Function of the hypothalamo-hypophyseal-thyroid system was investigated in 51 patients with euthyroid hyperplasia. Basal and TRH-stimulated levels of TSH and STH were investigated. Hyperergic ejection of STH and two pathological types of STH reaction (suggesting dysfunction of the hypothalamohypophyseal-thyroid system in euthyroid hyperplasia) were revealed. A group of patients with a low level of STH in response to TRH-stimulation demonstrated more pronounced changes of the hypothalamo-hypophyseal-thyroid system, characterized by a rise of a TSH level in response to stimulation up to 27.59 +/- 3.7 microU and a slightly reduced level of thyroid hormones. Changes in the level of STH indicated certain interest of growth hormone in the genesis of thyroid hyperplasia and could also serve an additional sign in the evaluation of a severity of disorders of the hypothalamo-hypophyseal-thyroid system.