Altitude-induced changes in plasma thyroxine, 3,5,3'-triiodothyronine, and thyrotropin in rats.

We have studied the thyroid function of high-altitude-acclimated (6,900 m for 5 wk) and nonaltitude-acclimated control (198 m) rats. After 5 wk of high-altitude exposure the plasma concentrations of both thyroxine (T4) and triiodothyronine (T3) were lower than those in control rats (4.7 +/- 0.3 vs. 3.3 +/- 0.2 microgram/dl and 96 +/- 5 vs. 74 +/- 6 ng/dl, respectively). The plasma thyrotropin (TSH) concentration was elevated in the high-altitude-acclimated rats compared with controls (52 +/- 4 vs. 29 +/- 3 micrograms/dl, respectively). Gross anatomical (thyroid wt/body wt) and histological observations of thyroid tissue were consistent with elevated plasma TSH concentrations. These results suggest that the fall in the plasma concentrations of T4 and T3 during acclimation to severe hypoxia is due to a primary block of thyroid secretion and not to a fall in plasma TSH levels.