The cause of thyroid cancer is still unknown, but it is thought to be related to chronic thyroid stimulating hormone. Pathological classification and biological characteristics: The development process and metastatic pathways of different types of thyroid cancer vary greatly, and their treatment also varies. The main route of dissemination is the lymphatic tract, generally the cervical lymph node metastasis is the most common, some believe that papillary carcinoma is multicentric, or has contralateral metastasis, about 80% of children and 2% of adult patients can find lymph nodes, followed by blood metastasis to lung or bone. Follicular carcinoma: It is the second most common type of thyroid cancer, accounting for about 20% of the cases, and its average age is older than that of papillary carcinoma. Well-differentiated follicular carcinoma is microscopically similar to normal thyroid gland, but with invasion of envelope, blood vessels and lymphatic vessels; poorly-differentiated follicular carcinoma has irregular structure, with cells densely packed into clusters or cords, rarely forming follicles. The route of dissemination can be through lymphatic metastasis, but mainly through blood to the lungs, bones and liver. Some follicular adenocarcinomas may recur long after surgical resection, but their prognosis is not as good as that of papillary adenocarcinoma. (3) Medullary carcinoma: It occurs in parafollicular cells (C cells) other than follicular epithelium, and it is sporadic or familial, accounting for about 2-5%. The cells are arranged in bands or bundles without papillae or follicular structures, and there are amyloid deposits in their interstitium. They secrete large amounts of 5-hydroxytryptamine and calcitonin. The histology is undifferentiated, but its biological characteristics are different from those of undifferentiated carcinoma. It is moderately malignant, with early cervical lymph node metastasis and distant metastasis in the late stage. Familial medullary carcinoma is mostly bilaterally involved at the same time. Undifferentiated carcinoma: It accounts for about 5% of thyroid cancer, including large cell carcinoma, small cell carcinoma, squamous cell carcinoma, sarcoma, carcinosarcoma, fibrosarcoma, malignant fibrous histiocytoma that originates from the thyroid gland, mostly occurs in the elderly. The thyroid cancer that we routinely treat with 131I is differentiated thyroid cancer (DTC), which includes papillary carcinoma type I and follicular carcinoma type II. The internationally recommended standardized treatment protocol for differentiated thyroid cancer: surgery + 131I therapy + thyroxine suppression therapy.