Effect of methimazole pretreatment on serum thyroid hormone levels after radioactive treatment in Graves' hyperthyroidism

Radioiodine (I-131) is the preferred definitive treatment for Graves' hyperthyroidism. Pretreatment with antithyroid drugs is often used to avoid thyroid hormone discharge after I-131 ablation. However, this may represent an unnecessary increase in risk and costs. Fifty-one patients with Graves' disease mere randomly assigned to receive I-131 alone (28 patients) or I-131 pins pretreatment with methimazole (30 mg/day; 23 patients). Methimazole was interrupted 4 days before I-131 therapy. Serum T-4, free T-4 (FT4), and T-3 were measured on days -4 and -1, on the day of treatment, and on days 2, 5, 7, 14, 20, and 30. In patients receiving I-131 alone, mean serum T-4 levels did not change after therapy. Mean serum FT4 and T-3 levels decreased significantly 5 days after I-131 administration (15% and 18%, respectively). Serum T-3 reached its lowest level on day 30 (38%). With pretreatment, mean serum T-4, FT4, and T-3 levels increased (38%, 39%, and 70%, respectively) after methimazole discontinuation and before I-131 administration. After I-131, serum T-4 levels peaked on day 7 (23% vs. treatment day; 70% vs, baseline); FT4 levels peaked on day 14 (53% vs. treatment day; 107% vs, baseline). The serum T-3 concentration increased 9% on day 2 (85% vs, baseline) and decreased from day 14 (15%) to day 30 (21%). We conclude that interruption of antithyroid drugs causes a short term increase in serum thyroid hormone levels in patients with Graves' hyperthyroidism receiving I-131. Thyroid hormone levels stabilize or decrease during the first 30 days after I-131 therapy.