GLOSSARY

A hormone is a chemical messenger secreted into the blood in very small amounts (usually nano- to picomolar) by endocrine cells or glands; by binding to specific receptors on target cells, it changes their activity.

Chemical Classification

• Peptide and protein: water-soluble, with membrane receptors (such as insulin, growth hormone, and ADH).
• Steroid: derived from cholesterol, fat-soluble, with intracellular receptors (such as cortisol, testosterone, and oestrogen).
• Amine: derived from amino acids (tyrosine or tryptophan), such as adrenaline (membrane receptor), thyroid hormone (nuclear receptor), and melatonin.

Characteristics