METABOLIC STUDIES WITH I131LABELED THYROID COMPOUNDS: DISTRIBUTION AND EXCRETION OF RADIODIIODOTYROSINE IN HUMAN BEINGS

Abstract

PREVIOUS studies of the metabolism of labeled thyroidal substances (1, 2, 3, 4, 5) have suggested that thyroxine is slowly metabolized to at least two substances—inorganic iodide and diiodotyrosine. The further fate of the inorganic iodide liberated from the catabolism of thyroxine is well known, but that of diiodotyrosine is not. Earlier studies dealing with large doses of diiodotyrosine in human subjects indicated that it was decomposed to iodide, since urinary excretion of iodide rose greatly after the administration of diiodotyrosine (6). On the other hand, Salter (7) listed values for the concentration of diiodotyrosine in both erythrocytes and serum of normal persons and of those having various thyroid disorders. The possibilities that diiodotyrosine may exist as a free amino acid in the blood or that it may be accumulated or secreted by the thyroid, merit a reinvestigation of the metabolism of diiodotyrosine in man. We have, therefore, prepared diiodotyrosine in radioactive form (labeled with I¹³¹) and have administered the radioactive amino acid to myxedematous patients who were brought to a euthyroid state by therapy with desiccated thyroid. The present paper reports the results of three such studies.