Medullary thyroid carcinoma (MTC) accounts for 5-7% of all thyroid cancers. It originates in the C cells of the thyroid gland. MTC is easier to treat and control if it is diagnosed before it spreads to other parts of the body. However, in some patients, MTC metastasizes and spreads before it is diagnosed. There are two types of medullary thyroid cancer: disseminated and familial. Disseminated medullary thyroid carcinoma accounts for approximately 80% of all medullary thyroid carcinomas, and patients have no significant family history. Familial medullary thyroid cancer may be associated with hypercalcemia and adrenal tumors (e.g., pheochromocytoma). Genetic testing should be performed in all patients with medullary thyroid cancer. Genetic testing is a criterion for diagnosis and treatment, not a scientific test. All immediate family members of patients with familial medullary thyroid cancer also need genetic testing to determine the presence of genetic factors that predict the progression of medullary thyroid cancer. The genetic assay focuses on the RET proto-oncogene. If this genetic alteration is present in the subject (including infants and children), removal of the thyroid gland before cancer may develop is an effective preventive treatment. Almost all individuals with a mutation in the RET gene (an abnormal change in the sequence of the RET proto-oncogene) will eventually develop medullary thyroid cancer. This specific mutation can be used to determine when the thyroid should be removed. Medullary thyroid cancer usually produces calcitonin and carcinoembryonic antigen, which can be detected by blood tests. Medullary thyroid cancer cannot take up iodine. Therefore, radioactive iodine therapy is not indicated for the treatment of medullary thyroid cancer. The main treatment for medullary thyroid cancer is surgery, and its long-term prognosis is not as good as that of differentiated thyroid cancer. However, in recent years, several new drugs have entered clinical trials that offer hope for effective treatment of medullary thyroid cancer. Caprelsa (vandetanib) has been approved by the FDA (U.S. Food and Drug Administration) for the treatment of certain patients with medullary thyroid cancer.