Impairment in cognitive and exercise performance during prolonged antarctic residence: Effect of thyroxine supplementation in the polar triiodothyronine syndrome

Humans who work in Antarctica display deficits in cognition, disturbances i n mood, increased energy requirements, a decline of thyroid hormone product s, and an increase of serum TSH. We compared measurements in 12 subjects, b efore deployment (baseline), with 11 monthly studies during Antarctic resid ence (AR). After 4 months of AR (period 1), half of the subjects (T-4 group ) received L-thyroxine [64 nmol.day(-1) (0.05 mg.day(-1))]; and the other h alf, a placebo (placebo group) for the next 7 months of AR (period 2). Duri ng period 1, there was a 12.3 +/- 5.1% (P < 0.03) decline on the matching-t o-sample (M-t-S) cognitive task and an increase in depressive symptoms, com pared with baseline. During the intervention in period 2, M-t-S scores for the T-4-treated group returned to baseline values; whereas the placebo grou p, in contrast, showed a reduced M-t-S score (11.2 +/- 1.3%; P < 0.0003) an d serum free T-4 (5.9 +/- 2.4%; P < 0.02), compared with baseline. The chan ge in M-t-S score was correlated with the change in free T-4 (P < 0.0003) d uring both periods, and increases in serum TSH preceded worsening scores in depression, tension, anger, lack of vigor, and total mood disturbance (P < 0.001) during period 2. Additionally, the submaximal work rate for a fixed O-2 use decreased 22.5 +/- 4.9% in period 1 and remained below baseline in period 2 (25.2 +/- 2.3%; P < 0.005) for both groups. After 4 months of AR, the L-thyroxine supplement was associated with improved cognition, which s eems related to circulating T-4. Submaximal exercise performance decrements , observed during AR, were not changed with this L-thyroxine dose.