Serum 3,5,3'-triiodothyronine and 3,3',5'-triiodothyronine concentrations during acute heat load

To determine the changes in thyroid hormone metabolism during short periods of exposure to heat, 30 euthyroid healthy male volunteers (aged 23-40 yr) were placed in a climatic chamber for 2 h (35 C, 50% relative humidity). The subjects were at complete rest during the first hour and performed light work (40 watts) during the second hour. Blood samples for T₄, T₃, and rT₃ were drawn at 0, 60, and 120 min. Rectal temperature and heart rate were monitored continuously. No significant changes in T₄, T₃, rT₃, rectal temperature, or heart rate were observed after the first hour (basal levels, 8.5 ± 0.3 μg/dl, 160 ± 5 ng/dl, 14.5 ± 2.5 ng/dl, 37.2 ± 0.1 C, and 78 ± 8 beats/min, respectively; mean ± SEM). During the second hour, a significant rise in body temperature was recorded (38.5 ±0.1 C), accompanied by a significant decrease in mean serum T₃ concentration and a rise in mean serum rT₃ concentration. T₄ concentration remained unchanged. Our findings suggest that, parallel to the elevation in body temperature, there is a shift in the conversion of T₄ to the noncalorigenic rT₃ metabolite rather than to T₃.