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Effect of Methimazole Pretreatment on Serum Thyroid Hormone Levels after Radioactive Treatment in Graves’ Hyperthyroidism¹

Endocrine Division, Hospital de Clínicas de Porto Alegre, Universidade Federal do Rio Grande do Sul, 90035–003 Porto Alegre, RS, 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, RS, Brazil. E-mail: almaia{at}vortex.ufrgs.br

Radioiodine (¹³¹I) is the preferred definitive treatment for Graves’ hyperthyroidism. Pretreatment with antithyroid drugs is often used to avoid thyroid hormone discharge after ¹³¹I ablation. However, this may represent an unnecessary increase in risk and costs. Fifty-one patients with Graves’ disease were randomly assigned to receive ¹³¹I alone (28 patients) or ¹³¹I plus pretreatment with methimazole (30 mg/day; 23 patients). Methimazole was interrupted 4 days before ¹³¹I therapy. Serum T₄, free T₄ (FT₄), and T₃ 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 ¹³¹I alone, mean serum T₄ levels did not change after therapy. Mean serum FT₄ and T₃ levels decreased significantly 5 days after ¹³¹I administration (15% and 18%, respectively). Serum T₃ reached its lowest level on day 30 (38%). With pretreatment, mean serum T₄, FT₄, and T₃ levels increased (38%, 39%, and 70%, respectively) after methimazole discontinuation and before ¹³¹I administration. After ¹³¹I, serum T₄ 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 T₃ 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 ¹³¹I. Thyroid hormone levels stabilize or decrease during the first 30 days after ¹³¹I therapy.

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