Objective To analysis the related factors in patients of hypothyroidism (late hypothyroidism) after 131I treatment of Grave 's disease (GD). Methods 442 clinical GD patients were treated by 131I, including 128 male cases (29%) and 314 female cases (71%). The treatment dose of 131I was determined by clinical experience and 131I uptake rate. Patients who were relapsed or not cured after first treatment got the same treatment more than 3 months later until cured. Follow up the curative effect for more than 1 year after the end of treatment, and the patients were divided into hypothyroidism group and non hypothyroidism group based on the occurrence of late hypothyroidism, then the difference of related factors and the clinical significance between two groups of patients were analyzed. Results 231 cases (52.3%) developed late hypothyroidism, including 56 male (43.8%) and 175 female (55.7%) patients (X2=4.7639, P<0.05). The anti-thyroid drug (ATD) treatment time of two groups were 43.6±42.0 and 64.6±50.1 months respectively (t=3.0290, P<0.01); and ATD withdrawal time before treatment were 27.2±25.3 and 18.7±16.4 weeks respectively (t=2.7709, P<0.01). There wais no statistically significant between two groups in age, course of disease, 131I uptake rate, thyroid hormone levels and autoantibodies before treatment, quality and texture of thyroid, treatment times, first and total treatment dose (t=0.2592~1.2363, P>0.05). Conclusion Late hypothyroidism has a certain relation with gender, but no significant relation with age, course of disease, 131I uptake rate, thyroid hormone levels and autoantibodies before treatment, quality and texture of thyroid, treatment times, first and total treatment dose. This suggests that the natural outcome of patients’ inherent law is the main reason of late hypothyroidism, and 131I dose does not need to be too conservative in clinical application.