How long a hormone lasts in the body

A hormone does not stay in the blood forever. It is broken down over time, and half-life is the measure used to track that disappearance: the time it takes for half of what was there to break down. It is like ink that fades on its own; the half-life tells you how long it takes for half the mark to go.

They do not all fade at the same rate, and there is a reason for the difference. Steroid hormones do not dissolve in water, so they travel bound to a carrier protein, a sort of escort that ferries them through the blood. That package, the hormone plus its escort, is a more complicated structure, and that is what makes the half-life of steroid hormones longer than that of hormones derived from amino acids, the building blocks that proteins are assembled from.

The difference is large. Cortisol takes an hour to an hour and a half to fall by half; epinephrine, the hormone also called adrenaline, about a minute.

And there are others that are also measured in minutes, such as GnRH or intact parathyroid hormone. Neither one stays in the blood for long.