Modifications of the pituitary-thyroid axis induced by hypobaric hypoxia.

The effects of simulated altitude induced by a decompression chamber upon the blood levels of thyrotropin (TSH), thyroxine (T4) and triiodothyronine (T3), measured by radioimmunoassay, were studied in male Wistar rats. When blood was extracted during the first 15 minutes after the rats were removed from the chamber and analysed, it was found that TSH levels decreased at 380 mmHg during 24 hours and sometimes at 440 mmHg during 24 hours, but this did not happen when the exposure lasted a week. Likewise, serum levels of T4 and T3 decreased at 440 mmHg during 24 hours, but they did not decrease using the same exposure during a week. When the blood was extracted 1 hour after the rats were removed from the chamber and analysed, TSH levels markedly increased at 380 mmHg and at 440 mmHg, during 1 day and during a week. Serum T4 and T3 levels were found to be normal. These results show that severe hypobaric hypoxia is needed in order to depress serum TSH levels. When the rats are removed from the decompression chamber, however, a rebound effect with marked increase in serum TSH levels can be seen before 1 hour elapses.