lipophilic hormone

A lipophilic hormone is a fat-loving, water-fearing messenger. The word literally means lipid-loving. Because the cell membrane is itself made of fat, these hormones pass through it as easily as a drop of oil merges with oil, slipping directly into the cell. Steroid hormones and thyroid hormones are the main examples.

Being fat-soluble dictates how these hormones travel and act. They cannot dissolve well in watery blood, so they ride bound to carrier proteins, with only a small free fraction available. Once they reach a target, they cross the membrane unaided and bind receptors inside the cell, frequently in the nucleus, where they act as switches that turn genes on or off.

Because their action involves reading and rebuilding the cell's protein-making instructions, lipophilic hormones tend to work slowly — over hours — but produce long-lasting, far-reaching changes. This stands in clear contrast to hydrophilic hormones, which act at the surface for fast, short-lived effects.