Principle: 131I treatment for thyroid cancer mainly targets the metastases of thyroid cancer. Some highly differentiated thyroid cancers, such as follicular and papillary thyroid cancers, have the function of selective 131I uptake, and the principle of 131I treatment for metastatic thyroid cancer is to kill cancer cells by using the β-rays radiated by 131I entering the cancer tissue. It is related to the uptake ability of 131I. Indications: (1) Risk stratification of patients, all patients with differentiated thyroid cancer with postoperative residual thyroid tissue iodine uptake rate greater than 1%, and residual thyroid tissue visualized in the thyroid bed on thyroid imaging, should use 131I to remove residual thyroid tissue if the following conditions are met: ①Patients with stage III and IV differentiated thyroid cancer; ②All patients with stage II differentiated thyroid cancer aged less than 45 years (3) Most patients older than 45 years with stage II differentiated thyroid cancer; (4) Patients with selective stage I differentiated thyroid cancer, especially those with multiple tumor lesions, lymph node metastases, extrathyroidal or vascular infiltration; (5) Patients with aggressive pathological types (hypercellular, islet cell or column cell types) (2) Patients with differentiated thyroid cancer in which residual thyroid tissue has been completely removed (3) Patients with differentiated thyroid cancer whose residual thyroid tissue has been completely removed, such as those with negative 131I imaging but elevated Tg levels (highly suggestive of metastatic thyroid cancer or small diffuse microscopic differentiated thyroid cancer lesions in vivo). Contraindications: Pregnant and lactating patients; Patients with unhealed postoperative wounds; Patients with WBC below 3.0×109/L; Patients with severe impairment of liver and kidney function.