Background and Objective: The success rate of I-131 therapy for hyperthyroidism is dependent on several factors. The aim of the present study was to evaluate the success rates, and the factors which affected them, in patients with hyperthyroidism at
Rajavithi Hospital who received I-131 therapy with doses based on thyroid gland weight (gram), clinical findings, and 3-hour
radioiodine uptake (3-hr RAIU).
Material and Method: A retrospective study was performed of 305 hyperthyroid patients who received initial I-131 therapy at the Nuclear Medicine Department of Rajavithi Hospital, Bangkok between January 2010 and December 2014. Baseline
characteristics, including thyroid gland weight (g), 3-hr RAIU, I-131 dose and all patient outcomes were reviewed. Success after initial I-131 therapy was defined as evidence of stable euthyroid or hypothyroid within 1 yr.
Results: A total of 305 hyperthyroid patients were enrolled. The mean (SD) age, gland weight, 3-hr RAIU, and I-131 dose
were 44.17 (15.14) years, 51.19 (31.95) grams, 50.62 (24.36) and 14.40 (5.71) mCi. respectively. The success rate within 12 months was 66.3 %, and the factors associated with success were I-131 dose (p = 0.014), gland size (p = 0.044), 3-hr
RAIU (p = 0.02) and underlying cardiovascular disease (p = 0.014).
Conclusion: The mean (SD) I-131 treatment dose was 14.4 (5.71) mCi., and the success rate within 1 year was 66.3%. I-131 therapy is an effective treatment for hyperthyroidism, and its rate of success rate depends on both patient factors and
I-131 treatment dose.

Keywords: Hyperthyroidism, Radioactive iodine (I-131), 3-hour radioiodine uptake (3-hr RAIU), Success rate