free hormone

Free hormone is the portion of a circulating hormone that is not stuck to a carrier protein — the part that is actually free to do something. For fat-soluble hormones, only this unbound fraction can leave the blood, enter cells, and act on receptors. The bound majority is held in reserve and is biologically silent until released.

This idea, the free hormone hypothesis, explains a clinical puzzle: the total hormone measured in blood can be misleading because it includes both free and bound. What the tissues feel is the free level. For many lipophilic hormones the free fraction is tiny — often only one to two percent of the total — yet it is the only part that matters for action and for feedback.

Free and bound hormone sit in a constant equilibrium. As cells consume free hormone, more is released from carriers to replace it, keeping the active level steady. This is why doctors often order free hormone measurements — free T4 or free testosterone — when carrier-protein levels are altered by pregnancy, illness, or medication and the total value could mislead.