Changes in Plasma Thyroxine Concentration and Metabolism, Catecholamine Excretion and Basal Oxygen Consumption in Man During Acute Exposure to High Altitude1

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Abstract
Plasma total and free thyroxine concentration, thyroxine turnover, catecholamine excretion and basal oxygen consumption were measured in 5 young male subjects during a control period at 5280 feet altitude and for 8 days at 14,100 feet (Pikes Peak, Colorado). Turnover data were analyzed in a manner enabling the calculation of daily rates of thyroxine degradation during the experimental period. Thyroxine degradation was increased during the first 3 days at high altitude and thereafter remained slightly elevated. Mean basal oxygen consumption increased from 125 to 147 ml per min per m2 on the first day at high altitude and then progressively decreased toward the control value. Plasma total and free thyroxine concentration and norepinephrine excretion were not altered during the first 2 days at altitude but then increased continually for the remainder of altitude exposure. Epinephrine and total metanephrine excretion were unchanged but the increased sympathetic activity was reflected by an increase in the excretion of vanilmandelic acid. The temporal association between the initial hypermetabolism and increased thyroxine degradation at altitude suggests that these changes may be causally related. The later alterations in sympathetic activity and circulating thyroxine concentrations may play a role in the adaptation to more prolonged exposure to this environment.