Reciprocal changes in serum concentrations of triiodothyronine and reverse triiodothyronine between summer and winter in normal adult men.

Abstract

Serum concentrations of thyroxine (T4), 3,5,3''-triiodothyronine (T3), 3,3'',5''-triiodothyronine (reverse[r]T3, thyronine-binding globulin (TBG), thyroid-stimulating hormone (thyrotropin, TSH) and cortisol in summer and in winter were compared in carefully selected normal adult men (aged 30-49 yr), 107 people from Sapporo, Japan (58 in summer and 49 in winter) and 15 hospital workers. A modest but significantly higher serum T3 concentration was recorded in winter than in summer; T4, TSH, TBG and cortisol levels were similar in both seasons. Serum rT3 concentration in the Sapporo subjects was slightly higher in summer than in winter but the difference was not statistically significant. A paired analysis of rT3 concentration in the hospital workers had significantly higher levels in summer than in winter and showed a significant rise in the rT3 to T3 ratio in summer compared to winter. These results were probably not due to changes in TBG, cortisol and hematocrit values. The present findings and a previous report on a similar TSH response to thyrotropin-releasing hormone [thyroliberin] in summer and in winter indicate that the changes in thyroid hormone metabolism rather than in thyroidal secretion would, at least partly, be responsible for the reciprocal alteration in T3 and rT3 concentrations.