As a part of a broad endocrine testing of patients with affective disorders the 24 h serum levels of thyrotropin (TSH) were investigated and correlated to the clinical history and disease symptoms. Thirty-two patients with the research diagnostic criteria of major depressive disorder were investigated. Twenty-six of these patients were reinvestigated in a state of full or partial clinical remission. Nine patients with unipolar and eight patients with bipolar affective disorders were also investigated in clinical remission. The control group comprised 32 healthy subjects. The results showed significantly lower 24 h serum levels of TSH and less variability of TSH levels during the 24 h period in the group with acute major depression compared with the controls. The 24 h serum levels of TSH normalized during clinical remission. Different subtypes of depression, different clusters of symptoms and severity of depression did not significantly correlate with the 24 h serum levels of TSH. There were also no significant correlations between abnormalities of the dexamethasone suppression test or nightly melatonin levels and the 24 h serum levels of TSH. The lower TSH levels seen in acute major depression could not be correlated to increased or decreased levels of peripheral thyroid hormones. The mechanism of the decreased 24 h serum levels of TSH is unclear. One possibility is an altered sensitivity in the thyrotrophs of the pituitary.