Objective:To investigate the changes in total radioactivity in patient body with differentiated thyroid carcinoma (DTC) after 131I treatment and the factors influencing its metabolism. Methods:The clinical data from 218 patients after DTC treatment in the Department of Nuclear Medicine, the Second Affiliated Hospital of Air Force Medical University from September 2021 to April 2022 were retrospectively analyzed. Based on administrated 131I dose, 171 patients were divided into low-dose group (≤ 3.7 GBq) and 47 into high-dose group (>3.7 GBq) . A whole body dynamic radiation monitoring system was used to measure the in vivo residual activity of 131I 24, 48 and 72 h after 131I administration and to explore their influencing factors. Results:24, 48 and 72 h after adimination of 131I, the residual activity of 131I in the low-dose group patients was significantly lower than in the high-dose group patients ( t= -7.46, -3.31, -2.01, P<0.05) . The discharge compliance rate at 24 and 48 h in the low-dose group was significantly higher than that in the high-dose group (21.0% vs. 4.3%, 98.2% vs. 89.4%, χ2 = 7.23, 5.91, P<0.05) , and all patients could meet the discharge criteria at 72 h. Univariate analysis showed that the residual 131I activity at 24 and 48 h was dependent on age, body mass index (BMI) , basal metabolism rate (BMR) and thyroid stimulating hormone (TSH) . As have been shown by multiple linear regression analysis, in the low-dose group, the older age, the higher BMR and the higher TSH level at 24 h tended to the higher 131I residual activity in the body. At 48 h, the higher BMI and the higher TSH level lead to the higher 131I residual activity in patient body. Meanwhile, in the high-dose group, the higher age and BMR at 24 h, tended to the higher in vivo131I residual activity. The influencing factors were analyzed in terms that 131I residual activity reaching 400 MBq in patient body at 24 and 36 h. The result showed that at 24 h the lower TSH level leaded to the lower 131I residual activity in patient body. At 36 h, the younger age, the lower TSH level, and the smaller 131I treatment dose tended to the lower in vivo131I residual activity. Conclusions:Age, BMI, BMR and TSH levels are the influencing factors for the change in total activity in patient body after 131I treatment of DTC. Radiation dose assessment based on the above indicators can provide a reference for adjusting the length of hospitalization time.