1.Introduction of thyroid cancer Thyroid cancer is the most common malignant tumor of endocrine system, accounting for 1.6% of all malignant tumors, and its incidence is continuously increasing. The causes of thyroid cancer are complex and may be related to genetic factors, environmental radiation, iodine intake, gender, age, etc. 2.How to detect thyroid cancer in early stage? Like other malignant tumors, the best way to prevent and treat thyroid cancer is early detection, early diagnosis and early treatment. Then how can we achieve early detection? When you find thickening of the neck or a lump, even if there is no uncomfortable symptom, you should think about whether thyroid disease has occurred or not. Ultrasound is simple, practical and inexpensive, and can detect tumors as large as 5 mm in diameter and even 2-3 mm in diameter. Ultrasound-guided fine needle aspiration cytology (FNAC) is now recognized as the most reliable and valuable diagnostic method to distinguish benign from malignant nodules. 3.What are the main types of thyroid cancer, how to confirm the diagnosis and what is the prognosis? Clinically, thyroid cancer is mainly divided into papillary thyroid cancer, follicular thyroid cancer, medullary thyroid cancer and undifferentiated thyroid cancer, the first two of which are collectively called differentiated thyroid cancer and account for more than 90% of thyroid cancer. At present, ultrasound-guided needle aspiration cytology is still the most effective diagnostic method, but for a small number of difficult-to-diagnose cases, postoperative paraffin section and immunohistochemistry are still required to make a definite diagnosis. Most of the differentiated thyroid cancers have a good prognosis after appropriate treatment, and the survival rate of non-advanced thyroid cancer patients under 45 years old can reach 85%-90% in 20 years after surgery. 4.Treatment of differentiated thyroid cancer Surgery is the first choice. The primary lesion and thyroid tissue as well as metastatic lesions in the neck are removed through surgery as much as possible; oral 131 iodine is used for internal radiation treatment of residual thyroid and occult lesions; some patients can be treated with oral 131 iodine again after six months for metastatic lesions; thyroid hormone replacement suppression therapy. 5.Why 131 iodine therapy is needed after differentiated thyroid cancer surgery? Domestic and foreign data show that after thyroid cancer surgery, 60-80% of the residual thyroid tissue or the contralateral thyroid gland can be seen as cancerous lesions under the microscope, leaving a hidden danger for future recurrence and metastasis. Because of the unique iodine uptake ability of thyroid (cancer) cells, radionuclide 131 iodine can target the residual thyroid (cancer) tissues after surgery, and the radiation released from it can produce ionizing radiation biology effect on postoperative hidden cancer foci and metastatic foci to kill these functional tissues, which will reduce the risk of tumor recurrence and metastasis. After the removal of normal thyroid tissue, its ability to compete for 131 iodine is significantly reduced, which helps to increase the uptake of 131 iodine by metastases (commonly metastases in cervical lymph nodes, lungs, bones, etc.), thus ultimately improving the efficacy of 131 iodine treatment for metastases of thyroid cancer. A whole-body scan after iodine treatment to understand the distribution of 131 iodine in the body can detect the presence of metastases of thyroid cancer, thus allowing for accurate staging of the disease and objective assessment of the prognosis. After 131 iodine is used to remove residual thyroid tissue, the normal source of serum thyroglobulin (Tg) in the body is gradually depleted, and Tg measurement can be extremely sensitive and specific to monitor the recurrence or metastasis of thyroid cancer in follow-up. 6.Why can 131 iodine treat thyroid cancer? The β-rays emitted by 131 iodine (with a range of about 2-3 mm) gathered at the site of the lesion destroy the residual thyroid gland and cancerous tissues, while having no significant effect on other surrounding organs and tissues. Since other tissues in the body do not have iodine uptake function, the unabsorbed 131 iodine is quickly eliminated from the body through urination, so there are no serious side effects.