According to the histopathological characteristics of thyroid cancer, there are generally four types. (1) papillary carcinoma: It is a malignant tumor originating from the parenchyma of the thyroid gland. It is the most common type of thyroid cancer, accounting for 50% to 89% of the cases. It is most common in women within 40 years of age and in children and adolescents under 15 years of age. The incidence rate can be bimodal, with the first peak around the age of 20 or 30 and another peak in later years. The ratio of females to males is about 3:1, and the cells are usually malignant at the beginning. There are a few cases of malignant transformation from benign adenomas. Most of the cancerous masses are single nodules, and a few are multiple or bilateral nodules with hard texture, irregular border and poor mobility. The masses grow slowly and most of them do not have obvious discomfort, so when they are seen, the average duration of the disease has reached about 5 years, or even more than 10 years. The size of the cancer varies greatly, with small ones less than 25 px in diameter, hard and sometimes not palpable, and often diagnosed due to metastasis to the cervical lymph nodes. Large papillary carcinomas can reach 250 px in diameter and are prone to cystic degeneration, fibrosis and calcification due to the long course of the disease. Large papillary carcinomas can cause local pressure symptoms, resulting in dysphagia, dyspnea and hoarseness. In case of cystic change of papillary carcinoma, yellow fluid can be extracted by puncture, which can be easily misdiagnosed as cyst. Papillary carcinoma is low grade malignant, with late metastasis and easy to invade lymphatic vessels, so early metastasis of cervical lymph nodes is mostly seen, especially in children. This invasive damage progresses slowly and is mainly located in bilateral cervical lymph nodes, and enlarged lymph nodes may remain undetected for years. In advanced stages, metastases to the upper mediastinum or axillary lymph nodes may also occur. Mass aspiration and lymph node biopsy can help establish the diagnosis. Microscopically, the tumor tissue is mostly composed of papillary nodes, with papillae of different sizes and lengths, and branches of 3 or more levels. The papillae have fibers and blood vessels in the center and are surrounded by single or multi-layered square cancer cells. The chromatin in the nucleus is sparse and red granular, evenly distributed, and resembles hairy glass, which is the characteristic of this type. (2) Follicular carcinoma: It is a thyroid carcinoma with follicular differentiation but without papillary structure, and its malignancy is higher than that of papillary carcinoma, accounting for about 20% of thyroid carcinoma and ranking second after papillary carcinoma. It is mainly seen in middle-aged and elderly people, especially in women over 40 years old. It can occur on the basis of thyroid adenoma or nodular goiter. The gross morphology shows that there is an intact envelope, mostly solid, fleshy and soft in texture. The infiltration of the envelope is not easily detected by the naked eye and can undergo degenerative changes, including hemorrhage, necrosis, cystic degeneration and fibrosis, often resembling benign follicular adenomas and not easily distinguished, even in pathological frozen sections. Microscopically, the tumor cells are differently differentiated and show diverse changes. Follicular structure is the main histological characteristic, resembling normal thyroid tissue, but it can also be a hypodifferentiated change without follicles and gelatinous material, with envelope and vascular infiltration. In some follicular adenocarcinomas, some or most of the cells are eosinophilic, and if eosinophils predominate, the diagnosis is eosinophilic adenocarcinoma. Sometimes a few of the cancer cells resemble clear cells, which is clear cell carcinoma. The cancer cells grow slowly and infiltrate into the surrounding area more easily, which is moderately malignant. (3) Medullary carcinoma: It originates from C cells (i.e. parafollicular cells) of thyroid gland and is a moderate malignant tumor, accounting for about 3%-8% of thyroid malignant tumors. The cancer cells may be round, polygonal or spindle-shaped, but they are uniform in shape in the same nest, without papillae or follicular structures. Eosinophilic granules in the cytoplasm and amyloid material in the interstitium are the characteristics of this type. Secretory granules can be seen in the plasma of cancer cells under electron microscopy. The first name was proposed by Hazard. There are many classification and staging methods, mainly