METABOLIC STUDIES WITH I131LABELED THYROID COMPOUNDS: COMPARISON OF THE DISTRIBUTION AND FATE OF RADIOACTIVEd-l-THYROXINE AFTER ORAL AND INTRAVENOUS ADMINISTRATION IN THE HUMAN*

Abstract

WE have been investigating the metabolism of thyroid compounds in the human by radioisotopic methods (1, 2, 3, 4). At the outset of our studies, the nature of circulating thyroid hormone was still uncertain and consequently we employed several different calorigenic compounds. These compounds were labeled with I¹³¹, and physiologic doses were administered orally to myxedematous patients both while they were hypothyroid prior to treatment and while they were euthyroid after adequate replacement therapy with unlabeled calorigen. Serial measurement of urinary and fecal elimination of I¹³¹, serial determinations of organic and of inorganic I¹³¹ in the blood, and serial in vivo measurements over several areas of the body were made. The nature of the radioiodine compounds in the urine was also studied. Three compounds—radioiodocasein, radiothyroglobulin and radiothyroxine—have been studied in this manner, and in each instance the behavior of labeled sodium iodide, equal in iodine content to the respective calorigen, was studied as a control.