Three major classes of molecules function as hormones in vertebrates: proteins and peptides (small polypeptides containing up to 30 amino acids); amines derived from amino acids; and steroids.

Most protein/peptide and amine hormones are water–soluble, whereas steroid hormones are not. For ex: Adrenalin is an amino acid, Glucagon is a Peptide, Insulin is a Protein, Thyroid stimulating hormone(TSH) is a Glycoprotein and Testosterone is a Steroid.

Hormones are secreted to regulate the physiological processes by chemical means, affect the enzyme systems of the body and act on target organs or cells usually away from their source. Hormones produced in one species usually show similar influence in other species.

Hormones are produced in very small quantities and are biologically very active. For example, adrenalin is active even in a concentration of 1 part in 300,000,000 parts. Chemically some hormones are peptides (proteins) which are water soluble, some are amines (derived from amino acids) again water – soluble, some are steroids lipid – soluble. Their excess (hypersecretion/ oversecretion) or deficiency (hyposecretion/undersecretion), both may lead to serious consequences. Hormones are not stored in the body and are excreted from the system.