THE EFFECT OF TRIIODOTHYRONINE AND THYROXINE UPON THE RATE OF RELEASE OF THYROID HORMONE IN VARIOUS THYROID STATES*

The effect of triiodothyronine (T3) or thyroxine (T4) upon the thyroidal secretion rate was studied in 12 normal subjects, 15 hyperthyroid patients, 2 patients in the euthyroid stage of Graves' disease, and 1 acromegalic patient with hyperthyroidism. All normal subjects demonstrated prompt and nearly complete inhibition of thyroid hormone release following the administration of small doses of T3 or T4, whereas in all of the other subjects there was only a slight alteration in hormonal secretion rate in spite of massive doses of T3 or T4. One hyperthyroid patient showed normal suppression of thyroidal I131 uptake during administration of 75 micrograms of T3 per day, but failure of suppression of the release rate when the dosage was increased up to 600 micrograms of T3 per day. The mean release rate during the pre-T3 control period was not significantly greater in the hyperthyroid group than in the normal group. The findings are interpreted as favoring the hypothesis that hyperthyroidism is due to autonomous hyperactivity of either the thyroid or the pituitary gland, rather than simply a quantitative decrease in the sensitivity of the hypothalamo-pituitary apparatus to suppression by circulating thyroid hormones. Certain limitations in the method for measuring thyroidal secretion rate are discussed.