Iodine Metabolism of the Thyroid Gland*

Because of the similarity in structure between diiodotyrosine and thyroxine, Harington and Barger (1) suggested that the former is the precursor of thyroxine in the thyroid gland. The formation of thyroxine from diiodotyrosine has been demonstrated lately in vitro (2, 3). Recent data (4, 5) obtained with radioiodine used without carrier indicate the rates of incorporation of iodine into the various thyroid fractions but do not indicate the sequence of formation nor the respective sources of the finished components. In a series of preliminary experiments with subphysiological doses of radioiodine, 1130 (without carrier), we have attempted to determine whether diiodotyrosine is a natural precursor of thyroxine, whether inorganic iodine is normally present in the normal non-iodized thyroid (6, 7), and whether this inorganic iodine in the thyroid gland is the source of the iodine in the diiodotyrosine molecule. Radioiodine (1130) injected in subphysiological amounts as iodide and without carrier should behave like iodide already present in physiological amounts in the blood stream and thus is actually representative of the circulating iodide. The radioactivity found in the thyroid gland iodine fractions is the result of absorption and subsequent reactions in the gland which utilize newly arrived radioiodine obtained from circulating iodide. The iodine fraction which is first formed or appears soon after