Principle of iodine-131 treatment for differentiated thyroid cancer: Iodine-131 is a radioactive drug which emits β-rays that can effectively destroy metastases and form fibrosis or calcification for therapeutic purposes without any significant effect on other tissues and organs. Indications for iodine-131 treatment of differentiated thyroid cancer: 1. primary cancer that cannot be removed surgically; 2. tissue remaining after surgery; 3. lymph node metastasis in the neck; 4. distal (lung, mediastinum, bone, liver, brain, etc.) metastasis; 5. cancer tissue found to have passed through the envelope after surgery, and cancer metastasis at the microscopic level cannot be excluded; 6. recurrence of thyroid cancer after treatment. After thyroid cancer surgery, the significance of iodine-131 treatment: 1. Iodine-131 can remove the microscopic thyroid cancer lesions that are difficult to detect in the thyroid tissue remaining after surgery; 2. Iodine-131 whole-body imaging after iodine-131 nail clearing treatment can detect new metastases that cannot be detected by other imaging examinations; 3. By detecting serum thyroglobulin (Tg), it can detect whether there is recurrence or metastasis, which is both 4. Most of the local or distant metastases (such as lung, bone, brain, etc.) have good results after multiple iodine-131 treatments, which can significantly reduce the size of the lesions, alleviate the symptoms, improve the quality of life, and cure some patients; 5. We recommend men to have contraception within six months after receiving Iodine-131 treatment, female patients to have contraception within one year, and female patients need to have their thyroid function tested regularly after pregnancy and delivery.