Therapeutic Effects in Hyperthyroidism from Repeated Diagnostic Doses of I131

Abstract

Several repeated tracer doses of I131Sare commonly used to follow the course of hyperthyroidism under treatment of various types. While it is generally assumed that these tracer amounts of the isotope are innocuous, evidence from three directions has suggested the need to test this assumption. The first observation in this clinic in conflict with the commonly accepted concept has been the consistent reporting by occasional patients with toxic goiter that they “felt better” following a diagnostic tracer test. The obvious explanation has been the facile one, that a factor of suggestion is involved. This has not always appeared adequate. The second cause for suspicion of a possible effect from tracer dosage is the finding that there is a wide biological variability in response to therapeutic doses of I131 in hyperthyroidism (1). Extreme sensitivity of the thyroid to internal radiation effect has been noted, as well as extreme resistance. As a consequence, it has appeared reasonable that smaller dosages of I131 than those used therapeutically might have some effect. This lower dosage range has remained unexplored since the failure of Soley and Miller to achieve remissions with doses of 250 microcuries repeated several times (2). Finally, interest in a possible effect from tracer dosage has been aroused by the conflict in results between groups investigating the effect of ACTH upon hyperthyroidism (3, 4, 5). The only apparent difference in procedure has been the number of tracer tests and the size of the dose of I131 used in each test. For these reasons, then, the present investigation was undertaken to determine the effect upon hyperthyroidism of several tracer doses of I131. It has been found that complete remission of thyrotoxicosis may result from as little as 240 microcuries I131, total dosage. The time interval from the beginning of the experiment to the achievement of remission has, in most instances, paralleled that of therapeutic doses of I131. The remissions following smaller dosage generally have been transient. Methods The methods employed by this laboratory for tracer testing with I131 and for direct measurement over the thyroid of uptake of the isotope at twenty-four hours have been described in detail elsewhere (6). In all cases the diagnosis was made and the patient was followed from the clinical and laboratory standpoints as previously detailed (7). Only patients with unquestioned hyperthyroidism due to toxic diffuse or toxic recurrent goiter were used in the present study. It should be pointed out that the series includes several children. These were deliberately treated after an initial observation indicated the possibility, in this age group, of unusual susceptibility of the thyroid to radiation effect. Basal metabolic rate determinations were conducted with the use of a Benedict Roth apparatus. Serum precipitable iodine was estimated by the method of Barker (8).