The prognosis of differentiated thyroid cancer is better than other malignant tumors, but it is prone to recurrence and metastasis. 2. For differentiated thyroid cancer, “surgery + iodine-131 (131I) therapy + thyroid hormone suppression therapy” is an internationally recognized effective treatment option. The main principle and function of 131I treatment are: 131I can destroy residual thyroid tissue and metastatic foci of differentiated thyroid cancer by using its highly targeted beta radiation. 4. The clinical values of using 131I to remove residual thyroid gland after differentiated thyroid cancer surgery include: a. Complete removal of microscopic tumor lesions that may be hidden, reducing local recurrence; b. Improving the sensitivity of thyroglobulin (Tg) to monitor recurrence and metastasis, facilitating long-term follow-up; c. Facilitating the treatment of metastases and improving the sensitivity of detecting metastases during 131I whole-body scan. 5.131I has both diagnostic and therapeutic functions for metastatic lesions of differentiated thyroid cancer. 6.The need for 131I treatment after surgery for differentiated thyroid cancer should be mainly based on the risk stratification of patients for recurrence and death. The European consensus of risk stratification for differentiated thyroid cancer is more reasonable with three categories: high risk, low risk and very low risk. For all patients with differentiated thyroid cancer except very low-risk patients, 131I therapy is recommended after surgical thyroidectomy. Although low-risk differentiated thyroid cancer has a good prognosis and can survive for a long time, its long-term follow-up is more important and necessary. 131I therapy is beneficial for long-term and lifelong follow-up of low-risk patients using Tg. Since many foods and drugs can affect the effect of 131I treatment, the following must be done in the preparation stage before hospitalization: 1) stop thyroid hormone replacement therapy; 2) strictly avoid iodine-rich foods (such as kelp, seaweed, sea fish, sea cucumber, shark fin, abalone, etc.); 3) take non-iodized salt if possible; 4) refrain from performing enhanced CT (because of the risk of iodine) for 2 months before treatment. Avoid performing enhanced CT within 2 months before treatment (because the contrast agent will affect the 131I treatment).