Inclusion Criteria:  Study time: June 1, 2021, to January 1, 2022; Study site: The First Affiliated Hospital of Shandong First Medical University; 18-75 years old (including upper and lower limits), male or female; Patients with differentiated thyroid carcinoma diagnosed pathologically after total thyroidectomy Patients with oral levothyroxine tablets ≥ 4 weeks after operation Patients who have stopped taking levothyroxine tablets and whose TSH < 4.2uIU/ml at the time of withdrawal Patients who meet the 131I treatment criteria  Exclusion Criteria:  Pregnant or lactating women Patients with various mental disorders, unconscious, etc. Cases with inaccurate information collection