Pheochromocytoma is a rare tumor of adrenal gland tissue. It results in the release of too much epinephrine and norepinephrine, hormones that control heart rate, metabolism, and blood pressure. In pheochromocytoma, the adrenal glands produce too much adrenaline, noradrenaline, or both. These hormones help balance heart rate, blood pressure, and stress response, including the fight-or-flight response. When levels of these hormones are too high, the body can enter a state resembling chronically high stress.