Primary organic disorders of the thyroid gland must be excluded in interpreting the thyrotropin (TSH)-releasing hormone (TRH) test in affective disease. Both endogenous depression and subclinical thyrotoxicosis are frequently associated with low basal TSH levels and a blunted (<5 mIU/L) TSH response to TRH despite thyroid hormone levels within the normal range. The present study was performed to establish whether a reduction of the hypothalamic somatostatinergic tone by treatment with the acetylcholinesterase inhibitor pyridostigmine before TRH might be useful to distinguish endocrine from affective diseases. Twelve male depressed patients (aged 41.4 +/- 3.1 years) and 12 men (aged 43.4 +/- 4.1 years) with subclinical thyrotoxicosis because of autonomous thyroid nodules were selected according to the presence of a low basal TSH level and a blunted TSH response to 200 microg TRH intravenously (IV) (TSH increment was <5 mIU/L at 30 minutes [peak] after TRH) but thyroid hormone levels within the normal range. All patients were tested again with TRH 60 minutes after treatment with 180 mg pyridostigmine orally. Eleven normal men served as controls. Basal TSH levels were 0.2 +/- 0.2 mIU/L (mean +/- SE) in depression and 0.1 +/- 0.2 in subclinical thyrotoxicosis (normal controls, 1.4 +/- 0.3). In both groups, the mean peak response to TRH was significantly higher than baseline; however, according to selection, the TSH increase was less than 5 mIU/L. Pyridostigmine did not change basal TSH levels in any group, but significantly enhanced the TRH-induced TSH increase in normal controls and in depressed subjects (TSH increment became >7 mIU/L in all depressed subjects). In contrast, no significant change in the TSH response to TRH was observed in subclinical thyrotoxicosis after pyridostigmine treatment. Basal and TRH- and pyridostigmine + TRH-induced TSH levels were significantly higher in the normal controls than in the other groups. These data show a cholinergic involvement in the blunted TSH response to TRH in patients with endogenous depression, but not in subjects with subclinical thyrotoxicosis, suggesting that these diseases could be separated on the basis of the pyridostigmine + TRH-induced TSH response test.