Hormone: a chemical messenger released in blood to cause an effect somewhere else in the body.

Function: Hormones help in growth and development, energy, metabolism and electrolyte and fluid balance etc.

Most hormones are manufactured in the body: usually released when gland is stimulated.  Two stimulations: a) intrinsic: neural factors eg control of growth; b) extrinsic: response to external conditions eg pain, temperature, food etc.

Feedback control: to prevent excess of hormones in the body most have negative feedback loops to stop synthesis or release of hormone.

Transport: usually through systemic circulation i.e. blood.  Most hormones attach to plasma proteins.

Mechanism of action:

1. Surface membrane: linked to enzymes on the cell membrane i.e. receptors are outside the cell eg insulin

2. Cytosolic receptor: protein receptors help hormone to enter the cell and can initiate gene expression.

3. Nuclear hormones: enter the chromatin in nucleus and cause DNA synthesis eg thyroid hormones (growth hormones)

Physiology: Can be divided according to:

Chemical structure: steroids (resemble cholesterol), peptides (amino acids chains)

Function: growth hormones, digestion etc.

Clinical Use:

1. Replacement therapy: usually in cases of deficiency of hormones

2. Diagnosis: antagonists or hormones can be administered to determine hyper or hypofunction of gland

3. Treatment of excessive endocrine function: inhibiting hormone synthesis

4. Exploiting hormone benefits: eg use of glucocorticoids in the treatment of arthritis.

5. Alter normal endocrine functions: eg birth control pills

6. Non-endocrine diseases: eg treatment of cancers using steroids 

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