Radioactive Iodine Therapy of Graves' Disease in Patients Pretreated With Methimazole Without Radioiodine Uptake for Dose Estimation.

Objective: To assess response predictors to radioactive iodine (RAI) therapy without using thyroid uptake for dose estimate in patients pretreated with methimazole.

Methods: Retrospective analysis was performed of patients with Graves' disease treated with RAI doses determined without using uptake studies.

Results: In 242 patients (median age, 41.9 years; 66.1% female), initial mean free thyroxine (FT4) level was 4.7 ng/dL with an estimated thyroid size of 49.15 g. Prior to RAI therapy, average methimazole dose was 22.7 mg/day. Mean RAI dose was 737.0 ±199.4 MBq (19.9 ± 5.4 mCi). Two hundred eight patients (85.9%) responded to RAI therapy; 185 (88.9%) became hypothyroid and 23 (11.1%) became euthyroid. The majority (90.4%) responded within 6 months of therapy with a quicker response (13.9 ± 8.3 vs 17.5 ± 13.5 weeks) for those treated with doses per gram of ≥14.8 MBq (0.4 mCi). Thirty-four nonresponders had a higher initial FT4 level and larger thyroid size with a lower RAI dose per gram of thyroid tissue. In multivariate analysis, the independent response predictor to therapy was dose per gram of thyroid tissue of ≥14.8 MBq (0.4 mCi) (hazard ratio, 3.18; 95% CI, 1.1-9.7). Doses per gram of 14.8 to 18.1 MBq (0.4-0.5 mCi) achieved maximal response rate without added advantage of higher doses. Thyroid size prior to RAI therapy, FT4 levels at diagnosis, and age were inversely related to response.

Conclusion: RAI therapy for Graves' disease without uptake studies for dose estimates is an effective treatment method. In patients pretreated with methimazole, an RAI dose per gram of thyroid tissue of ≥14.8 MBq (0.4 mCi) showed high response rate. Prospective studies are needed to confirm the viability of this simplified and cost-effective approach.