corticosteroid-binding globulin

Corticosteroid-binding globulin, also called transcortin, is the main carrier protein for cortisol in the blood. It clings to most of the circulating cortisol, leaving only a small free fraction able to enter cells and act. In effect it is cortisol's dedicated taxi service through the watery plasma.

Like other carriers it is made in the liver and binds reversibly. CBG has a clever feature: it has limited capacity and binds cortisol tightly only up to a point. When cortisol surges — as during severe stress — CBG becomes saturated and the excess spills over into the free, active pool, so the free fraction can rise sharply just when the body needs cortisol most.

CBG levels themselves shift with circumstances: estrogen and pregnancy raise them, while severe illness and inflammation can lower them. As with all carrier proteins, this means total blood cortisol can be misleading, and the free fraction is what truly determines the hormone's effect on tissues.