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Anti-diuretic hormone

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Anti-diuretic hormone acts to maintain blood pressure, blood volume and tissue water content by controlling the concentration of urine made by the kidneys.

Alternative names for anti-diuretic hormone

Vasopressin; arginine vasopressin; AVP; ADH.

What is anti-diuretic hormone?

Anti-diuretic hormone is made by special nerves in the hypothalamus which deliver it to the pituitary gland from where it is released into the bloodstream.  Anti-diuretic hormone helps to control blood pressure by acting on the kidneys and the blood vessels.  Its most important function is to conserve the fluid volume of your body by reducing the amount of water passed out in the urine.  It does this by increasing the permeability of a specific region of the kidney through which urine flows.  Thus, more water returns to the bloodstream, urine concentration rises and water loss is reduced. Higher concentrations of anti-diuretic hormone cause blood vessels to constrict (become narrower), which causes an increase in blood pressure.  The deficiency of body fluid can only be finally restored by increasing water intake.

How is anti-diuretic hormone controlled?

The release of anti-diuretic hormone from the pituitary into the bloodstream is regulated by a number of factors.  A decrease in blood volume or low blood pressure, which occurs during dehydration or a haemorrhage, is detected by receptors in the heart and large blood vessels, and stimulates anti-diuretic hormone release.  Secretion of anti-diuretic hormone also occurs if the concentration of salts in the bloodstream increases, for example, as a result of not drinking enough water on a hot day.  This is detected by special sensors in the hypothalamus which simulate anti-diuretic hormone release from the pituitary.  If the concentration of salts reaches abnormally low levels, this condition is called hyponatraemia.  Anti-diuretic hormone is also released by thirst, nausea, vomiting and pain, and acts to keep up the volume of fluid in the bloodstream at times of stress or injury.  Alcohol prevents anti-diuretic hormone release which causes an increase in urine production and dehydration.

What happens if I have too much anti-diuretic hormone?

High levels of anti-diuretic hormone cause the kidneys to retain water in the body.  There is a condition called Syndrome of Inappropriate Anti-Diuretic Hormone secretion (SIADH; a type of hyponatraemia) where excess anti-diuretic hormone is released when it is not needed (see the article on hyponatraemia for more information).  With this condition, excessive water retention dilutes the blood, giving a characteristically low salt concentration.  Excessive levels of anti-diuretic hormone might be caused by drug side-effects and diseases of the lungs, chest wall, hypothalamus or pituitary.  Some tumours (particularly lung cancer), can produce anti-diuretic hormone.  

What happens if I have too little anti-diuretic hormone?

Low levels of anti-diuretic hormone will cause the kidneys to excrete too much water.  Urine volume will increase leading to dehydration and a fall in blood pressure.  Low levels of anti-diuretic hormone may indicate damage to the hypothalamus or pituitary gland, or primary polydipsia (compulsive or excessive water drinking).  In primary polydipsia, the low level of anti-diuretic hormone represents an effort by the body to get rid of excess water.  Diabetes insipidus is a condition where you either make too little anti-diuretic hormone (usually due to a tumour, trauma or inflammation of the pituitary or hypothalamus), or where the kidneys are insensitive to it.  Diabetes insipidus is associated with increased thirst and urine production.

 

Written: September 2011

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