Medullary thyroid carcinoma is rare in China, but clinically it is more malignant than papillary carcinoma, prone to lymph node metastasis, and difficult to treat. The diagnosis of medullary carcinoma relies heavily on laboratory tests for calcitonin and carcinoembryonic antigen (CEA). This can be used as a screening test when thyroid cancer is suspected. Calcitonin and CEA are usually elevated prior to treatment, and absolute values do not vary from patient to patient, but once elevated, a diagnosis of medullary carcinoma is considered. Both show a significant progressive decrease after treatment, which is considered an indicator of effective treatment. The rate of decline is inconsistent and usually drops to a low level 1 month after treatment. Calcitonin and carcinoembryonic antigen can be used as indicators for post-treatment follow-up, usually with a 3-6 month check to monitor treatment efficacy. If both hover consistently at low levels, there is no cause for concern. If the test value is progressively increasing, it indicates cancer recurrence and further examination is necessary. The absolute values of calcitonin and carcinoembryonic antigen are not very important, but more important is the trend of their changes and the magnitude of their increase. an increase in CEA should also be considered for other diseases, and this value is not very specific in medullary thyroid cancer.