Hormone
A chemical messenger released by one tissue that regulates the activity of cells elsewhere in the body.
Unit
mol/L
In depth
Hormones act at extremely low concentrations by binding to specific receptors. Steroid hormones cross the membrane and act on gene expression directly, while peptide hormones bind surface receptors and act through second messenger cascades that amplify the signal enormously.
Examples in the real world
Insulin lowering blood glucose; adrenaline preparing the body for exertion; thyroxine setting metabolic rate.