Pheochromocytoma is a tumor derived in the inner part of the adrenal gland (the medulla). It secretes hormones called catecholamines (the breakdown of catecholamines is called metanephrines). Norepinephrines and to a lesser extent, epinephrine are released into the body in excess amounts. Under normal circumstances, this is what is known as the "fight or flight" response. Individuals with a pheo are constantly in fight or flight response. This creates a deadly scenario if left untreated.