Factors influencing the success of radioiodine therapy in patients with Graves' disease.

Objective: The first dose of radioiodine (I) does not always cure hyperthyroidism in patients with Graves' disease (GD). Our aim was to evaluate the factors influencing the success of I therapy.
Patients and Methods: We reviewed the medical records of 724 patients who were first diagnosed with GD between 2005 and 2009 and were subsequently treated with I in a fixed-dose manner considering the thyroid volume (TV). TSH, fT(4), and fT(3) were measured. TV was measured by means of ultrasonography. Successful therapy was followed by euthyroidism or hypothyroidism.
Results: Out of 724 patients, 656 (90.5%) were successfully (Group 1) and 69 (9.5%) were unsuccessfully (Group 2) treated with the first dose of (131)I. In Group 1, the applied dose of (131)I was lower than that in Group 2 [626±107, 95% confidence interval (CI) 618-634, and 709±140, 95% CI 675-742 MBq, respectively; P<0.001]. At presentation, patients in Group 1 were younger than those in Group 2 (45.5±14.9, 95% CI 44.4-46.6, and 50.1±15.8, 95% CI 46.3-53.9 years, respectively; P=0.031). They had a lower fT(4) (54.9±26.1, 95% CI 52.9-56.9, and 72.1±34.1, 95% CI 63.9-80.3 pmol/l, respectively; P<0.001), a lower fT3 (20.9±8.2, 95% CI 20.3-21.5, and 23.9±8.2, 95% CI 21.9-25.9 pmol/l, respectively; P<0.001), and a smaller TV (21.5±13.2, 95% CI 20.2-22.8, and 35.6±22.3, 95% CI 28.2-42.9 ml, respectively; P<0.001). Before I therapy, patients in Group 1 had a lower fT(3) (9.6±6.0, 95% CI 9.2-10.1, and 11.3±7.6, 95% CI 9.5-13.2 pmol/l, respectively; P=0.038).
Conclusion: Successfully treated GD patients were younger, less severely hyperthyroid, and had a smaller TV at presentation. They were also less severely hyperthyroid before I therapy.