Hormones are chemicals that control the activities of responsive tissues. Some hormones exert their effects in the vicinity of their release; others are released into the extracellular fluids of the body and affect distant tissues. Similarly, some hormones affect only specific tissues (target tissues), whereas others affect nearly all cells of the body. Chemically, hormones are classified as polypeptides, amines, and steroids.
Hormones act on responsive tissues by (1) altering the rate of synthesis and secretion of enzymes or other hormones, (2) affecting the rate of enzymatic catalysis, and (3) altering the permeability of cell membranes. Once the hormone has accomplished its function, its rate of secretion normally decreases. This is known as negative feedback. After sufficient reduction in hormonal effects, negative feedback decreases, and the hormone is again secreted.