The time course of thyroid function and hypophyseal thyrotropic activity were investigated in 182 patients with diffuse toxic goiter of severe and moderately severe forms of thyrotoxicosis before operation and in short- and long-term periods after operation using radioimmunoassay in a hospital and outpatient clinic. The determination of blood T3, T4 and TSH at the height of clinical manifestations of thyrotoxicosis and before operation was shown to reflect the gravity of disease and could serve as a diagnostic test of the efficacy of preoperative preparation. A T4 concentration index varied at different time after operation. A considerable decrease in blood TSH on the first days after the operation and its rise in the following 6 mos. resulted from the patients' adaptation to new conditions and were of transient nature in the overwhelming majority of the patients. Final assessment of the thyroid and pituitary function was possible only 12 mos. after operation.