• "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).