Thyroid hormone responses to military winter exercises in the Arctic region

The effects of military field exercises in the arctic on thyroid hormone concentrations were examined in Norwegian soldiers (n = 35). Originally the soldiers were divided into 4 groups having low (2 h) or high (6 h) levels of daily sleep, and low or high physical work requirements during the field exercises. The operations were three days of stimulated combat scenarios during winter. Day 1 consisted of 30 h without sleep. Sleep was then provided at approximately 18-22 h intervals thereafter. Blood samples were taken immediately prior to the exercises (BASELINE), during the exercises at 24 h, 48 h, and 72 h, and at 24 h and 48 h of recovery from the exercises (REC1, and REC2). Hormonal analysis consisted of total (T) thyroxine (TT4), free (f) T4, total triidothyronine (TT3), fT3, and thyroid stimulating hormone (TSH). No significant group interactions were observed therefore groups were pooled. However, main effects over time (i.e., days) were observed for all hormones (p < 0.001). TSH declined throughout the exercises, as did fT3. TT4, fT4, and TT3 increased in the first 24 to 48 h of the exercises, then progressively declined thought REC1. By REC2, TT4, fT4, and TSH showed trends towards returning to BASELINE levels, although recovery was not complete. The findings from this study demonstrates that thyroid hormone concentrations become reduced with as little as 72 h of military field operations in an arctic winter environment. However, short-term sleep deprivation and differing levels of physical work seem to have no major impact on the magnitude of the hormonal changes.