Study of the Pituitary-Thyroid Functions at High Altitude in Man

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Abstract
The alterations in serum levels of T3, T4, TSH and TBG, TSH response to 100 μg iv TRH, and urinary excretion of T3 and T4 were studiedin 8 healthy men at sea level (SL), on days 1, 2, 4, 8 and 16 after arrival by air at high altitude (3, 700 m, HA), and during days 5 to 7 after their return to SL. No significant alterations in serum levels of TSH and TBG or TSH response to TRH were observed during exposure to HA or on return to SL. There was, however, an acute elevation in both serum total T3 and T4. Serum total T3 from amean basal ± SE value of 128 ± 13ng/dlincreased to 320 ± 18 on day 1 and remained significantly elevated at 225 ± 48 up to day 8 after arrival at high altitude. Similarly serum total T4 increased from basal level of 9 ± 0.92 ug/dl to15.2 ±1.2 and remained elevated till day 16 and it was 11 ± 1.19 μg/dl during days 5 to 7 after return to SL. The urinaryexcretion of both T3 and T4 was decreased. These changes perhaps were theresult of complex physiologic adjustments on acute exposure to high altitude, like shrinkage of the T3 and T4 distribution pools, altered binding capacities of tfiyroid hormones binding proteins, and a reduction in clearance of thyroid hormones from the plasma compartment; and were probably not suggestive of an enhanced thyroid activity. Their actual significance in high altitude adaptation in man is not clearly understood.