Objective:To summarize the clinical feature of functional distant metastasis (DM) of differentiated thyroid cancer (DTC) and observe the efficacy of 131I treatment. Methods:Between August 2008 and January 2021, a total of 13 DTC patients (4 males, 9 females; age 26-75 years) with functional DM from Henan Provincial People′s Hospital were retrospectively analyzed. Clinical data of patients were collected, including pathological type, metastasis size, metastasis location, and thyroid stimulating hormone (TSH) before the first 131I treatment. Efficacy of 131I treatment in patients with functional DM-DTC was evaluated by response evaluation criteria in solid tumors (RECIST) 1.1 and thyroglobulin (Tg). Complete remission (CR) and partial remission (PR) were considered as effective. Wilcoxon signed rank test was used to analyze the maximum diameter change of metastatic lesions before and after 131I treatment. Results:Among 13 DM-DTC patients, 8 were follicular thyroid cancer (FTC), 5 were papillary thyroid cancer (PTC). Metastasis lesions were mainly located in lungs ( n=12) and bones ( n=6). There were 12 patients with maximum metastasis diameter ≥1 cm, and 3 patients with TSH≥30 mU/L before the first 131I treatment. Nine patients were assessed as PR by RECIST 1.1, 3 patients were assessed as CR by RECIST 1.1 and the value of Tg, and 1 patient was assessed as PR by the changing of Tg. The effective rate of 131I treatment for patients with functional DM-DTC was 13/13. The maximum metastasis diameter was significantly decreased after 131I treatment (2.6(1.6, 3.3) vs 1.2(0.1, 2.2) cm; z=-3.06, P=0.002). Conclusion:Patients with functional DM-DTC are characterized by high proportion of FTC and the maximum metastasis diameter ≥1 cm, low proportion of TSH ≥30 mU/L before the first 131I treatment, and high effective rate of 131I treatment.