A pheochromocytoma (phaeochromocytoma outside of the US) is a neuroendocrine tumor of the medulla of the adrenal glands originating in the chromaffin cells, which secretes excessive amounts of catecholamines, usually epinephrine and norepinephrine. Extra-adrenal paragangliomas (often described as extra-adrenal pheochromocytomas) are closely related, though less common, tumors that originate in the ganglia of the sympathetic nervous system and are named based upon the primary anatomical site of origin.