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OBJECTIVE To analyze the incidence and associated factors of hypothyroidism after radioiodine treatment for hyperthyroidism during a 13-year follow-up period. SUBJECTS AND METHODS This was a retrospective study of consecutive patients with hyperthyroidism who were treated using a single dose of radioactive iodine (RAI) with a calculated dose regimen from 07/2005 to 12/2012. Univariate and multivariate Cox regression models were used to examine the factors that are associated with the occurrence of hypothyroidism after RAI therapy. Kaplan-Meier analysis was used for confirming associations between these models. RESULTS A total of 182 patients were included during a 7.5-year median follow-up (range: 6-13 years). They were 36.4±11.1 years. The mean radioactive iodine dosage was 308.2±104.3 (range: 129.5-740.0) MBq. The rates of euthyroidism, early hypothyroidism, improvement, and ineffective treatment at 6 months were 48.4%, 37.9%, 8.8%, and 4.9%, respectively. The cumulative incidence of hypothyroidism in all patients with hyperthyroidism was 45.6% at 1 year, 48.9% at 5 years, and 52.3% at 10 years. Thyroid weight >46g (HR=0.643, 95%CI: 0.422-0.981, P=0.040) and a course of disease of 0.5-3 years (HR=0.592, 95%CI: 0.358-0.981, P=0.042) were identified as independent factors associated with an increased risk of hypothyroidism after radioactive iodine therapy. CONCLUSION Radioactive iodine treatment with a calculated dose has a high cure rate for hyperthyroidism and has a low annual increase of hypothyroidism. Hypothyroidism after radioactive iodine treatment is more likely to occur in patients with small thyroid and a short disease course.