Acclimation of a non-indigenous sub-Arctic population: seasonal variation in thyroid function in interior Alaska.

Total, as well as free, T4 and T3 levels were obtained over four seasons for young male infantry soldiers assigned to interior Alaska. Significant seasonal variations were found in both T3 and T4. Total T4 and T3 levels were highest in winter, while free T4 and T3 levels were highest in early spring. Correlations with melatonin levels from a concurrent study showed an association between late day (17.00) mean spot melatonin levels during the preceding summer and T3 levels in winter and spring. Differences in seasonal T4 and T3 levels between indigenous and newly arrived people in the sub-Arctic may be related not only to cold acclimation but also to light.