The levels of thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (RT3), thyroxine-binding capacity (TBC), a free thyroxine index (FT4I) and thyroid-stimulating hormone (TSH) were studied by a radioimmunoassay in the blood of healthy schoolchildren aged 14 to 17 with stage I-II of thyroid hyperplasia residing in a mountain region of the Transcarpathian area, the Ukrainian SSR, with noticeable environmental iodine deficiency. Similar studies were conducted in a lowland region with a sufficient iodine level. A decrease in the concentration of T4, T3, RT3 and FT4I and higher TBC and TSH values were established in the healthy schoolchildren from the mountain region as compared to corresponding values in the schoolchildren from the lowland region indicating a lower level of homeostasis of thyroid hormones in the mountain region. A decrease in indices of thyroid function was observed in hyperplasia as compared to indices of the healthy, schoolchildren (T3 and TSH levels were higher) both in the mountain and lowland regions. More pronounced shifts were revealed in the blood of the schoolchildren from the mountain region. Less iodine was excreted with daily urine in the healthy schoolchildren from the mountain region than in the schoolchildren from the lowland region. Schoolchildren with hyperplasia excreted less iodine than healthy children, iodine excretion being lower in the schoolchildren from the mountain region. The results obtained suggested the hypothyroid nature of thyroid hyperplasia in the mountain region. The importance of iodine prophylaxis was emphasized.