Thyroid function in sojourners and acclimatised low landers at high altitude in man.

The circulatory levels of T4, T3, rT3, TSH as well as TSH response to TRH, thyroid hormone binding proteins and T3 concentration of erythrocytes were studied in (i) healthy euthyroid sea level residents (SLR) at sea level, (ii) during three weeks of stay of SLR at an altitude of 3500 m (sojourners, SJ), (iii) SLR staying at high altitude (HA) for 3 months to 10 years (acclimatised low landers. ALL), (iv) high altitude natives (HAN) and (v) euthyroid men during intermittent exposure to simulated altitude of 3500 m in a hypobaric chamber maintained at an ambient temperature of 22 degrees C to 24 degrees C. Hypoxic stress either simulated or natural, produced marked elevation in plasma T4 and T3 within 4 h and the increased levels were maintained during the entire period of exposure. The circulatory levels of T4 and T3 were higher in HAN and ALL compared to SLR values. The T3 concentration of erythrocytes was decreased (P less than 0.01) at HA, whereas plasma rT3, TBG and T4 binding capacities of TBG and TBPA did not show any appreciable change. Plasma TSH at high altitude in SJ, ALL and HAN was not significantly different from the SLR values. Furthermore, when L-eltroxine treated (L-T4, 0.5 mg/d for 11 days) euthyroid men were subjected to simulated altitude, there was an elevation in both T4 and T3 suggesting that the rise in hormone levels was independent of pituitary secretion of thyrotropin. Both T4 and T3 returned to SLR values when SJ and HAN were brought down to SL.(ABSTRACT TRUNCATED AT 250 WORDS)