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Impairment in Cognitive and Exercise Performance during Prolonged Antarctic Residence: Effect of Thyroxine Supplementation in the Polar Triiodothyronine Syndrome¹

Endocrine Service (H.L.R., N.V.D., N.S.F., H.J.L.), Departments of Medicine and Clinical Investigation (J.W.), Madigan Army Medical Center, Tacoma, Washington 98431; U.S. Food and Drug Administration (K.R.R.), Rockville, Maryland 20857; Department of Family and Preventive Medicine (L.A.P.), University of California at San Diego, La Jolla, California 92093; Department of Exercise Science and Physical Education (H.S.C.), Western Maryland College, Westminster, Maryland 21157; and Office of Naval Research (J.T.), Arlington, Virginia 22217

Address all correspondence and requests for reprints to: H. Lester Reed, Division of Medicine, Middlemore Hospital, Private Bag 93311, Otahuhu, Auckland 6, New Zealand. E-mail: lreed{at}middlemore.co.nz

Humans who work in Antarctica display deficits in cognition, disturbances in mood, increased energy requirements, a decline of thyroid hormone products, and an increase of serum TSH. We compared measurements in 12 subjects, before deployment (baseline), with 11 monthly studies during Antarctic residence (AR). After 4 months of AR (period 1), half of the subjects (T₄ group) received L-thyroxine [64 nmol·day^-1 (0.05 mg·day^-1)]; and the other half, a placebo (placebo group) for the next 7 months of AR (period 2). During period 1, there was a 12.3 ± 5.1% (P < 0.03) decline on the matching-to-sample (M-t-S) cognitive task and an increase in depressive symptoms, compared with baseline. During the intervention in period 2, M-t-S scores for the T₄-treated group returned to baseline values; whereas the placebo group, in contrast, showed a reduced M-t-S score (11.2 ± 1.3%; P < 0.0003) and serum free T₄ (5.9 ± 2.4%; P < 0.02), compared with baseline. The change in M-t-S score was correlated with the change in free T₄ (P < 0.0003) during 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 O₂ 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 seems related to circulating T₄. Submaximal exercise performance decrements, observed during AR, were not changed with this L-thyroxine dose.

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