Endocrinology is a discipline of medicine that studies the endocrine system and diseases caused by its inadequate functioning. Some of the diseases that endocrinology deals with are diabetes mellitus caused by insulin deficiency or resistance to its action, hypothyroidism due to a deficiency in the production of thyroid hormones, hyperthyroidism due to excessive production of thyroid hormones, Cushing's disease due to generally due to excessive production of cortisol by the adrenal glands, and hyperaldosteronism or aldosteronism characterized by the overproduction and secretion of the hormone aldosterone.