Objective: To analyze the effectiveness of the first dose of radioiodine (I-131) treatment in patients with hyperthyroidism in Southern Thailand. The second objective was to predict the factors implicated in treatment failure.

Materials and Methods: Patients with hyperthyroidism who received first dose of I-131 between January 2010 and December 2019 at Songklanagarind Hospital were enrolled in the present retrospective study. The data collected included age, gender, duration of the disease, type of disease, thyroid gland size (by palpation), type of antithyroid drug (ATD), ATD dose, and time of withholding ATD before and after I-131 treatment. The effectiveness of the first dose of I-131 treatment was assessed after six months of treatment. Treatment success was defined as a euthyroid or hypothyroid status.

Results: One thousand four hundred twenty-four patients with hyperthyroidism at a median age of 44.5 years and including 73.2% women were included in this study. Graves’ disease was the most common cause at 95.8%. The median I-131 dose was 7 mCi. Six months after treatment, 703 patients or 49.4%, achieved treatment success, which increased to 859 patients (60.3%) after 12 months. The authors found three factors that influenced treatment failure, duration of disease of seven months or longer (p=0.001), thyroid gland size of 100 g or larger (p=0.004), and I-131 treatment dose 10 to 19 mCi (p=0.021). The patient cohort was divided into three groups based on thyroid gland size at less than 50 g, 50 to 99 g, and more than 100 g, the I-131 treatment dose that increased the success rate of treatment in each group was 5 mCi or more, 6 mCi or more, and 15 mCi or more, respectively.

Conclusion: A single dose of I-131 could successfully treat hyperthyroidism. The duration of the disease, size of the thyroid gland, and the dose of I-131 treatment had a substantial impact on the result. Thus, the authors recommended early treatment with I-131 at a suitable dose for patients with hyperthyroidism.

Keywords: Radioiodine treatment; I-131; Hyperthyroidism; First dose

DOI: 10.35755/jmedassocthai.2022.12.13710

Received 24 May 2022 | Revised 30 September 2022 | Accepted 11 October 2022