• "A chemical substance, synthesised by a specific organ or tissue and secreted into the blood stream, in which it is carried to other specific sites in the body, where it exerts its action(s)".

  • Chemically, hormones may be conveniently divided into the following classes of organic compound:

  1. proteins (eg. insulin) and peptides (eg. oxytocin)
  2. amine derivatives (eg adrenaline)
  3. amino acid derivatives (eg. thyroxine)
  4. steroids (eg. cortisol)
  • Hormones are usually transported in the blood bound to plasma proteins or to proteins specific to the particular hormone

Blood Levels of Hormones will depend on:

  • The rate of secretion, which may be divided into:
    • the rate of synthesis, dependent on chemical precursors from diet or from other cells in the body.
    • the rate of release of preformed hormone into the blood, dependent on the stimulus for release.
  • The rate of removal, which may be divided into:
    • the rate of inactivation, dependent on the rate at which the hormone is used up at its site of action or rendered inactive during passage through the liver or kidney.
    • the rate of excretion, dependent upon renal handling of either the inactivated hormone or its metabolic breakdown products (often used as a basis for "drug" testing).