The nocturnal serum thyrotropin surge is abolished in patients with adrenocorticotropin (ACTH)-dependent or ACTH-independent Cushing's syndrome.

TSH secretion was evaluated in 10 patients with ACTH-dependent (pituitary microadenoma, n = 5) or ACTH-independent [adrenal adenoma (n = 4) or carcinoma (n = 1)] Cushing's syndrome, and in 12 normal controls matched for age and sex. Serum TSH concentration was assayed at night, from 2200-0200 h, and in the morning, both basally and 30 min after iv injection of 200 micrograms synthetic TRH. Patients with hypercortisolism showed significantly reduced serum total T4 and T3 and free T3 concentrations and increased serum reverse T3 levels. Their mean baseline serum TSH concentration in the morning, albeit slightly lower, did not significantly differ from those of controls. The mean peak TSH value after TRH was significantly reduced, and a blunted TSH response to TRH was found in 4 out of 10 patients. At variance with normal controls, who showed nighttime TSH values 63-228% higher than morning values, 9 out of 10 patients had nighttime levels not different from or even lower than those in the morning; the remaining patient had nighttime TSH values marginally (33%) higher than in the morning. An inverse relationship (r = 0.80, P less than 0.001) was found between serum cortisol and TSH values both at night and in the morning. No differences were found either in the pattern of TSH secretion or in the TSH response to TRH between patients with ACTH-dependent and those with ACTH-independent Cushing's syndrome. These results show a substantial impairment of TSH secretion, and in particular the loss of the nocturnal surge of the hormone, in patients with Cushing's syndrome. Although the origin of the nocturnal TSH rise is probably multifactorial, cortisol, at least when secreted in excess, appears to play an important role in its regulation.