Seasonal variation of serum TSH and thyroid hormones in males living in subarctic environmental conditions.

To evaluate the seasonal influence upon thyroid hormone dynamics, we took blood samples every two months during a period of 14 months from 20 healthy males living in Northern Finland (69-70 degrees N), where the mean daily temperature ranges from a winter minimum of -40 degrees C to a summer maximum of 2 degrees C, while the photoperiod changes from a polar night of 6 days in winter to a polar day of 45 days in summer. The subjects were allowed free choice of diet, exercise, and outdoor exposure. Serum free T3 levels were lower in February than in August (3.9 vs. 4.4 pmol/l, p < 0.05) and TSH levels higher in December than during other months (2.1 vs. 1.5-1.7 mU/l, p < 0.05). Serum total and free T4 and total T3 levels were unchanged. Serum free T3 correlated significantly to the mean outdoor temperature of the preceding month. Serum TSH did not show any correlation with the mean temperature of the month or with free T3. Low serum free T3 in winter suggests that the disposal of thyroid hormones is accelerated in cold as described in the Polar T3 Syndrome. Elevations in serum TSH are not accounted for by changes in circulating thyroid hormones, suggesting that other influences, such as photoperiod, may mediate this fluctuation.