Multiple endocrine neoplasia type 2 is part of a group of disorders that affect the endocrine system (the network of glands that produce hormones). These disorders greatly increase the risk of developing multiple cancerous and noncancerous tumors in glands such as the thyroid, parathyroid, and adrenals. Multiple endocrine neoplasia occurs when tumors are found in at least two endocrine glands. Tumors can also develop in organs and tissues other than endocrine glands. If the tumors become cancerous, some cases can be life-threatening. Although many different types of hormone-producing tumors are associated with multiple endocrine neoplasia, the most common sign of multiple endocrine neoplasia type 2 is a form of thyroid cancer called medullary thyroid carcinoma. This tumor secretes an inactive hormone called calcitonin. Many people with this disorder also develop pheochromocytoma, which is a tumor of the adrenal glands (located above each kidney) that can cause dangerously high blood pressure. In addition, overactivity of the parathyroid gland (hyperparathyroidism) occurs in some cases of multiple endocrine neoplasia type 2. Hyperparathyroidism disrupts the normal balance of calcium in the blood, which can lead to kidney stones, thinning of bones, weakness, and fatigue. The two major types of multiple endocrine neoplasia, type 1 and type 2, are often confused because they have similar names. These types are distinguished by the genes involved, the hormones that are affected, and their characteristic signs and symptoms. They are also very different in their options for cancer prevention and treatment. Multiple endocrine neoplasia type 2 can be further divided into three subtypes: type 2A, type 2B, and familial medullary thyroid carcinoma (FMTC). These subtypes differ in their characteristic signs and symptoms and risk of specific tumors, but are relatively consistent within any one family.