Low-risk patients: 1. No local or distant metastasis of the tumor after initial surgery and radioiodine removal of the residual thyroid gland; 2. The tumor visible to the naked eye was removed; 3. The tumor did not invade into local tissues and surrounding blood vessels; 4. The pathological type of the tumor does not belong to aggressive types such as high-cell carcinoma, insular carcinoma and columnar carcinoma; 5. No 131I uptake outside the thyroid bed on whole-body imaging after the first 131I treatment. Low-risk patients do not need more aggressive methods to monitor tumor recurrence. Intermediate-risk patients: Tumor invasion into soft tissues around the thyroid gland or tumor pathology of aggressive type or tumor invasion into blood vessels are found under microscope at the time of initial surgery. Long-term follow-up and treatment of intermediate-risk patients should adopt a more aggressive strategy with close monitoring and early detection of tumor recurrence for effective treatment. High-risk patients: Tumor invasion into the surrounding tissues at the time of initial surgery, tumor not completely removed, or tumor with distant metastasis or the presence of 131I uptake outside the thyroid bed during radioiodine imaging after removal of the residual thyroid gland. The purpose of long-term follow-up and treatment for high-risk patients is to cure or delay tumor-related disease and reduce the rate of death; or to reduce the size of the tumor by using therapies that prevent tumor growth.