Medullary thyroid cancer: a type of thyroid cancer. Medullary thyroid cancer originates from the so-called “C-cells” of the thyroid gland, also known as parafollicular cells. These parafollicular cells do not make thyroid hormones like the cells of the thyroid follicles (glandular units); instead, they produce a hormone known as calcitonin. Medullary thyroid cancers account for only about 1%-2% of all cancers of the thyroid. Medullary thyroid cancer frequently spreads to lymph nodes and other organs. Up to 25% of cases run in families. The inherited forms can occur together with other endocrine tumors, in syndromes called multiple endocrine neoplasia (MEN) 2A and MEN 2B. See also: thyroid cancer.