Follicular carcinoma of the thyroid accounts for 5% to 20% of all thyroid cancers. In areas with endemic goiter, follicular carcinoma accounts for a larger proportion. The average age of patients at the time of presentation is 45-50 years, which is slightly older than the average age of papillary carcinoma. However, it is also well documented that patients with follicular carcinoma are between 35-40 years of age at the time of presentation. Seventy percent of follicular carcinomas are well differentiated and those with intact envelopes are not easily distinguished from follicular adenomas. Measurement of intracellular DNA content may help in the differential diagnosis. Hypodifferentiated types account for about 15% of cases, and the cancer masses are often large (more than 3 cm), mostly seen in women over 40 years of age. Eosinophilic cell carcinoma composed of eosinophilic cells accounts for 3%-9% of follicular carcinomas and has a poor prognosis. Clear cell carcinoma, which is composed of clear cells, is relatively rare. It is more malignant. Follicular carcinoma invades less lymphatic system, and the rate of lymph node metastasis at the time of consultation is less than 10%; however, it invades more blood vessels, and the rate of distant metastasis (hematologic metastasis) at the time of consultation is 15%-20%; there are also very few cases that can spread within the gland to form satellite nodes. Follicular carcinoma has the highest iodine uptake among thyroid malignancies, and the amount of TSH receptors in cancer tissue is about 70% of that in normal thyroid tissue, and these 2 features should be used for treatment. The clinical manifestations of follicular carcinoma are similar to those of papillary carcinoma, but the cancer mass is generally larger, with less local lymph node metastasis and more distant metastasis (metastasis to lung, bone, liver, etc.), and the presence of distant metastases is detected earlier than the primary foci. The diagnosis of follicular carcinoma mainly relies on pathological diagnosis, but pathological diagnosis is sometimes quite difficult. It is often difficult to distinguish benign follicular adenoma from follicular adenocarcinoma by thyroid aspiration cytology, and the false-negative rate is over 20%, and the misdiagnosis of benign adenoma can be up to 20% in rapid ice-storage. Therefore, for all tumors with follicular structures, even if the cytologic or histologic findings are benign, it is necessary to be vigilant. Serum Tg level is helpful for diagnosis, and DNA content of tumor cells and its ploidy can also be used as an auxiliary indicator, and CEA positive rate is about 35%. Estrogen and progesterone receptor assay is also sometimes used for differential diagnosis. However, the positivity rate is not high. The principles of surgical treatment for follicular carcinoma are the same as those for papillary carcinoma. It seems to have its own special aspects. Generally speaking, thyroidectomy for follicular carcinoma should be as complete as possible. If the diagnosis is confirmed, the affected lobe plus the isthmus should be removed, and at least most of the contralateral lobe should be removed, preferably completely or nearly completely; if it is not certain to be malignant, complete removal of the affected lobe plus the isthmus is also preferable. This can reduce the difficulty of re-operation. If the lymph nodes in the neck have metastases, the lymph nodes in the middle cervical region should be cleared. However, local lymph node metastasis in follicular carcinoma is rare, and in fact, less than 10% of lymph nodes need to be cleared. The biological characteristics of follicular carcinoma (iodine uptake, tumor cells rich in TSH receptors) determine that I131 therapy and TSH suppression therapy (thyroxine administration) are important components of its treatment after surgery. I131 therapy is effective not only for possible residual primary cancer but also for local recurrence and metastasis, but the premise is that the minimum amount of residual thyroid gland (zero residual is ideal), and if there is a lot of residual glandular tissue, the residual gland must be killed first before I131 can be applied to the tumor tissue. If necessary, Co60 or high-energy X-rays can also be helpful for external irradiation of tumors that cannot be completely removed. After careful physical examination and relevant adjuvant examinations, recurrent and metastatic lesions can be detected in time and treated accordingly. In general, serum Tg is reviewed every 6 months for 2 years after surgery and every 1 – 2 years thereafter. In follicular carcinoma patients, if total thyroidectomy has been performed or residual thyroid tissue has been killed by I131 after surgery, the serum Tg level should be 0. If it is greater than 0, it indicates the presence of metastatic foci or residual cancer. In patients with papillary carcinoma, if the serum Tg level is greater than 10ug/L, it indicates the possibility of cancer recurrence or metastasis and should be examined closely to find the lesion.