One of the reasons why the incidence of thyroid cancer varies so much around the world and its pathological type is the different iodine intake in the diet of the population. In regions with sufficient iodine intake, only 0.9 per 1,000 people suffer from thyroid cancer, and mainly papillary carcinoma, which has a good prognosis, accounts for 70% to 90% of the cases; in regions with insufficient iodine intake, 1.3 per 1,000 people suffer from thyroid cancer, of which papillary carcinoma accounts for only 41%, while follicular carcinoma and undifferentiated carcinoma, which have a poor prognosis, are more common. In Japan, after iodization of salt in 1920, there were no iodine-deficient areas in 1950, and the incidence of thyroid cancer decreased significantly. Papillary carcinoma is the most common type of thyroid cancer. Papillary carcinoma can be divided into two categories: high-risk and low-risk. High-risk category has the following characteristics: 1) thyroid cancer invades beyond the peritoneum; 2) symptoms worsen rapidly in the last 6-12 months; 3) masses increase significantly in 6-12 months; 4) distant metastasis is present. The high-risk category accounts for 10%-15% of papillary carcinoma and 18%-46% of cancer deaths; after treatment, most of them are recurrence and distant metastasis at the same time; the patients are almost always men over 40 years old and women over 50 years old. The low risk category accounts for 85% to 90% of papillary carcinoma, of which only 1% to 2% die from cancer and the recurrence rate is only 1.5% to 2%. What are the types of thyroid cancer? What are the characteristics of each? Thyroid cancer accounts for about 2% of all cancers and is mainly divided into the following four types. Papillary carcinoma: It is a well-differentiated type of thyroid cancer and the most common one, accounting for about 3/4 or more of thyroid cancers. This type can occur at any age and can occur in both men and women, but is more common in young and middle-aged women (18 to 40 years old). Papillary thyroid carcinoma is more common with lymph node metastasis in the neck and less common with hematogenous metastasis to distant organs. It is a tumor with a very high cure rate, and it does not affect the natural life expectancy after surgery in more than 80% of cases. Follicular carcinoma: It is also a kind of well-differentiated thyroid cancer, accounting for about 10%-15% of thyroid cancer, and is most common in middle-aged and elderly women aged 40-60. Follicular carcinoma can have early hematogenous metastasis, but lymphatic metastasis is less common than papillary carcinoma. The treatment effect is still good, and the cure rate is about 70%. Undifferentiated carcinoma: It is a highly malignant tumor, accounting for 5% to 10% of thyroid cancer, mostly seen in elderly men. Hematogenous and lymphatic metastases are early and common. The prognosis of this type is poor, most patients die within 1 year, and the 5-year cure rate is less than 5%. Medullary carcinoma: It is a moderately malignant tumor, accounting for about 3% to 10% of thyroid cancer. It can occur at any age and there is no significant difference in incidence between men and women, and about 10% have family history. This type of cancer cells can secrete many substances, such as calcitonin, adrenocorticotropic hormone, prostaglandin and 5-hydroxytryptamine, etc. The corresponding symptoms can appear, which doctors call carcinoid syndrome. The clinical manifestations are chronic diarrhea, facial flushing, low blood calcium and high blood pressure. This type of tumor belongs to the so-called dopa precursor tumor (APUD tumor). The 5-year cure rate of this type is about 40%. The main symptom of thyroid cancer is rapid neck swelling, including thyroid lumps and lumps formed by metastases from cervical lymph nodes. However, some patients may develop a neck lump years after the onset of diarrhea and may have pressure symptoms (such as breath-holding) in advanced stages. How to treat thyroid cancer High-risk papillary thyroid cancer often invades the adjacent trachea, followed by the esophagus, and the common sites of distant metastases are bone and lung. For this type of high-risk thyroid cancer, combined resection of adjacent organs is feasible. The classic treatment for low-risk papillary thyroid cancer is total thyroidectomy and cervical lymph node dissection, followed by systemic treatment with 131 iodine and lifelong thyroxine tablets. Since about 30 years ago, it has been advocated to perform thyroidectomy on one side only in the low-risk group; if there are lymph node metastases, functional clearance (i.e., preservation of the sternocleidomastoid muscle of the neck, internal jugular vein and paraneoplastic nerves) is added; postoperative thyroxine tablets are taken to prevent recurrence and metastasis. Oral thyroxine tablets of 80-160 mg daily for 3 years can shrink the residual thyroid gland. The level of thyroid stimulating hormone in blood should be checked frequently after surgery. If the level of thyroid stimulating hormone is elevated, it may promote the recurrence of cancer metastasis. Medullary thyroid carcinoma is a cancer arising from calcitonin-secreting parafollicular cells and should be surgically removed and cervical lymph node dissection performed. If the calcitonin value decreases, the cancer is confined to the thyroid gland and can be cured by surgical resection; if the value remains high after surgery, the cancer has residual lymph node metastasis and the prognosis is poor. Follicular thyroid cancer can be divided into two types: enveloped and infiltrative type. The latter type is prone to bone metastasis and other hematogenous metastasis, so thyroidectomy should be actively performed. The higher the value of thyroglobulin in blood, the higher the possibility of metastasis, and systemic treatment with 131 iodine should be used as soon as possible, which has better efficacy. Undifferentiated thyroid cancer should be radically resected, and radiotherapy and chemotherapy should be used before and after surgery as appropriate.