This aim of this retrospective study was to determine the impact of glucocorticoid therapy on the effective 131 I half-life in radioiodine therapy for Graves disease. Methods: Three hundred fifteen consecutive Graves disease patients undergoing radioiodine therapy at our institution between August 2004 and January 2009 were enrolled. We investigated the influences of thyroid state (hypothyroidism, euthyroidism, hyperthyroidism), antithyroid drug dose before 131 I therapy, thyroid-stimulating hormone receptor antibody (TRAb) level, and qualitative and quantitative factors of prednisolone therapy on the effective 131 I half-life, applying univariate (paired t test) and multivariate (multiple-regression) analyses. Results: Multivariate analyses revealed independent significant effects of the thyroid metabolic state (P 5 0.004), antithyroid drugs (P , 0.001), presence of TRAb (P 5 0.004), and glucocorticoids (P 5 0.046) on thyroidal radioiodine halflife. Compared with euthyroidism, thyrotoxicosis and hypothyroidism reduced the effective half-life; high doses of antithyroid drugs and high TRAb levels had the same effect. Also, glucocorticoid therapy shortened the effective thyroidal radioiodine half-life in a dose-dependent manner. Pharmacologically, this effect is attributable to the prednisolone-induced increase of renal plasma 131 I clearance and the resulting reduction of plasma 131 I available for reuptake into the thyroid during radioiodine therapy. Conclusion: Oral treatment with prednisolone results in a reduction of effective thyroidal 131 I half-life in Graves disease, especially at higher doses.Key Words: Graves disease; radioiodine; endocrine ophthalmopathy; effective half-life; prednisolone Med 2010; 51:1917 51: -1922 51: DOI: 10.2967 Inrandomi zed trials, glucocorticoids have been shown to reduce the risk of endocrine ophthalmopathy (EO) after radioiodine therapy (RIT) (1). European and national statements on RIT in patients with Graves disease (GD) and EO recommend short courses of oral glucocorticoids during and after RIT (2,3). The German guidelines for treatment of GD with radioiodine propose low-dose glucocorticoid pulse therapy with 0.5 mg of prednisolone per kilogram in patients at risk of developing EO (3). In patients already presenting with EO-related symptoms, doses of 1.0-1.5 mg/kg are suggested. Although prednisolone has beneficial effects on the development or course of EO during RIT of GD, it is well established that glucocorticoids themselves change thyroid metabolism and triiodothyronine (T 3 ) and levothyroxine (T 4 ) levels and modulate the concentration of stimulating antithyroid antibodies (thyroid-stimulating hormone receptor antibody [TRAb]) (4-7). Therefore, the aim of our study was to investigate the influence of prednisolone on the thyroidal 131 I half-life in RIT of GD patients and to assess this effect in comparison to other factors already known to change effective 131 I half-life during the course of RIT in GD patients-namely the thyroidal metabolic state, antithyroid med...