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Radioiodine and natural iodine are chemically and physiologically identical, providing the former is present in quantities so small as to be without biologic effects due to radiation per se. A minute quantity of radioiodine may be used to trace, by its radioactivity, the course of a given quantity of iodine through various chemical and biologic reactions. Hertz (1, 2) was the first to call attention to the unique possibilities of this method in the study of thyroid function. Hamilton and Soley ( 12) were the first to apply it to the clinical investigation of the human thyroid. Radioiodine tracers have subsequently been utilized in many studies dealing with the thyroid of laboratory animals, in vitro experiments and clinical investigations. The method has been applied to the study of the biosynthesis of thyroid hormone (1, 3 to 9), the action of thyrotrophic hormones (4, 10), the action of goitrogens (5, 11) and of other factors influencing the function of the thyroid (6), the functional activity of various thyroid tumors and a number of other problems.It is possible that radioiodine may prove most useful in the study of thyroid function in man. To date, relatively few definitive clinical studies employing this technique have been published. The first was that of Hamilton and Soley (12), who compared the collection by the thyroid of iodine labeled with radioiodine and its excretion in the urine and feces in normal subjects and in patients having various thyroid disorders. Radioiodine in the thyroid was measured directly by the recording of its gamma radiation by means of a Geiger-Muller counter placed over the trachea and in some instances by analysis of surgically removed specimens of thyroid tissue. It was found that an orally administered dose of labeled iodine was absorbed very rapidly and could be detected in the thyroid within 20 minutes. Normal subjects excreted 74 to 89 per cent of the dose in the urine during a period of 5 days, 53 to 81 per cent appearing during the first 24 hours. Two myxedematous patients excreted 91 and 94 per cent, respectively, in the urine during 5 days but at a slower rate than did the normals. Patients who had hyperthyroidism excreted as much in the urine as did the normals. This finding may have been due to the fact that most of the patients had received strong solution of iodine (Lugol's solution) and in part to the very large amount of iodide (14 mgm.) which was administered as a labeled dose. Fecal excretion was variable but averaged only about 1 per cent of the dose.In another series of in vivo measurements employing the same dose of iodide, Hamilton and Soley (13) recorded characteristic collection curves for various thyroid states. The curve typical of normal thyroids was a smooth curve which leveled off to a flat plateau in 2 days. Iodine collection by thyroids of hyperthyroid patients was greater and much more rapid than in normal thyroids. It rose to a peak within 4 to 8 hours and thereafter decreased almost as rapidly to a plateau lower than that of the normal thyro...

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