hormones

Hormones are chemical messengers produced by glands and specialized cells, released into the circulatory system to regulate physiology and behavior. They coordinate metabolism, growth, development, reproduction, and responses to stress. Hormones can act at distant sites (endocrine) as well as locally (paracrine/autocrine signaling) in some contexts.

Hormones are broadly categorized by chemical structure into peptide and protein hormones, amine hormones, and steroid hormones. Peptide/protein hormones (such as insulin, glucagon, and growth hormone) are water-soluble and typically bind to cell-surface receptors, triggering signaling cascades. Steroid hormones (such as cortisol, estrogens, and testosterone) and thyroid hormones are lipophilic; they pass through membranes and act by binding intracellular receptors that regulate gene transcription.

Target cells are defined by receptors; the hormone-receptor interaction depends on concentration, affinity, and receptor availability. Signaling can alter enzyme activity, gene expression, or ion channel function, producing diverse responses like metabolism changes, cell growth, and secretion.

Regulation: The hypothalamus and pituitary gland form major control centers, integrating signals and using negative feedback to maintain homeostasis. Circadian rhythms and stress can modulate secretion patterns.

Production and transport: Hormones are released into the bloodstream and distributed throughout the body, with many bound to carrier proteins. Degradation occurs mainly in the liver and kidneys, determining half-life and duration of action.

Clinical relevance: Disorders arise from hypo- or hypersecretion, receptor defects, or signaling pathway problems, e.g., diabetes mellitus, hypothyroidism, adrenal disorders. The study of hormones covers endocrinology, pharmacology, and physiology.

History: The term hormone was introduced in the early 20th century by Bayliss and Starling.