This study aimed to describe the characteristics of Graves' patients treated with I131 and the hypothyroidism after radioiodine I131 therapy at Endocrinology department of Thai Nguyen National Hospital from July 2021 to July 2022. This was a cross-sectional descriptive and prospective study was performed on 50 patients with radioiodine I131 therapy at Endocrinology department of Thai Nguyen National Hospital from July 2021 to July 2022. Results: After 6 months of follow-up, it was found that most of the study subjects were female (82%), the mean age was 38.04 ± 15.01 years old. The average therapeutic dose is 9.1± 1.6mCi. After 3 months treatment, rate hypothyroidism 36.0%. hyperthyrodisum was 58,0%, euthyroidism was 6.0%. After 6 months treatment, rate hypothyroidism 26.0%, euthyroidism was 46.0% and hyperthyrodisum was 28%.