To elucidate the features of thyroid hormone secretion under conditions of impaired circulation, blood samples were withdrawn at operation from thyroid and peripheral veins of 23 patients with thyroidal and non-thyroidal diseases. They were taken from the respective veins simultaneously, when the arterial supply to the thyroid tissue was partially occluded by surgical manipulation. It was found that triiodothyronine was preferentially, as compared with thyroxine, secreted from the thyroid, irrespective of the disease. Serum levels of thyroglobulin were also markedly elevated in the thyroid vein. These results suggest that episodic degenerative discharge of thyroid hormone from nodular goiters may be a cause of impaired or blunted TSH response to TRH, frequently observed in patients with common nontoxic nodular goiters.