Antidiuretic Hormone (ADH)

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 symptoms and complications 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, and the blood becomes more concentrated. This can cause excessive thirst, frequent urination, dehydration, and – if you do not drink enough water to replace what is being lost – high blood sodium.

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.

The ADH test is not widely used to diagnose these conditions. Often, a diagnosis is made on the basis of medical history and other laboratory tests, such as urine and blood osmolality and electrolytes.

ADH deficiency, also called diabetes insipidus, is caused by a lack of ADH or the kidney’s inability to respond to ADH.

• 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 large volumes of dilute urine eliminated by the kidneys.

Excessive ADH is seen in “syndrome of inappropriate antidiuretic hormone” (SIADH) when ADH is released in unregulated quantities. SIADH is caused by inappropriate production of too much ADH, resulting in water retention, low blood sodium, and decreased blood osmolality.

• The increased production is not due to the normal response to high blood osmolality or low blood volume.
• SIADH 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, excess ADH causes low blood sodium and osmolality because water is retained and blood volume is increased.