Determination of factors influencing the outcome of radioiodine therapy in patients with Graves' disease

Aim of this study was to determine whether success of radioiodine therapy (RIT) in Graves' disease depends on thyroid volume, function, thyroideal receptor antibodies (TRAK), thyreostasis, therapeutic dosage, I-131 uptake, or effective half-life. Method: 78 patients received an average of 626 +/- 251 MBq of iodine-131 orally for thyroid ablation. 60 were assessed for successful therapy 3 months after RIT. Results: In patients showing hyperthyreosis or a TRAK value > 11 U/I at the beginning of RIT, a significantly lower therapeutic dosage and effective iodine half-life were found than in non-hyperthyreotic patients or patients with TRAK less than or equal to 11 U/I. Patients with a thyroid volume less than or equal to 25 ml showed a significantly lower I-131 uptake, but: a significantly higher relative uptake (I-131 uptake/volume) than patients with a thyroid volume > 25 ml. All failures were treated thyreostatically during RIT and showed a significantly lower therapeutic iodine dosage and relative uptake, as well as a significantly higher thyroid volume than patients with a successful therapy. RIT caused a thyroid volume reduction of 44%, with therapy failures showing a significantly lower volume reduction. Patients who received a therapeutic dosage of less than or equal to 250 Gy showed significantly worse results than did those who had received > 250 Gy. Only one case of therapy failure received a dosage > 250 Gy, while 50% of failures received dosages > 200 Gy but < 250 Gy. Multivariate analyses (MANOVA, factor analyses) showed thyreostasis as the decisive negative factor for a successful course of therapy. Conclusions: Since most treatment failures occurred in patients under thyreostatic medication, we recommend raising the target dosage to 250 Gy for these cases.