Evaluation of Methods of Measuring the Accumulation of I131by the Thyroid Gland

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Abstract
A little over fifteen years has passed since it was shown by Hertz et al. (8) and Hamilton (7) that radioactive iodine could be put to good use in studying thyroid physiology. Most of the earlier thyroid function studies consisted in indirect measurements of the amount of a tracer dose of radioactive iodine excreted in the urine over a twenty-four to seventy-two-hour period (10, 23, 24, 25, 28). These tests were based on the premise that the thyroid gland was the only tissue which utilized iodine to any extent, and that amounts which were not metabolized were rapidly excreted in the urine. Keating and his group (11) extended these studies to show that more diagnostic information could be obtained if serial samples of urine were collected during the early hours of the test and assayed for radioactivity. If these samples were plotted against time, the renal excretion rate could be calculated, and from this the extrarenal (or thyroidal) rate of accumulation could be determined. While fairly accurate results were obtained by these methods, they appeared to offer several disadvantages. The principal handicap was the necessity of assuming that normal renal function existed and that no abnormal pooling of the iodide ion was present. (Secondary was the necessity of relying on the patient to collect total samples of urine.) With the development of improved counting methods, radioactive iodine studies of thyroid function could be made by direct measurements over the thyroid gland, following relatively small tracer doses of the isotope. As a consequence, there have appeared in the literature a variety of diagnostic tests dealing with the physiological facets of the iodine cycle (2, 3, 4, 9, 12–15, 27, 29). The use of one or more of these tests has proved clinically useful in separating euthyroidism and hyperthyroidism. Extensive reviews and analyses of the physiological aspects of thyroid function as it pertains to iodine metabolism have been reported by Riggs (21) and Oddie (17, 18). While it is not the purpose of this paper to discuss the fundamental aspects of the iodine cycle, it would appear that, in essence, the ideal objective of all studies of I131 uptake by the thyroid gland is information as to the metabolic turnover rate of thyroxine. Unfortunately, this is not a directly measurable quantity and clinically it is beyond the limits of practicability, since it would involve simultaneous thyroid arterial and venous sampling, together with quantitative estimates of iodine and protein-bound iodine within the gland itself. For that reason, other approaches have been made to this fundamental assay. When there is an increased rate of formation of thyroxine (as in hyperthyroidism) the gland will necessarily be more avid for one of the fundamental constituents of the hormone, such as iodine.