A hormone (from Greek ὁρμή, "impetus") is a class of regulatory biochemical that is produced in all multicellular organisms by glands, and transported by the circulatory system to a distant target organ to coordinate its physiology and behavior. Hormones serve as a major form of communication between different organs and tissues. Hormones regulate a variety of physiological and behavioral activities, including digestion, metabolism, respiration, tissue function, sensory perception, sleep, excretion, lactation, stress, growth and development, movement, reproduction, and mood. Generally, only a small amount of hormone is required to alter cell metabolism. The brain is often a target organ for many of the hormones, and the brain, in turn, regulates the secretion of these hormones.
Hormone formation may arise at localized clusters of specific cells known as endocrine glands, or at other specialized cells with several functions. Hormone synthesis occurs in response to specific biochemical signals induced by a wide range of regulatory systems. In some cases, the rate at which these systems act on a hormone depends on the particular effect or properties of the hormone. For instance, ionized calcium concentration modulates PTH synthesis, whereas glucose concentration modulates insulin synthesis. Contrarily, regulation of hormone synthesis of gonadal, adrenal, and thyroid hormones is often dependent on a complex set of direct influences and feedback interactions involving the hypothalamic-pituitary axis, such as the HPA, HPG, and HPT axes.
Upon secretion, certain hormones, including protein hormones and catecholamines, are water soluble and are thus readily transported through the circulatory system. Other hormones, including steroid and thyroid hormones, are lipid soluble; to allow for their widespread distribution, these hormones must bond to carrier plasma glycoproteins (e.g., throxine-binding globulin (TBG)) to form ligand-protein complexes. Some hormones are completely active when released into the bloodstream (as is the case for insulin and growth hormones), while others must be activated in specific cells through a series of activation steps that are commonly highly regulated. The endocrine system secretes hormones directly into the circulatory system typically into fenestrated capillaries, whereas the exocrine system secretes its hormones indirectly using ducts. Hormones with paracrine function diffuse through the interstitial spaces to nearby target tissues.
Hormones purposefully affect the target tissue of interest by binding to specific receptor proteins to elicit a specified action in the cellular target. Cells respond to a hormone when they express a specific receptor for that hormone. When a hormone binds to the receptor protein, it results in the activation of a signal transduction mechanism. This ultimately leads to cell type-specific genomic responses that cause the hormone to activate genes that regulate protein synthesis (e.g., up-regulation: synthesis of a receptor for that hormone).
Plant hormones are known as phytohormones.
Endocrinology is a branch of science concerned with the biosynthesis, storage, chemistry, biochemical and physiological function of hormones and with the cells of the endocrine glands and tissues that secrete them.