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 T₃, rT₃), thyronine-binding globulin (TBG), thyroid stimulating hormone (TSH) and cortisol in summer and in winter were compared in carefully selected normal adult men (aged 30-49 yr); 107 people from Sapporo (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, while 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 difference was not statistically significant. However, a paired analysis of rT₃ concentration in the hospital workers had significantly higher levels in summer than in winter, showed a significant rise in the rT₃ to T₃ ratio in summer compared to winter. These results were not likely to be due to changes in TBG, cortisol and hematocrit values. The present findings and our previous report on a similar TSH response to thyrotropin-releasing hormone 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.