DISCONTINUING ANTITHYROID DRUG-THERAPY BEFORE ABLATION WITH RADIOIODINE IN GRAVES-DISEASE

Objective: To determine the relative effects on thyroid hormone levels of discontinuing antithyroid drug therapy and subsequent ablation with radioiodine in patients with hyperthyroid Graves disease. Design: A clinical trial with a prospective analysis of the relative change in thyroid hormone levels over time in response to therapy in two study groups. Setting: An outpatient endocrine clinic at a tertiary care hospital. Patients: 21 patients with a clinical diagnosis of hyperthyroid Graves disease scheduled to receive ablation therapy with radioiodine (I-131): 17 patients were pretreated with antithyroid drugs, and 4 were not. Methods: Antithyroid drugs were stopped 6 days before radioiodine therapy. Patients were monitored clinically and biochemically with measurement of free and total levels of thyroxine (T-4) and triiodothyronine (T-3) on days -6, -3, -1;the day of radioiodine therapy; and days 1, 2, 3, 4, 5, 7, and 14. Results: Before radioiodine treatment and compared with baseline measurement, the mean increase in free T-4 levels after discontinuation of antithyroid therapy was 86% (95% CI, 16.1% to 156%), with a concurrent mean increase in free T-3 levels of 71.6% (CI, 31% to 112%). Radioiodine therapy resulted in a mean decrease in free T-3 levels of 28.7% (CI, -44.1% to -13.2%), a mean decrease in total T-3 levels of 22.9% (CI, -39.4% to -6.4%), and stability in free and total T-4 levels rather than aggravation of thyrotoxicosis. A smaller group of patients not receiving antithyroid drugs experienced a course qualitatively similar to that of pretreated patients after I-131 treatment, with a mean reduction in free T-4 levels of 39.8% (CI, -69.9% to -9.7%) and a mean decrease in free T-3 levels of 49.4% (CI, -93.7% to -5.1%). Conclusion: Short-term increases in thyroid hormone levels in patients with Graves disease receiving radioiodine ablation occur primarily as a result of discontinuing antithyroid therapy rather than as a result of treatment with I-131. Stability or decrease in thyroid hormone levels, rather than further elevation, occurs during the 2-week interval after ablation therapy with I-131. Antithyroid drug therapy before radioiodine ablation may have little effect on the short-term biochemical course after I-131 therapy for Graves disease. The homogeneity of our sample regarding age, diagnosis, and general health may prevent application of these findings to other populations without further study.