OBJECTIVE: To investigate and analyze the characteristics of early adverse drug reaction (ADR) of 131I in the treatment of hyperthyroidism. METHODS: 468 hyperthyroidism patients treated with 131I were followed up for 1 year. RESULTS: After 2~7 days of treatment, different reactions occurred in 54 cases (11.50%) including rash, itch of skin and alopecia of 16 cases (3.40%), paroxysmal muscle spasm of 19 cases (4.10%), eye discomfort of 13 cases (2.80%), aggravated symptom of 3 cases (0.60%) and thyroid pain of 3 cases (0.60%). These ADRs can last about 2 weeks to one year, and then relieved themselves. CONCLUSION: 11.50% of hyperthyroidism patients treated with 131I complain early systemic or local adverse reactions which are relieved within several months.