Overview

Hormones are chemical messengers that coordinate activity between cells, tissues and organs. Produced by specialized endocrine cells or by dispersed tissues, hormones transmit information that adjusts physiological processes to internal and external conditions. Some act locally on nearby cells, while others enter the blood to reach distant targets. Hormonal control works alongside the nervous system to regulate growth, metabolism, reproduction, water and salt balance, and responses to injury or stress. For summaries of the organ systems and interactions involved, see the endocrine system and its relationship with the nervous system.

Chemical classes and examples

Hormones differ in chemical structure and how they are carried in the body. Major classes include:

  • Peptides and proteins: chains of amino acids (for example, insulin and many pituitary hormones).
  • Steroid hormones: lipids derived from cholesterol (for example, sex steroids and adrenal corticosteroids).
  • Amine hormones: small molecules derived from single amino acids (for example, adrenaline and the thyroid hormones).

Each class influences how a hormone is synthesized, stored, released and how it reaches its targets. Many animals and plants, and other multicellular organisms, use similar biochemical strategies for signalling, though specific hormones and receptors differ across groups.

Sources and glands

Cells that specialize in hormone production are often grouped into organs known as glands. Classic endocrine glands include the pituitary, thyroid, adrenal glands and pancreas, but many other tissues (such as adipose tissue and the gastrointestinal tract) also secrete hormones. When hormone-producing cells cluster they form a glandular structure; see a basic description of a gland. The term endocrine refers to secretion directly into the blood from an endocrine gland, while exocrine secretions pass through ducts to a surface or cavity, as with some exocrine glands.

Receptors and mechanisms of action

Target cells respond to hormones only if they express the appropriate receptor. Receptors may be on the cell surface or located inside the cell. Binding of a hormone to a receptor protein initiates cellular processes that change the cell's activity. Surface receptors often trigger rapid signalling through second messengers, while intracellular receptors commonly influence gene transcription and thus have slower but longer-lasting effects. The biochemical and cellular processes set in motion by hormone–receptor interaction are part of broader signalling pathways.

Modes of signalling and regulation

Hormonal action is described by its range and target: endocrine (long-range via the bloodstream), paracrine (nearby cells), autocrine (acting on the secreting cell itself), and intracrine (active within the producing cell). Hormone levels are tightly regulated by feedback systems: negative feedback is common, where the effect of a hormone reduces further secretion, while positive feedback amplifies an effect in specific circumstances. Transport proteins in blood can affect hormone half-life and delivery; metabolic clearance in the liver and kidneys removes hormones from circulation.

Physiological roles and clinical importance

Hormones affect development, metabolism, reproduction, behavior and homeostasis. Examples include regulation of blood glucose by insulin, control of metabolic rate and development by thyroid hormones, and preparation for acute stress by catecholamines such as adrenaline. Disruption of hormone synthesis, secretion, transport or receptor function causes endocrine disorders; common clinical issues include diabetes mellitus, thyroid disorders and conditions of adrenal excess or deficiency. Treatments may involve hormone replacement, drugs that modify synthesis or receptor activity, and surgical approaches in some cases. Laboratory tests measure hormone concentrations or the response of target tissues to assess endocrine function.

Hormones in plants and other organisms

Plants use hormone-like substances to regulate growth, development and responses to the environment. Widely known plant hormones include auxins, gibberellins, cytokinins, abscisic acid and ethylene; these influence processes such as cell elongation, flowering and stress responses. In fungi and many invertebrates, small molecules and peptides play comparable regulatory roles. Although the molecules and mechanisms vary, the general principle of chemical signalling to coordinate multicellular function is shared across life.

Historical note and continuing research

The first hormone to be identified was secretin, discovered in 1902, and the term "hormone" was coined a few years later (1905) to describe these messenger substances. Since then, research has expanded into molecular receptor biology, synthetic analogues used therapeutically, and environmental concerns such as endocrine-disrupting chemicals. Contemporary study integrates physiology, molecular biology and clinical medicine to understand how hormonal networks maintain stability and how they can be modulated for health or agricultural benefit.

Further reading and resources

For introductory material on organs and systems that produce and respond to hormones, follow links to the endocrine system, the nervous system, and basic entries on cellular signalling and multicellular organisms. For information on glands and anatomy see the entry on gland, and for distinctions between secretion styles see endocrine glands and exocrine glands. The discovery history of secretin can be explored under secretin.