endocrines

Endocrines, or hormones, are chemical messengers produced by endocrine glands and released into the bloodstream to act on distant target cells. They coordinate many physiological processes and help maintain homeostasis, growth, and reproduction. The term endocrine is used to describe the systems that secrete these signaling molecules, in contrast to exocrine secretions released into ducts.

Most endocrine molecules fall into three broad chemical classes: peptide or protein hormones (such as insulin, growth hormone, and glucagon), steroid hormones (such as cortisol, estrogen, testosterone, and aldosterone), and amine derivatives formed from amino acids (such as epinephrine and thyroid hormones). Glands that secrete endocrine substances include the pituitary, thyroid, parathyroid, adrenal glands, pancreas, gonads, and hypothalamus, with other organs contributing hormones as well.

Mechanism: Hormones reach target tissues via the bloodstream and bind to specific receptors. Water-soluble peptide hormones bind cell-surface receptors, triggering signaling cascades. Lipid-soluble steroid hormones and thyroid hormones cross the cell membrane and interact with intracellular receptors to influence gene expression. The resulting cellular responses adjust metabolism, growth, or function of organ systems.

Regulation: Hormone levels are controlled by feedback mechanisms, often via the hypothalamic-pituitary axis. Circadian rhythms and environmental cues can modulate secretion. Hormone action is dynamic and can be amplified or inhibited by interactions with other hormones and signaling pathways.

Clinical relevance and terminology: Endocrines are distinguished from exocrines, which secrete into ducts, and from autocrine and paracrine signals that act locally without entering the bloodstream. Disorders of endocrine signaling include diabetes mellitus (insulin deficiency or resistance), thyroid disease (hypo- or hyperthyroidism), and adrenal disorders such as Cushing's syndrome or Addison's disease. The term hormone was coined in the early 20th century to describe these chemical messengers, and the broader concept of endocrines has evolved with advances in physiology and medicine.