bound hormone

Bound hormone is the fraction of a circulating hormone that is attached to a carrier protein. Think of it as money in a savings account rather than cash in hand: it is real and important, but it cannot be spent until it is withdrawn. While bound, the hormone cannot enter cells or trigger receptors — it is temporarily inactive.

For lipophilic hormones, the bound fraction is usually the large majority. The attachment is reversible and held in a moving equilibrium with the free fraction: as free hormone is used up, bound hormone is released to replenish it. In this way the bound pool serves as a circulating reservoir that steadies the active level minute to minute.

Binding also protects and prolongs. A carrier shields the hormone from enzymes and from filtration by the kidney, so bound hormones generally have longer half-lives. The trade-off is that anything changing carrier-protein levels — pregnancy, liver disease, certain drugs — shifts the bound total and can confuse interpretation of a blood test that measures the total rather than the free hormone.