Serum concentrations of thyroid hormones in patients with nonseasonal affective disorders during treatment with bright and dim light

Serum concentrations of thyroxine (T-4), triiodothyronine (T-3), and thyrotropine were measured in 34 patients with nonseasonal affective disorders before and after 1 week of light treatment, Nineteen of these patients received bright white light (2500 lx) and 15 dim red light (50 lx) for 2 hours daily in the mornings over a 1-week period, Slight but significant reductions in the rating scores for the depressive symptomatology were found for both the bright- and dim-light groups, but there were no significant differences between the two groups, The improvement is thus most likely a placebo effect, Surprisingly the small changes in the severity of the depressive symptoms in the group as a whole were significantly correlated to the changes in the serum levels of T-4 during the weeks of bright- and dim-light treatment, respectively, The more a patient improved the further his or her T-4 level fell and vice versa. The fluctuations in the concentrations of T-4 during light treatment were significantly greater in the depressed patients than in a group of 12 healthy controls who also received bright or dim light, whereas the changes in T-3 were significantly smaller than those of the healthy controls, The pronounced fluctuations in T-4 levels were probably not secondary to changes in mood, Rather, they are likely to reflect changes in tissue (intracellular) metabolism of T-4, which may be involved in the mechanisms underlying the fluctuations in mood in these patients. (C) 1996 Society of Biological Psychiatry.