The final diagnosis of MTC was made by fine-needle aspiration cytology of the thyroid swelling. To briefly introduce MTC, medullary thyroid carcinoma arises from parafollicular cells of the thyroid (also known as C cells), which are neuroendocrine cells. It is characterized by the secretion of calcitonin and various substances including carcinoembryonic antigen and the production of amyloid, and was first proposed by Hazand et al. in 1959 as an independent clinicopathological type with unique features in its pathogenesis, diagnosis and treatment. Medullary thyroid carcinoma is less common, accounting for about 3-10% of thyroid carcinomas and is moderately malignant. The tumor invades blood vessels and distant metastases occur in up to 15% of cases, and liver, lung and bone metastases are common. The clinical presentation is similar to that of other thyroid cancers, except for the combination of endocrine syndrome, which presents with a slow-growing neck mass, including enlarged lymph nodes in the neck and a hard thyroid mass. The diagnosis of MTC should be differentiated from the following diseases: 1. malignant thyroid tumors: undifferentiated thyroid cancer, follicular thyroid cancer, lymphoma and papillary carcinoma; 2. non-cancerous thyroid tumors: such as thyroid adenoma. If the above diseases need to be differentiated, the following methods are commonly used: Fine needle aspiration cytology (FNAC): used to clarify the pathological type of MTC tumor tissue, and is one of the most effective methods for preoperative qualitative diagnosis, if DNA can be extracted, RET gene mutation analysis can also be performed; Calcitonin test: can be used as a specific tumor marker for MTC, and is closely related to tumor load; Imaging methods Ultrasound, CT, MRI, nuclear medicine examination (thyroid imaging and PET/CT whole body scan) all play an important role in the localization and qualitative differential diagnosis of MTC tumors. It is worth noting that the possibility of medullary thyroid carcinoma should be considered when patients present with carcinoid syndrome such as unilateral or bilateral thyroid masses, dyspnea, dysphagia, masses in the lateral neck area, hoarseness, hand and foot twitching, facial flushing, palpitations, diarrhea, and emaciation. Calcitonin and carcinoembryonic antigen are important indicators for the diagnosis of medullary thyroid carcinoma and the judgment of surgical efficacy and postoperative recurrence, which should attract the attention of clinicians.