hormone

Hormones are chemical messengers produced by glands or other cells of the endocrine system. They are released into the bloodstream and travel to distant target tissues to regulate physiology and behavior. Hormone action depends on receptor presence on or in target cells, and the response is typically specific, graded, and reversible.

Most hormones are peptides or proteins, steroids derived from cholesterol, or amine derivatives. Lipid-soluble hormones such as steroids and thyroid hormones diffuse across cell membranes and bind intracellular receptors to influence gene expression. Water-soluble hormones, including peptide hormones and catecholamines, bind cell surface receptors and activate intracellular signaling cascades.

Target cells respond according to receptor number, affinity, and intracellular signaling pathways. Secretion is regulated by feedback mechanisms often involving the hypothalamus and pituitary, and can be influenced by circadian rhythms, nutrition, stress, and activity. Hormone signaling can be endocrine, paracrine, or autocrine in scope.

Major endocrine organs include the pituitary, hypothalamus, thyroid, parathyroid, adrenal glands, pancreas, ovaries, and testes, with additional hormone producers in adipose tissue, the gut, and elsewhere. Common hormones include insulin, glucagon, cortisol, adrenaline, thyroid hormones, estrogen, progesterone, testosterone, growth hormone, and prolactin.

Hormones are central to metabolism, growth and development, reproduction, fluid and electrolyte balance, immune function, and behavior. Dysregulation leads to conditions such as diabetes mellitus, hypothyroidism or hyperthyroidism, Cushing's syndrome, Addison's disease, and endocrine neoplasia. Therapies include hormone replacement, antagonists, and receptor modulators.