Acute Effect of Inorganic Iodide After 131i Therapy for Hyperthyroidism

SUMMARY Patients treated with inorganic iodide weeks to years following 131I therapy for hyperthyroidism do not adapt to its antithyroid effect. To determine whether such adaptation occurs soon after 131I therapy, serum thyroxine (T4) and triiodothyronine (T3) concentrations were measured daily for 9‐14 days following 131I therapy in seventeen hyperthyroid patients. Nine patients received 150 mg KI daily starting 48 h after 131I administration; eight received only 131I. Serum T4 and T3 concentrations did not change significantly in the patients who received only 131I. In the patients who received 131I and KI, serum T4 and T3 concentrations fell promptly, reaching nadir values 2‐10 days after initiation of iodide, and then increased despite continuation of KI therapy. The mean maximal fall in serum T4 was 34% and in serum T3 42%. These results show that ‘escape’ from the acute antithyroid effect of iodide occurs when it is given immediately after 131I therapy, thus limiting the utility of iodide as a therapeutic agent at this time. 131I‐iodide is a widely used and effective form of therapy for hyperthyroidism. Reduction in thyroid hormone does not occur within the first weeks after 131I therapy. Exacerbation of hyperthyroidism may occur shortly after 131I administration, with frequency ranging from 0 to 11% in several large species (Chapman et al., 1954; Cassidy & Astwood, 1959; Lamberg et al., 1959; Green & Wilson, 1964). Such exacerbations are thought to reflect acute thyroid radiation necrosis and subsequent hormone release, and may limit the use of 131I therapeutically. Inorganic iodide is a rapidly, but usually transiently, effective antithyroid agent when used alone. No reports are available concerning the efficacy of iodide when given immediately following 131I therapy. Since some hormone release after 131I therapy may reflect thyroid destruction, rather than secretion, it is possible iodide might be less effective in this setting. On the other hand, if iodine is effective immediately after 131I therapy, especially if its action is sustained (as occurs in patients treated with 131I months or years previously; Hagen et al., 1967; Braverman et al., 1969), it might be a useful adjunct for certain hyperthyroid patients treated with 131I. This report describes the result of a study of patients so treated.