9th February 2024

Classification of hormones

Hormones are chemical messengers produced by cells, glands or organs into the circulation, exerting their effects elsewhere in the body.

Classification of hormones is according to their chemical composition:

• Polypeptides – E.g. the hypothalamic and pituitary hormones, growth hormones
• Monoamines (i.e., single amino acid) – E.g. catecholamines (adrenaline and noradrenaline), thyroxine, serotonin
• Steroids – E.g. aldosterone, sex hormones

Comparing and contrasting the three classifications:

Mechanism of hormone action

Hormones exert their action via receptors on cell membranes, mainly G-protein receptors. G protein-coupled receptors are transmembrane proteins with an extracellular site to which hormone binds and an intracellular site to which G protein attaches.

G proteins couple hormones to second messenger systems such as cAMP, IP3 and Ca2+-calmodulin to carry out physiological actions. G proteins are heterotrimeric, i.e., consist of 3 subunits: α, β and γ. G proteins can be bound to GTP (guanosine triphosphate) or GDP (guanosine diphosphate). 

Hormone diffuses through the cell membrane and binds to intracellular receptors. This complex of hormone and receptor then enters the nucleus, altering DNA transcription and protein synthesis.

Hormones are classified according to their mechanism of action:

Regulation of hormones

Regulation of hormone secretion can be achieved by the negative feedback mechanism or the positive feedback mechanism.