Thyrotrophin and prolactin responsiveness to thyrotrophin releasing hormone in Cushing's disease.

Serum thyrotrophin response to thyrotrophin releasing hormone was impaired in all of eight subjects with untreated Cushing's disease; while all of six similar subjects tested after adequate treatment by a pituitary implant of 198Au then showed a normal response. In all five of the untreated patients studied, the serum prolactin response to TRH was normal. Thus chronically excessive cortisol levels as seen in Cushing's disease inhibit pituitary TSH responsiveness to TRH without affecting the prolactin response to TRH. Evidently the release of TSH and of prolactin by TRH are governed by independent mechanisms.