carrier protein

A carrier protein is a plasma protein that acts as a chauffeur for fat-soluble hormones, ferrying them through the watery bloodstream where they could not otherwise travel. Lipophilic hormones like steroids and thyroid hormones barely dissolve in water, so without a carrier they would clump up and clear out fast.

Most carrier proteins are made in the liver. Some are highly specific — sex hormone-binding globulin grabs sex steroids, corticosteroid-binding globulin grabs cortisol, thyroxine-binding globulin grabs thyroid hormone — while albumin binds many hormones loosely as a general-purpose carrier. The bond is reversible, so a hormone is constantly hopping on and off its carrier.

Carriers do more than transport. By holding a large bound reservoir in equilibrium with the small free fraction, they buffer hormone levels, smooth out swings, extend half-life by shielding the hormone from breakdown, and act as a circulating store that can release hormone as the free portion is used up. Only the free, unbound hormone can actually enter cells and act.