Assessment of thyrotropin-releasing hormone and thyrotropin reserve in man

Endogenous thyrotropin-releasing hormone (TRH) reserve and pituitary thyrotropin (TSH) reserve were assessed in four normal subjects, three patients post-cryohypophysectomy, one patient with a hypothalamic lesion secondary to trauma, and four patients with Sheehan's syndrome. TSH reserve was determined by the immunoassayable TSH response to 500 mug TRH given i.v. (TRH stimulation test). TRH reserve was assessed by the rebound response in thyroidal iodine release (TIR) following withdrawal of pharmacologic doses of prednisolone (glucocorticoid withdrawal test). When compared with normals, the post-cryohypophysectomy patients demonstrated parallel impairment of TRH stimulation and glucocorticoid withdrawal testing. The patient with the hypothalamic lesion and the four patients with Sheehan's syndrome all had normal TRH stimulation tests, indicating adequate TSH reserve capacity, yet had abnormal glucocorticoid withdrawal tests, indicative of impairment in endogenous TRH reserve or neurohumoral transport. Three of the patients (hypothalamic injury and two Sheehan's) with impaired TRH reserve were euthyroid. THE FOLLOWING CONCLUSIONS WERE REACHED: (a) A combination of the TRH stimulation test and glucocorticoid withdrawal test may allow for differentiation between pituitary and suprahypophyseal disorders. (b) Certain cases of Sheehan's syndrome appear to have impaired endogenous TRH reserve or transport. (c) Euthyroidism can be maintained in spite of diminished TRH reserve.