Seasonal variation of peripheral thyroid hormone levels in major depression

The seasonal variation in thyroid function tests was examined in 138 patients with major depression. No alterations in thyroxine, free thyroxine index, triiodothyronine, T3 resin uptake and thyrotropin were observed across the four seasons. This applied to both male and female subgroups. These data suggest that seasonality does not account for the wide variability in abnormalities of thyroid function reported in depression.