circulating hormone

Once a hormone enters the blood, it is like a passenger on a busy train. Some passengers ride alone and can hop off at any stop; others are escorted by a chaperone and can only get off where the chaperone lets them. A circulating hormone is the hormone in transit through the blood, traveling either freely or held by a carrier protein.

A circulating hormone is one present in the bloodstream, where it exists in two forms: free hormone (dissolved directly in plasma) and bound hormone (attached to carrier proteins such as albumin, SHBG, or thyroxine-binding globulin). The two forms exist in equilibrium. Crucially, only the free fraction can leave the blood, reach receptors, and act — the bound fraction is a circulating reserve.

Carrier proteins extend a hormone's half-life, smooth out its blood levels, and keep poorly soluble hormones dissolved, but they also complicate measurement. A caveat: a standard blood test often reports total hormone (free plus bound), yet it is the free level that matters biologically — so conditions that change carrier-protein amounts, such as pregnancy, can shift the total without truly altering hormone action.