The pituitary-thyroid axis in healthy men living under subarctic climatological conditions

In order to evaluate the effects of climatic factors on the secretion of th yroid hormones and TSH in a high latitude population, we have taken serum a nd urine samples From 20 healthy men from northern Finland (67 degrees -68 degrees N) every 2 months for a period of 14 months. Serum free triiodothyr onine (T-3) levels were lower in February than in August (3.9 vs 4.4 pmol/l , P<0.05) and TSH levels were higher in December than during other months ( 2.1 vs 1.5-1.7 mU/l, P<0.01). Serum total and free thyroxine (T-4), total T -3 and reverse T-3 levels and urinary T-4 levels were unchanged. Urinary T- 3 levels were significantly higher in winter than in summer. Serum free T-3 correlated highly significantly with the outdoor temperature integrated ba ckwards weekly for 7-56 days (r=0.26 for 1-56 days) from the day when the b lood samples were taken. Serum TSH did not show any significant correlation with the thyroid hormones or with the integrated temperature of the previo us days, but it did show an inverse and significant correlation (r= -0.31) with the ambient luminosity integrated backwards for 7 days front the day w hen the blood sample was taken. The gradually increasing correlation betwee n outdoor temperatures and serum free T-3 suggests that the disposal of thy roid hormones is accelerated in winter, leading to low serum bet T-3 levels and a high urinary free T-3 excretion. Since there was no correlation betw een thyroid hormones and serum TSH, the feedback mechanism between TSH and thyroid hormones may not be the only contributing factor, and other factors such as ambient luminosity may at least partly determine serum TSH in thes e conditions. Also urinary free T3 appears to be a novel and non-invasive i ndicator for thyroid physiology.