A reappraisal of the role of methimazole and other factors on the efficacy and outcome of radioiodine therapy of Graves’ hyperthyroidism

The outcome of radioiodine therapy of Graves’ hyperthyroidism was retrospectively evaluated in 274 consecutive patients treated from 1975 to 1984. At 1-yr follow-up, permanent hypothyroidism occurred in 36.9% of patients and the cumulative incidence of hypothyroidism progressively increased up to 79.3% after 7–10 yr. At the end of the follow-up period, 148 patients (54%) were hypothyroid, 115 (42%) euthyroid and 11 (4%) still hyperthyroid. The prevalence of hypothyroidism was significantly higher in patients with small goiters (≤ 50 g) than in those with large goiters (> 90 g). Moreover, hypothyroidism was more frequent in patients with high thyroglobulin antibodies titers (≥ 1: 25,600) than in those with low titers or negative tests, and occurred earlier in the former group than in the latter ones Correction of thyrotoxicosis was obtained after the administration of a single dose of 131I in 187 patients (63.6%); 69 patients required two doses and 11 three or more doses. Seven patients refused further treatment with 131I after the first dose. In an effort to identify possible factors affecting the efficacy of 131I therapy, we avaluated the results obtained after the administration of the first dose of radioiodine. We found that large goiters, rapid iodide turnover and adjunctive therapy with methimazole shortly after radioiodine were associated with a higher rate of persistence of thyrotoxicosis, whereas an increased prevalence of hypothyroidism was observed in patients with small goiters and in those not treated with methimazole up to one week after 131I. On the other hand pretreatment with methimazole before radioiodine administration did not affect the outcome of treatment. Thus, if correction of thyrotoxicosis, irrespective of the occurrence of hypothyroidism, has to represent the goal of radioiodine therapy of Graves’ disease, the following points should be kept in mind: i) Large goiters and goiters with a rapid iodine turnover should receive larger 131I doses/g thyroid tissue; ii) Methimazole should not be given shortly after radioiodine: if thyrotoxicosis is severe, the antithyroid drugs may be given prior to 131I therapy to restore euthyroidism, without limiting the efficacy of radiodiodine.