Levels of circulating total thyroxine (TT4), free thyroxine (FT4), total triiodothyronine TT3 and thyrotropin (TSH) were determined in 27 men with unipolar major depressive disorder ages 24-50, mean +/- SEM 36.9 +/- 2.9 years, and 38 healthy controls (HC) ages 20-50, mean +/- SEM 34.2 +/- 3.1 years. No significant differences were observed between HC and depressed men with regard to TT4 and FT4. Mean TT3 levels were lower, and mean TSH levels higher in depressed patients than in HC, p less than 0.05 for both, compatible with possible subclinical primary hypothyroidism in depressed patients. Consistent with this, an inverse correlation between basal TSH values and TT3 (r = -0.38, p less than 0.05) was noted in depressed but not in HC subjects.