Acute and chronic effects of high glucocorticoid levels on hypothalamic-pituitary-thyroid axis in man

It is known that glucocorticoids can influence anterior pituitary hormones other than ACTH. Their effects on the hypothalamic-pituitary-thyroid axis are controversial. To further investigate this issue, the acute and chronic effects of high plasma cortisol levels on TSH secretion were evaluated in 20 normal subjects and in 14 patients with Cushing's syndrome. In normals, high plasma cortisol levels were obtained by giving ACTH 250 micrograms or CRH 100 micrograms iv as a bolus or by an hydrocortisone 500 mg infusion over 1 h. Acute cortisol increase produced no effect both on basal and TRH-stimulated TSH secretion. In patients with Cushing's syndrome, basal TSH levels, low or suppressed, showed an impaired response to TRH, inversely correlated with urinary cortisol values. After successful surgery, TSH and its response to TRH became normal in concomitance with the normalization of plasma and urinary cortisol levels. Our data show the lack of an acute inhibitory effect of high plasma cortisol levels on TRH-TSH axis. However, after long-term exposure to high plasma cortisol levels, i.e. Cushing's syndrome, inhibition of both basal and TRH-stimulated TSH secretion was demonstrated. These findings indicate that only prolonged hypercortisolism does interfere with pituitary TSH secretion. The underlying mechanisms are still unclear.