously reported (Kd, 56, 30, and 49 nM; maximum binding, 187, 46, and 94 fmol/mg protein, respectively). In contrast, no specific TRH binding was found in the two TSH-secreting adenomas. We conclude that the unresponsiveness of TSH after TRH administration is related to the absence of specific TRH-binding sites in these thyrotropic tumors.