Objective:To explore the relationship between level of urinary iodine excretion (UIE) before 131I treatment and excellent response (ER) in low-to-intermediate risk differentiated thyroid carcinoma (DTC) patients. Methods:A retrospective analysis was performed with 432 DTC patients (124 males, 308 females, age: (42.1±11.0) years) who were treated with 131I for the first time after total thyroidectomy from June 2017 to October 2018 in Department of Nuclear Medicine, the Second Hospital of Shandong University. All patients were divided into 4 groups: G1, group 1, UIE<50 μg/L; G2, group 2, 50 μg/L≤UIE<100 μg/L; G3, group 3, 100 μg/L≤UIE<200 μg/L; G4, group 4, UIE≥200 μg/L. Patients were given 131I with a fixed dose (3 700 MBq). Response was evaluated 6 to 8 months after 131I treatment: ER, indeterminate response (IDR), biochemical incomplete response (BIR), and structural incomplete response (SIR). χ2 test and Kruskal-Wallis rank sum test were used to analyze the data. The adjusted standardized residual (residual) and Cramer′s V between G1-G4 and different treatment reactions were calculated to judge the difference among groups. IDR, BIR and SIR were classified into non-ER (NER) group, and binary logistic regression analysis and receiver operating characteristic (ROC) curve analysis were performed to find the influencing factors of treatment reactions. Results:There were 51.9%(41/79), 64.9%(98/151), 53.8%(63/117), 30.6%(26/85) patients achieved ER in G1-G4, and the proportion of G4 was significantly lower than that of G1-G3 ( χ2 values: 7.695-25.697, all P<0.05), and there was no significant difference among G1-G3 ( χ2 values: 0.072-3.667, all P>0.05). The UIE level of patients in ER, IDR, BIR, SIR group was 87.5(57.0, 129.0), 97.0(55.7, 211.5), 141.0(74.0, 231.0), 148.0(68.5, 221.0) μg/L( H=15.977, P=0.001), and there was significant difference between those of patients in ER and SIR groups ( χ2=8.729, P=0.019). There was a certain correlation between UIE levels and different treatment reactions (Cramer′s V=0.151, P=0.001). UIE (≥200 μg/L), gender and preablative stimulated thyroglobulin could be used as independent factors affecting ER ( Wald values: 4.029, 7.185, 56.301, all P<0.05). Conclusion:Among DTC patients with low-to-intermediate risk, 131I treatment does not affect ER when the UIE level is less than 200 μg/L, while 131I treatment should be performed carefully when the UIE level is more than 200 μg/L.