Thyroid function in changing weather in a subtropical region

Serum and 24-hr urine samples were collected on 2 consecutive days during the first week of each month for 1 yr from eight healthy euthyroid men aged 25–37 yr. The means of minimum and maximum environmental temperature for the 30 days period preceding the sample collection represented the temperature for that month. Total serum thyroxine (T₄), triiodothyronine (T₃), thyrotropin (TSH), and urinary T₃ and T₄ were measured by specific radioimmunoassays and serum thyroxine-binding globulin (TBG) by the radioligand binding assay. The serum TSH and urinary T₃ and T₄ responses to 100 μg intravenous TRH were studied in five subjects during summer and again during winter. The serum concentration of these hormones and TBG did not reveal significant variations throughout the year. However, the mean urinary excretion of both T₃ and T₄ during coldest months (January and February), at 0.97 and $\text{1.95 μ}\text{g}\text{24 hr}$, respectively, were significantly higher than the corresponding values (T₃, 0.48; T₄, $\text{1.18 μ}\text{g}\text{24 hr}$) during the hottest months (May–July). The TSH, and urinary T₃ and T₄ responses to identical doses of TRH during summer and winter did not differ significantly. Since urinary T₃ and T₄ indirectly reflect the prevailing unbound serum levels of these hormones, it is likely that the greater availability of free and biologically active thyroid hormones could help the body to adapt to cold by increasing nonshivering thermogenesis.