Comparisons of the Distribution of Radioactive Iodine

One of the earliest uses of radioiodine was as a tool for the study of thyroid physiology (1-7). Estimations of the relative quantities of isotope stored in the thyroid have been made by means of a Geiger-Muller tube held over the gland and by determining the amount present in the urine and blood. The rate with which the level of radioiodine changes in these compartments has also been studied (7, 8). In subjects with euthyroidism, radioiodine was found (8) to be excreted rapidly, within a few hours after its administration, but a plateau tended to appear after 48 hours. In myxedematous individuals, excretion was slower initially but it was more persistent, requiring four days or more; the total amount excreted after several days was greater in myxedematous subjects than in normal or thyrotoxic ones. In untreated thyrotoxic patients the rate of excretion was less than in either of the other two groups. The curves describing excretion became asymptotic relatively quickly. The differences in the metabolism of iodine in various functional states of the thyroid have prompted an investigation of the usefulness of radioiodine as a diagnostic test. One of the best tests not dealing with radioiodine that has been available is the determination of the protein-bound iodine of the plasma (9). This test is so long and difficult that it can be performed satisfactorily only under the direction of a few highly qualified individuals and even then some overlapping in the values for different degrees of thyroid function may be found (10). Moreover, the test is unreliable if the patient has recently received iodine in organic form and if highly scrupulous technique is not followed in the collection of the plasma as well as in the determination. It seemed to us that in some institutions it might be possible to use radioiodine in tests which would be more accurate, simpler and more rapid. It appears logical to assume that the quantity of protein-bound radioiodine in the serum, after the administration of tracer doses, might indicate the relative rate of manufacture of the thyroid hormone and its release into the blood stream. Of course, the values obtained would represent only an approximate balance between these two factors and the quantity of the hormone stored in the various fluids and tissues of the body, including the thyroid; also between the amount metabolized and excreted. Nevertheless, with methods not using the radioiodine, not only do these difficulties in interpretation exist, but also it is not possible to determine the time required for the changes to occur. The latter aspect would seem to be a very important one.