Radioiodine therapy in Graves' disease patients with large diffuse goiters treated with or without carbimazole at the time of radioiodine therapy.

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We sought to ascertain how high the success rates of radioiodine therapy are for Graves' disease patients with large diffuse goiters when aiming for a constant absorbed dose of 250 Gy. Thirty-six patients with a thyroid volume of 50-110 mL were evaluated for changes in thyroid function and appearance 3, 6, and 12 months after radioiodine therapy. Success was defined as definitive elimination of hyperthyroidism following therapy (hypothyroidism corrected with thyroxine on diagnosis); failure as persistent/recurrent hyperthyroidism after 12 months. Overall success rate was 50%. However, a subgroup of 20 patients without simultaneous carbimazole (carbimazole-off) showed a highly significantly larger success rate (85%) than the 16 patients with simultaneous carbimazole (carbimazole-on) at the time of radioiodine therapy (6.3%, p < 0.000005). Successful cases showed a significantly higher volume reduction after radioiodine than failures (75.5% vs. 35.4%, p < 0.00005). Stepwise logistic regression showed that therapy failure was related to administration of carbimazole during radioiodine therapy (p < 0.0250 and absorbed dose (p < 0.05), but not thyroid function (free triiodothyronine [FT3] and free thyroxine [FT4]), initial thyroid volume or thyrotropin-receptor antibody (TRAb) value. However, a significant correlation of therapy success to absorbed dose (r = 0.69, p < 0.005) could be shown only for carbimazole-off patients, but not for the others. Finally, multivariate factor analysis consistently showed that therapy success was correlated only to absorbed dose and antithyroid drugs, not to initial thyroid volume, TRAb value, or thyroid function. Thyroid volume per se is not responsible for the lower success rate in Graves' disease patients with large goiters because even a comparable group of 32 Graves' disease patients with small thyroid glands (< or =20 mL) and without simultaneous carbimazole showed a success rate of 87.5%. The high failure rate in the carbimazole-on patients (absorbed dose comparable to carbimazole-off) is due to the simultaneous administration of carbimazole. Therefore, if clinically feasible, we recommend discontinuing carbimazole at least one day before beginning radioiodine therapy.