THE RADIOIODINE-ACCUMULATING FUNCTION OF THE HUMAN THYROID GLAND AS A DIAGNOSTIC TEST IN CLINICAL MEDICINE*

Abstract

The radioiodine-accumulating function of the thyroid, studied by tests of several types in 790 patients who had various conditions, was found to be significantly increased in patients who had exophthalmic goiter, or adenomatous goiter with hyperthyroidism, and significantly reduced in patients who had myxedema. Significant elevation of radioiodine accumulation was also encuntered in some patients who did not have hyperthyroidism. These included patients who had adenomatous goiter without hyperthyroidism, colloid goiter, hyperplastic thyroid nodules and thyroid hyperplasia resulting from antithyroid drugs. Significant depression was encountered in many situations unaccompanied by clinical hypothyroidism. These included renal insufficiency, cardiac decompensation, Addison''s disease, acute diffuse thyroiditis and Riedel''s thyroiditis. Depression of I accumulation as measured with l131 occurred when inorganic I had been ingested within a week. It was observed many months after admn. of gallbladder dye. Antithyroid drugs, as well as other organic iodine compounds including desiccated thyroid, were also found to interfere with I131 accumulation. In some instances relative or absolute suppression of Il31 accumulation which could not be explained was encountered in normal or hyperthyroid patients. Normal values for I131 accumulation were observed in most cases of adenomatous goiter without hyperthyroidism, in about half the cases of adenomatous goiter with hyperthyroidism, in a few cases of exophthalmic goiter and in some cases of myxedema. Patients found to have Hashimoto''s thyroiditis associated with clinical myxedema characteristically had normal values for I131 accumulation. In spite of the foregoing, measurement of I131 accumulation by any one of several means was found to be a highly efficient diagnostic tool for separating more than 90% of cases of exophthalmic goiter from normals. It was found to be comparatively inefficient as a method for establishing the presence or absence of hyperthyroidism in cases of adenomatous goiter. I131 accumulation was found to be less useful in the diagnosis of myxedema, providing unequivocal evidence of hypofunction in about half the cases and equivocal evidence in the rest. 4 methods for measuring radioiodine accumulation were compared: Measurement of the quantity of radioiodine excreted in the urine within 48 hrs. after its admn.; detn. of extrarenal disposal rate from analysis of the curve of urinary I131 excretion; in vivo measurement of the quantity of I131 accumulated in the thyroid 24 hrs. after admn. of the dose; and detn. of an in vivo accumulation rate. All 4 values proved similar in diagnostic sensitivity. Extrarenal disposal rate provided the clearest picture of the state of radioiodine function, particularly in situations complicated by altered renal function. In vivo observations provided more accurate information in the presence of reduced or absent I accumulation than did urinary observations, but in other circumstances were less efficient, possibly owing to the intrinsic inaccuracies of in vivo measurements. Detns. of 48-hr. urinary excretion proved to be a less specific measure of uncomplicated hyperthyroid states than the others, and in some conditions it provided inexact and misleading information. It was, however, the simplest and least expensive of the procedures employed. Measurement of I131 accumulation was considered to be comparable but not superior to detn. of basal metabolic rate as a measure of thyroid function. In view of the difficulties and hazards which attent the use of I131 it appears likely that it will supplement rather than supplant other diagnostic aids.