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Thyroid Hormone Economy in Response to Extreme Cold Exposure in Healthy Factory Workers^*

Department of Medicine, Division of Endocrinology, Dr. M. Stojanouic University Hospital Zagreb, Yugoslavia

Address requests for reprints to: M. Solter, M.D., Department of Medicine, Division of Endocrinology, Dr. M. Stojanovic University Hospital, Vinogradska c 29, YU-41000 Zagreb, Yugoslavia.

The effects of cold exposure on serum total T₄ (TT₄), total T₃ (TT₃), free T₄ (FT₄), free T₃ (FT₃), rT₃, TSH, T₄-binding globulin (TBG), and T₃ resin uptake were investigated in 82 euthyroid factory workers. Twenty-five workers (group 1) were exposed intermittently (3.5 h daily) to extreme cold (–40 to –20 C) during the 8-h work shift, and 47 (group 2) were exposed to moderate cold (–10 to 8 C) for the entire 8 h. Ten individuals working at room temperature for the same period also were studied. After cold exposure, serum TT₄ decreased in group 1 and did not change in group 2, whereas FT₄ did not change in group 1 and increased in group 2. After exposure, serum TT₃ and rT₃ decreased significantly in both groups, while FT₃ did not change in either. The basal serum TT₄levels in groups 1 and 2 were significantly lower than those in the control group, whereas those of FT₄ and FT₃ were higher. Thus, cold exposure had opposite effects on total thyroid hormones and their free fractions, consistent with a cold-induced decrease in thyroid hormone-binding capacity. A postexposure decrease in serum TBG was found in women in group 2, but not in men in either group 2 or group 1, suggesting that factors other than decreased TBG are also involved. The results suggest the possibilities that 1) decreased thyroid hormone-binding capacity is an adaptive response to cold exposure, and/or 2) increased free thyroid hormone levels in response to cold exposure result in a new higher equilibrium between extracellular and intracellular FT₄ and FT₃.

^* This work was supported by grants from Gavrilovic Meat Industries (Petrinja, Yugoslavia) and Croatian Council for Scientific Research Grant 1-08-03-08-08.

Received November 25, 1988.

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