Hormones are chemical messengers secreted from the ductless endocrine glands directly into the blood stream. Hormones have distinct structures that can sometimes be synthetically replicated to provide hormone replacement therapy in people who have certain types of hormone deficiency. The main classes of hormones include:

Monoamines

The monoamines are hormones derived from aromatic amino acids such as phenylaline, tyrosine and tryptophan and are involved in neurotransmission. Examples include catecholamines such as adrenaline, noradrenaline and dopamine and the tryptamines serotonin and melatonin.

Peptide hormones

The peptide hormones are proteins composed of amino acid chains. One examples of a small peptide is thyrotropin releasing hormone (TRH) secreted by the hypothalamus to stimulate the release of thyroid stimulating hormone (TSH) by the anterior pituitary gland. In turn, TSH stimulates the thyroid gland to secrete thyroxine and prolactin.

Another example is vasopressin, secreted by the posterior pituitary gland. This regulates the salt and water balance and therefore level of water retention in the body, among other functions. Longer peptide chains include insulin and growth hormone.

Insulin is secreted by the pancreatic beta cells and regulates glucose metabolism. Growth hormone regulates growth and development.

Peptide hormones that have a covalently attached carbohydrate moiety are called glycoproteins. Examples of glycoproteins include luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH play major roles in the regulation of the reproductive systems in both males and females.

Lipid derived and phospholipid hormones

These are hormones derived from fats or lipids such as linoleic acid, arachidonic acid and the phospholipids. The main class of lipid-derived hormone is the steroids which are derived from cholesterol and the eicosanoids. Steroid hormones include testosterone, estrogen and cortisol. Testosterone and estrogen are important regulators of reproductive function, secreted by both the testes and the ovaries.

Cortisol is a hormone released by the adrenal gland in response to stress or lowered levels of blood glucocorticoids and is involved in stimulating glucose synthesis and anti-stress and anti-inflammatory processes.