9418466 Reed People who live and work in Antarctica for longer than four to five months develop a characteristic constellation of symptoms and hormonal changes called the Polar T3 Syndrome. These people have previously been described as having a 40 percent increase in energy requirement, frequent mood disorders, doubling of the production, utilization, and tissue stores of the most active thyroid hormone, triiodothyronine (T3), a decline in central nervous system thyroxine (T4), and acquisition of physiologic cold adaptation. These apparent discordant and compartmentalized tissue responses will be studied over a four-year period utilizing a multidisciplinary approach carried out by experienced polar physiologists, endocrinologists and psychologists. The possible cognitive and metabolic changes in performance from declines in central nervous system T4 and elevations in skeletal muscle T3 content will be studed. Placebo-controlled T4 replacement directed at the central nervous system deficit will be carried out and measured with cognitive instruments. T3 content within the cardiovascular system will be evaluated by utilizing submaximal exercise testing to differentiate resting from activity mediated energy use contributions by the skeletal muscles. Additionally, tissue samples of skeletal muscle will provide information regarding the genetic coding for T3 responsive proteins to help better characterize the thyroid status of these muscles. Moderate energy restriction will be used along with T4 supplementation to study the dependence of T3 production, distribution and tissue stores upon both pituitary generation of thyrotropin and energy intake. Analysis will be carried out using each subject's baseline determined in the predeployment situation of California and compared with periods and standardized measures obtained during the antarctic summer and winter. It is proposed that a correction of the low T4 state in the central nervous system can be managed w ith T4 supplementation without dramatically changing energy requirements as suggested by previous human studies using cold air chamber experiments. If this thesis is correct, then characteristic declines in mood and memory during winter seasons in circumpolar regions may be attenuated by T4 supplementation without disadvantageous effects upon their energy metabolism. Additionally, this project will expand information regarding the ultimate regulation and maintenance of the increased T3 production which is a central determinant of the Polar T3 Syndrome.