The treatment of differentiated thyroid cancer (DTC) with 131I has a history of 60 years internationally and has been carried out in China for nearly 50 years. At present, this treatment has been carried out in all provinces, municipalities and autonomous regions in China, and has become one of the most important methods for DTC. The combination of surgical resection, 131I therapy and thyroid hormone suppression therapy is an internationally recognized ideal solution for the treatment of DTC. 131I therapy includes post-surgical 131I removal of residual thyroid tissue after DTC (clear nail) therapy and treatment of recurrent and metastatic lesions. After clear nail treatment, when the patient’s serum Tg≤2ng/ml (in the state of not taking thyroid hormone), the treatment objective is achieved and thyroid hormone suppression therapy is started with regular follow-up. During the follow-up, Tg≥10ng/ml (in the state of not taking thyroid hormone or Tg>5ng/ml when taking thyroxine TSH suppression therapy) or recurrent and metastatic lesions are found, treatment of 131I metastatic lesions should be performed. Since differentiated thyroid cancer cells have iodine uptake function, the lesions can gather 131I and exert therapeutic effect through the radiation biological effect of β-rays. Clinical practice proves that most papillary carcinomas and follicular carcinomas are sensitive to 131I, and the clinical efficacy is certain. I. 131I removal of residual thyroid tissue after DTC surgery (nail clearing) 1. Reasons for nail clearing: ①131I can destroy tiny thyroid cancer lesions that are difficult to detect in the residual thyroid tissue after surgery; ②It is beneficial to perform whole-body 131I imaging; ③It is beneficial to monitor thyroid cancer by measuring Tg level; ④Papillary carcinoma has the tendency of bilateral, tiny multifocal, local lymph node metastasis, local latent and ⑤ DTC is characterized by local infiltration and the possibility of recurrence is increased. A retrospective study found that 131I clearance of residual thyroid tissue after DTC can reduce tumor recurrence and mortality; similar effects were not found in low-risk patients; however, there is a lack of prospective study results. 2. Indications ①Patients with stage III and IV (TNM stage) differentiated thyroid cancer. ②All patients aged less than 45 years with stage II differentiated thyroid cancer. ③Most of the patients with stage II differentiated thyroid cancer who are older than 45 years old. ④Patients with selective stage I differentiated thyroid cancer, especially those with multiple tumor lesions, lymph node metastasis, extra-thyroidal or vascular infiltration. ⑤ Patients with aggressive pathological types (hypercellular, islet cell or column cell types).