hormone

A hormone is a chemical message in a bottle, dropped into the bloodstream and carried by the body's currents until it reaches a shore that knows how to read it. Only cells equipped with the right receptor — the right reading glasses — can respond, so a single hormone can travel everywhere yet act only in a few specific places.

Formally, a hormone is a molecule secreted by a gland or specialized cell into the blood, where it acts on distant target cells bearing a matching receptor. Hormones come in several chemical families: peptides and proteins (like insulin), steroids (like cortisol and estrogen), amines (like adrenaline and thyroid hormone), and lipid-derived eicosanoids. Their chemistry determines how they travel and how their receptors work.

Hormones are potent at vanishingly low concentrations — often nanomolar or picomolar — which is why the body controls their release and breakdown so tightly. A useful caveat: the strict textbook definition (secreted into blood, acting at a distance) is fuzzy at the edges, because many of the same molecules also act locally on neighbors or even on the cell that made them.