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High Dose ¹³¹I Therapy for the Treatment of Hyperthyroidism Caused by Graves’ Disease

Thyroid Division, Department of Medicine, Brigham and Women’s Hospital, Boston, Massachusetts 02115

Address all correspondence and requests for reprints to: P. Reed Larsen, M.D., Thyroid Division, Brigham and Women’s Hospital, H. I. M. Building, 77 Avenue Louis Pasteur, Room 560, Boston, Massachusetts 02115. E-mail: . plarsen{at}partners.org

Abstract

Radioactive iodine (¹³¹I) has become the most widely used therapy for patients with hyperthyroidism caused by Graves’ disease in the United States. There remains, however, significant variability among ¹³¹I dosing regimens, and it is clear that most patients ultimately develop hypothyroidism after therapy. To avoid persistent hyperthyroidism, we adopted a high dose ¹³¹I therapy protocol based on measurement of 24-h thyroid ¹²³I uptake designed to deliver 8 mCi (296 MBq) to the thyroid gland 24 h after ¹³¹I administration. To evaluate the efficacy of this protocol, we reviewed our clinical experience over a 7-yr period.

We treated 261 patients (219 women and 42 men) with hyperthyroidism caused by Graves’ disease with ¹³¹I [mean dose, 14.6 mCi (540 MBq)] between 1993 and 1999. Before treatment, 207 (79%) had received an antithyroid drug (109 propylthiouracil and 98 methimazole). We determined their thyroid status 1 yr after treatment in relation to age, pretreatment with an antithyroid drug, pretreatment thyroid size, and dose of ¹³¹I retained in the thyroid 24 h after treatment.

Among the 261 patients, 225 (86%) were euthyroid or hypothyroid 1 yr after treatment, and 36 patients (14%) had persistent hyperthyroidism and required a second treatment. The patients who had persistent hyperthyroidism were younger (P < 0.01), had larger thyroid glands (P < 0.01), higher pretreatment thyroid ¹²³I uptake values (P < 0.01), and higher serum T₄ concentrations (P < 0.01) and were more likely to have taken antithyroid medication before administration of ¹³¹I (P = 0.01). Five of these patients developed transient hypothyroidism, followed by thyrotoxicosis. There was an asymptotic, inverse relationship between the retained dose of ¹³¹I at 24 h and persistent hyperthyroidism, revealing a 5–10% failure rate despite delivery of up to 400 µCi (14.8 MBq)/g.

A dose of ¹³¹I that results in accumulation of 8 mCi (296 MBq) in the thyroid gland 24 h after administration is an effective treatment for the majority of patients with Graves’ hyperthyroidism. Young patients with larger thyroid glands, higher serum T₄ concentrations, and higher 24-h thyroid ¹²³I uptake values, and those pretreated with antithyroid medication for greater than 4 months are at higher risk for treatment failure. A higher dose of ¹³¹I may be advisable in such patients.

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