Hormones are circulating peptide/proteins (pituitary hormones, insulin, glucagon), amino acid derivatives (catecholamines, TH), or steroids (cortisol, aldosterone) that induce responses on target organs typically through a soluble medium like the extracellular fluid or blood. by binding to their specific receptors (Figure 8.2).
Hormones fall into one of a number of different hormonal classes eg, steroids, monoamines, peptides, proteins, and eicosanoids.
They signal through a variety of general (eg, nuclear vs cell surface) and specific (eg, tyrosine kinase vs phosphoinositide turnover) mechanisms in target cells.