Resumption of methimazole after 131I therapy of hyperthyroid diseases:: effect on thyroid function and volume evaluated by a randomized clinical trial

Objective: Retrospective studies have indicated that anti-thyroid drugs (ATD) might possess a radioprotective effect, leading to a higher rate of recurrence of hyperthyroidism after iodine-131(I-131) therapy. Design: A randomized clinical trial was performed to clarify whether resumption of methimazole after I-131 influences the final outcome of this treatment. Methods: We assigned 149 patients with Graves' disease or a toxic nodular goitre to groups either to resume (+ATD) or not to resume (-ATD) methimazole 7 days after I-131. Before I-131 therapy, all patients were rendered euthyroid by methimazole, which was discontinued 4 days before the I-131 therapy. Results: During the follow-up period of 12 months, 13 patients developed hypothyroidism, 42 were euthyroid, and 18 had recurrence of hyperthyroidism in the +ATD group; the respective numbers in the -ATD group were 16, 42 and 18 (P = 0.88). At 3 weeks after I-131 therapy, the serum free-thyroxine index was slightly decreased (by 5.7%; 95% confidence interval (CI) -15.5 to 5.4%) in the +ATD group, in contrast to an increase of 35.9% (95% CI 18.8 to 55.5%) in the -ATD group (P < 0.001 between groups). In the subgroup that remained euthyroid during follow-up, thyroid volume reduction, assessed by ultrasonography, was smaller in the +ATD group [38.7% (95% CI 33.3 to 44.1%)] than in the -ATD group [48.6% (95% CI: 41.5 - 55.6%)] (P < 0.05). Conclusion: No radioprotective effect could be demonstrated, with regard to final thyroid function, for the resumpton of methimazole 7 days after I-131 therapy Although resumption of methimazole slightly reduced the magnitude of shrinkage of the goitre obtained by I-131, the prevention of a temporary thyrotoxicosis in the early period after radiation favours this regimen.