hormone half-life

Imagine pouring dye into a steadily flushing tank. Even if you stop pouring, the dye does not vanish at once; it fades by half over some fixed stretch of time, then halves again, and again. A hormone behaves the same way once it is in the blood: its half-life is the time it takes for its concentration to drop by half.

Hormone half-life is the time required for the concentration of a circulating hormone to fall to half its value, assuming no new secretion. It depends on how fast the hormone is broken down and cleared — by the liver, kidneys, target tissues, or blood enzymes — and on whether it is protected by binding to carrier proteins. Half-lives vary enormously: adrenaline lasts seconds, most peptide hormones minutes, and thyroid hormone days.

Half-life sets the tempo of a hormone's signal: short half-lives allow rapid, precisely timed control (useful for stress responses), while long half-lives give steady, slow-changing background regulation. A caveat: half-life shapes drug dosing too — levothyroxine, with a roughly week-long half-life, can be taken once daily, whereas a very short-lived hormone may need continuous infusion to maintain a steady level.