Radioactive Iodine: Its Use as a Tool in Studying Thyroid Physiology

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Abstract
IN 1915, MARINE and his associates (1-3) demonstrated that the thyroid is capable of concentrating iodine to an approximate level of 5,000 times that of the blood. This unique function has provided students of biology and medicine with a valuable means for studying the physiology of the thyroid. In recent years the physicists have provided dynamic tools, in the radioactive isotopes of iodine, with which certain functions of the thyroid can be investigated at physiologic levels never before obtained. Though several radioactive isotopes of iodine have been described, only three have been widely used in studies of thyroid function. The earliest studies with radioactive iodine were done with I128, an isotope having a half-life of only twenty-five minutes, which limited the time of study. This isotope is prepared by slow neutron bombardment of iodine. Iodine130; the twelve-hour half-life isotope, has been used in biological studies and in treating certain cases of Graves' disease. This isotope is a product of the cyclotron and is prepared by bombarding metallic tellurium with deuterons. Iodine131 is likewise prepared in the cyclotron by deuteron bombardment of tellurium, but has also been formed by the bombardment of tellurium with slow neutrons which have their source in the chain-reacting pile. The eight-day half-life of this isotope has made it a particularly convenient tool in physiologic studies of the thyroid. The earliest reported studies of thyroid physiology with radioactive iodine were those of Hertz, Roberts, and Evans (4). In their studies with isotopes, these investigators repeated some of the classic studies reported by Marine and noted the unique avidity of the thyroid for iodine. Subsequently they observed that thyroids made hyperplastic by the administration of thyroid-stimulating hormone, or by the ingestion of cabbage, concentrated a greater percentage of the administered iodine than did their controls. Leblond and Sue (5) also observed that the thyroid has an affinity for iodine which is increased after treatment with thyrotropic hormone and is greatly decreased following hypophysectomy. Chaikoff and his associates (6) observed that thyroids of hypophysectomized rats concentrate less iodine than do those of control animals and that such thyroids are capable of converting the collected iodine to diiodotyrosine. During the twenty-sixhour period that they followed their animals, practically none of the administered labeled iodine was demonstrated in the thyroxin fraction. This same group of investigators have observed that the thyroids of guinea-pigs previously treated with thyroid-stimulating hormone not only have an increased avidity for iodine but that they secrete it into the blood stream as a radioactive iodine thyroxine at a much greater rate than do the thyroids of untreated controls.