The Test Sample
What is being tested?
Antidiuretic hormone (ADH), also called arginine vasopressin (AVP), is a hormone that helps regulate water balance in the body by controlling the amount of water the kidneys reabsorb while they are filtering wastes out of the blood. This test measures the amount of ADH in the blood.
ADH is produced by the hypothalamus in the brain and stored in the posterior pituitary gland at the base of the brain. ADH is normally released by the pituitary in response to sensors that detect an increase in blood osmolality (number of dissolved particles in the blood) or decrease in blood volume. The kidneys respond to ADH by conserving water and producing urine that is more concentrated. The retained water dilutes the blood, lowers its osmolality, and increases blood volume and pressure. If this is not sufficient to restore the water balance, then thirst is also stimulated so that the affected person will drink more water.
There are a variety of disorders, conditions, and medications that can affect either the amount of ADH released or the kidneys' response to it. ADH deficiency and excess can cause acute and chronic symptoms that, in rare cases, may become life-threatening.
If there is too little ADH or the kidneys do not respond to ADH, then too much water is lost through the kidneys, the urine produced is more dilute than normal, and the blood becomes more concentrated. This can cause excessive thirst, frequent urination, dehydration, and – if not enough water is ingested to replace what is being lost – high blood sodium (hypernatremia).
If there is too much ADH, then water is retained, blood volume increases, and the person may experience nausea, headaches, disorientation, lethargy, and low blood sodium (hyponatremia).
The ADH test is not widely used to diagnose these conditions. Often, a diagnosis is made on the basis of clinical history and other laboratory tests, such as urine and blood osmolality and electrolytes.
ADH deficiency is called diabetes insipidus. There are two types of this disorder: central and nephrogenic.
- Central diabetes insipidus is associated with a lack of ADH production by the hypothalamus or release from the pituitary and may be due to a variety of causes, including an inherited genetic defect, head trauma, a brain tumor, or due to an infection that causes encephalitis or meningitis.
- Nephrogenic diabetes insipidus originates in the kidney and is associated with a lack of response to ADH, causing an inability to concentrate urine. It may be inherited or caused by a variety of kidney diseases.
Both types of diabetes insipidus lead to the excretion of large quantities of dilute urine.
Increased levels of ADH are often seen with "syndromes of inappropriate antidiuretic hormone" (SIADH) secretion. SIADH is characterized by inappropriate (that is, not due to high blood osmolality or low blood volume) production of too much ADH, resulting in water retention, low blood sodium, and decreased blood osmolality. It may be due to a wide number of diseases and conditions that either stimulate excessive ADH production and release or that prevent its suppression. SIADH may also be seen with cancers that produce ADH or ADH-like substances independent of the hypothalamus and pituitary glands. Regardless of the cause or source, excessive ADH causes low blood sodium and osmolality because water is retained and blood volume is increased.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed for an ADH test. However, talk to your health care provider about required preparation when the ADH test is performed as part of a water deprivation ADH stimulation test or a water loading ADH suppression test.