1. All patients with differentiated thyroid cancer who have postoperative thyroid imaging with residual thyroid tissue visualized should use iodine-131 to remove residual thyroid tissue if they meet the following conditions (1) Patients with stage III and IV differentiated thyroid cancer; (2) All patients with stage II differentiated thyroid cancer who are younger than 45 years old; (3) Most patients with stage II differentiated thyroid cancer who are older than 45 years old; (4) Patients with selective stage I differentiated thyroid cancer, especially those with multiple tumor lesions and the presence of lymph node metastasis, extrathyroidal or vascular infiltration; (5) Patients with aggressive Patients with pathological types (hypercellular, islet cell or column cell types). 2.Patients with differentiated thyroid cancer whose residual thyroid tissue has been completely removed, recurrent foci or metastases that cannot be surgically removed, and foci that are concentrated with iodine-131. 3.Patients with differentiated thyroid cancer whose residual thyroid tissue has been completely removed, such as negative iodine-131 imaging but elevated Tg levels (highly suggestive of metastatic thyroid cancer or small diffuse microscopic differentiated thyroid cancer foci in vivo).