text: Alternative names for anti-diuretic hormone
  Vasopressin; arginine vasopressin; AVP; ADH
  What is anti-diuretic hormone?
  Anti-diuretic hormone is made by special nerve cells found in an area at the base of the brain known as the hypothalamus. The nerve cells transport the hormone down their nerve fibres (axons) to the posterior pituitary gland where the hormone 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 role is to conserve the fluid volume of your body by reducing the amount of water passed out in the urine. It does this by allowing water in the urine to be taken back into the body in a specific area of the kidney. 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) and this increases blood pressure. A deficiency of body fluid (dehydration) can only be finally restored by increasing water intake.
  How is anti-diuretic hormone controlled?
  The release of anti-diuretic hormone from the pituitary gland into the bloodstream is controlled by a number of factors. A decrease in blood volume or low blood pressure, which occurs during dehydration or a haemorrhage, is detected by sensors (baroreceptors) in the heart and large blood vessels. These stimulate 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 nerve cells in the hypothalamus (osmoreceptors) which simulate anti-diuretic hormone release from the pituitary. . 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 to stop the blood becoming too dilute. 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 the production of large amounts to pale urine which can lead to rapid dehydration if untreated.