Thyroid function in seasonal affective disorder.

Morning serum levels of triiodothyronine (T3), free thyroxine (F-T4) and thyroid-stimulating hormone (TSH) were measured in Winter-Seasonal Affective Disorder patients in nondepressed state in summer and before and after light treatment in winter. Practically all patients had hormone levels well within the reference ranges. T3 and F-T4, but not TSH, were significantly higher in winter than in summer; this is considered a normal phenomenon. There was no significant correlation between hormone levels and severity of winter depression. Light treatment did not alter serum hormone levels.