Hormones are substances released into the bloodstream by glands or organs, which affect activity in cells at another site. Most are proteins composed of amino-acid chains of various lenghts. Others are steroids . Hormones (similar to the cytokines and other mediators ) act at low concentrations with very large responses in the body and bind with high affinity to specific receptors . These receptors are on the surface or inside the cell and the result of such binding causes an alteration in the cell's functioning.
Hormones control growth and development, reproduction and sexual characteristics as well as exercising a large influence over the body's use of energy.

Examples include the Catecholamines (such as Epinephrine and Norepinephrine) and the Corticosteroids released by the adrenal glands .