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The Effect of Methimazole Pretreatment on the Efficacy of Radioactive Iodine Therapy in Graves’ Hyperthyroidism: One-Year Follow-Up of a Prospective, Randomized Study

Endocrine Division, Hospital de Clínicas de Porto Alegre, Universidade Federal do Rio Grande do Sul, 90035-003 Porto Alegre, Brazil

Address all correspondence and requests for reprints to: Ana Luiza Maia, M.D., Ph.D., Serviço de Endocrinologia, Hospital de Clínicas de Porto Alegre, Rua Ramiro Barcelos 2350, 90035-003 Porto Alegre, Rio Grande do Sul, Brazil. E-mail: almaia{at}vortex.ufrgs.br

Abstract

The effect of antithyroid drugs on the efficacy of radioiodine (¹³¹I) treatment is still controversial. This study evaluated the effect of methimazole pretreatment on the efficacy of ¹³¹I therapy in Graves’ hyperthyroidism. Sixty-one untreated patients were randomly assigned to receive ¹³¹I alone (32 patients) or ¹³¹I plus pretreatment with methimazole (30 mg/d; 29 patients). ¹³¹I was administered 4 d after drug discontinuation. The calculated ¹³¹I dose was 200 µCi/g thyroid tissue as estimated by ultrasound, corrected by 24-h radioiodine uptake. Serum TSH, T₄, and free T₄ were measured 4 d before ¹³¹I therapy, on the day of treatment, and then monthly for 1 yr. Considering cure as euthyroidism or hypothyroidism, based on free T₄ measurement, approximately 80% of patients from both groups were cured 3 months after beginning ¹³¹I treatment. After 1 yr the groups were similar in terms of persistent hyperthyroidism (15.6% vs. 13.8%), euthyroidism (28.1% vs. 31.0%), or hypothyroidism (56.3% vs. 55.2%). Relapsed patients presented larger thyroid volume (P = 0.002), higher 24-h radioiodine uptake (P = 0.022), and T₃ levels (P = 0.002). Multiple logistic regression analysis identified T₃ values as an independent predictor of therapy failure. In conclusion, pretreatment with methimazole had no effect on either the time required for cure or the 1-yr success rate of ¹³¹I therapy.

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