Within 5 days of hypoxia there was an increase in thyroid functional activity, but on days 15-30 there was a decrease in T3 and T4 concentrations and an increase in thyroid-stimulating hormone. Clear correlations were found between the morphometric indices of the thyroid epithelium and the blood concentration of T3 and T4. There was a relationship between the levels of thyroid hormones and the state of the cardiovascular and respiratory systems.