Circannual changes in thyroid hormone physiology: the role of cold environmental temperatures.

Hypothyroid subjects given a constant dose of thyroxine (T4) show both an increase in serum thyrotrophin (TSH) of approximately 50% and a decline of serum T4 by approximately 15% during the winter months. Euthyroid subjects who are older than 41 years of age show an increase of approximately 30% in serum TSH during the winter months without a change in T4. Young euthyroid subjects, between the ages of 19 and 41 years, show neither a circannual pattern of TSH nor T4 but have an increase in triiodothyronine (T3) plasma clearance rate and production rate of approximately 30% during the winter seasons. Additionally, young subjects living for greater than 5 months in Antarctica show a doubling in T3 plasma clearance rate and production rate, as well as elevations of approximately 30-50% in TSH and small declines in T4. Repeated exposure to cold air has recently been reported to induce declines in serum T3, and T4 and increases in T3 clearance and production rate, thus linking a possible physiological stimulus to these seasonal observations. This collection of studies from several laboratories supports the concept that T3 kinetic changes have a seasonal dependence and that cold exposure provides one possible mechanism. As the subjects age, have a fixed thyroid hormone dosage for hypothyroxinemia, or spend an extended number of months in polar conditions, a reflex TSH rise occurs. This rise in TSH signals a hypothalamic-pituitary hypothyroxinemia whose clinical significance is unknown.