Thyroid function during a prolonged stay in Antarctica

Adaptation of the thyroid gland to the Antarctic environment was studied in nine healthy euthyroid tropical men of the Sixth Indian Antarctic Expedition during 1 year of their residence at polar latitudes. Circulatory concentrations of thyroid hormones, total T-4 (TT4), total T-3 (TT3), free T-4 (FT4)), free T-3 (FT3), reverse T-3 (rT(3)), thyroxine binding globulin (TBG), T-3 uptake and thyroid stimulating hormone (TSH) were estimated in New Delhi and during the first week of each month of the stay in Antarctica. At the end of the Austral summer in March, the TT3 concentrations were found to be significantly lower (P < 0.01) compared to values recorded in New Delhi and showed a significant increase (P < 0.05) during the Austral winter in August. The mean TT3 concentrations from May to December were found to be significantly higher than the March or April values. Plasma TT4 and rT(3) concentrations tended to decline in March but remained unaltered during the entire period in Antarctica. The FT4, FT3, TBG and T-3 uptake did not show any appreciable change. Though, the TT3:TT4 ratio tended to decline in March and April suggesting decreased peripheral conversion of T-4 to T-3 as the possible mechanism for a decline in TT3 in March, physical exertion and prolonged exposure to extreme cold appeared to be the major contributory factors. The TSH concentration in March, April, November and December were found to be significantly higher than the New Delhi values. The morning as well as evening cortisol concentrations during the Austral winter were higher than the March values suggesting that cortisol rhythmicity was well maintained in Antarctica, albeit at a higher level. These observations indicated that the subtle changes in thyroid hormones during a prolonged stay at polar latitudes are related not only to the extreme cold but also to other factors such as physical activity, polar days and polar nights.