from the European Organization for Research and Treatment of Cancer (EORTC), the National Thyroid Cancer Treatment Collaborative Study Group (NTCTCS) and Thyroid Cancer Surveillance, Epidemiology and Referral Criteria (SEER). Medullary thyroid carcinoma can generally be divided into two categories: disseminated and familial. The sporadic type accounts for about 80% and the familial type accounts for about 20%. MEN2A includes medullary thyroid carcinoma, pheochromocytoma and hyperparathyroidism, while MEN2B includes medullary thyroid carcinoma, pheochromocytoma and mucosal neuroma. Category 3 is a family type unrelated to MEN. Medullary thyroid carcinoma is sporadic, with an average age of about 50 years. The carcinoma is often solitary and mostly confined to one side of the thyroid. The familial type is often bilateral and multiple. The cancer is usually a round or oval nodule with a hard texture and clear margins, and the duration of the disease varies (months to more than 10 years). The cancer tumor can easily erode the lymphatic ducts in the thyroid gland and metastasize via lymph nodes, often in the cervical lymph nodes, paratracheal soft tissue, paraesophageal or mediastinal lymph nodes, which can produce pressure symptoms and metastatic masses. It may also metastasize to the lungs, bones or liver via bloodstream. Calcification of the tumor and the involved lymph nodes is an important clue to the diagnosis. Parafollicular cells of the thyroid gland belong to the APDD cell system (APUD tumor). Therefore the tumor produces biologically active substances such as calcitonin (CT), 5-hydroxytryptamine, diastolic intestinal peptide (VIP) and prostaglandins (as shown in Figure 4). It may be accompanied by symptoms of carcinoid syndrome such as intractable diarrhea, dizziness, weakness, tachycardia, precordial urgency, shortness of breath, facial flushing and decreased blood pressure. When the cancer is removed, diarrhea and other carcinoid syndromes disappear, but may reappear when the cancer recurs and metastasizes. The diagnostic marker of myeloid carcinoma is blood CT level. Especially in the familial type, CT measurement can be used to screen family members. Lately, ret gene mutation analysis has been used to diagnose the disease and to screen for high-risk subjects among family members. Girelli summarized the medical records of 78 cases of medullary thyroid carcinoma in Italy from 1969 to 1986, with the following results: age 15-89 years, average 45 years, male to female ratio 1:2.9. 70 cases of disseminated type, 3 cases of familial non-MEN type, 3 cases of MEN2A type and 2 cases of MEN2B type. The average follow-up was 15.9 years, and 34 cases died (4 of them died of other diseases unrelated to the disease). Their mean survival period was 6 years, and the postoperative survival time of the 22 cases who still survived was 10-24 years. The length of survival time was mainly closely related to the stage of the tumor and the age at the time of consultation and treatment, and the efficacy of early treatment was good. Patients with normal pentagastrin test and postoperative blood CT were recurrence-free, while those with abnormalities recurred in different periods after surgery, and the higher the blood CT level, the earlier the recurrence. However, 30% of patients had only elevated blood CT (individually for up to 15 years) without recurrence of the lesion. (4) Undifferentiated carcinoma: Clinically, it includes giant cell carcinoma, small cell and other types of thyroid cancer with higher malignancy (squamous cell carcinoma, adenoid cystic carcinoma, mucinous carcinoma, poorly differentiated papillary carcinoma and follicular carcinoma). It is the most malignant type of thyroid cancer and develops rapidly. Local lymph node metastasis, invasion of the laryngeal nerve, trachea or esophagus, and bloodstream metastasis to lung and bone are common in elderly people aged 60 to 70 years old, accounting for about 5% of thyroid cancer. The goiter or thyroid nodule may be present before the onset of the disease, but the mass increases rapidly within a short period of time and rapidly develops extensive local infiltration to form bilateral diffuse thyroid masses. The local skin temperature of the mass is increased, and the mass is large, hard, poorly defined, and fixed with adhesions to the surrounding tissues, accompanied by pressure pain. It often metastasizes to local lymph nodes and results in lymph node enlargement. Clinically, it may show hoarseness, dysphagia and dyspnea. In addition to lymph node metastasis, undifferentiated carcinoma may also spread to distant places through bloodstream.