The ADH test measures how much antidiuretic hormone (ADH) is in the blood. ADH is a hormone that helps your kidneys manage the amount of water in your body. The test is seldom used alone, but is often combined with other tests to find out what is causing too much or too little of this hormone to be present in the blood.
ADH is also called arginine vasopressin. It is a water-regulating hormone made by the hypothalamus (in the brain), and stored in the posterior pituitary gland. ADH is the chemical that tells the kidneys how much water to conserve.
ADH constantly regulates and balances the amount of water in the blood. Higher water concentration increases the volume and pressure of the blood. Osmotic sensors and baroreceptors work in tandem with ADH to maintain water metabolism.
Osmotic sensors in the hypothalamus react to the concentration of particles (molecules of sodium, potassium, chloride, and CO2) in the blood. When particle concentration is imbalanced, or blood pressure is too low, these sensors and baroreceptors signal the kidneys to store or release water to maintain a healthy range of these substances. They also regulate the body’s sense of thirst.
When ADH levels are too low or too high, it can be caused by a number of different problems.
When there is too little ADH in the blood, the cause may be compulsive water drinking, or low blood serum osmolality (particle concentration).
A rare water metabolism disorder, called central diabetes insipidus, is sometimes the cause of ADH deficiency. Central diabetes insipidus is marked by a decrease in either the production of ADH by the hypothalamus, or the release of ADH from the pituitary.
Common symptoms are: excessive urination (polyuria) followed by extreme thirst (polydipsia).
Patients with central diabetes insipidus are often extremely tired because sleep is frequently interrupted by the need to urinate. Their urine is clear, odorless, and has an abnormally low concentration of particles.
Left untreated, central diabetes insipidus can lead to severe dehydration. Your body won’t have enough water to function.
This disorder is not related to the more common diabetes mellitus, which affects the level of insulin hormone in the blood.
When there is too much ADH in the blood, syndromes of inappropriate ADH (SIADH) may be the cause. If the condition is acute (water intoxication), the patient may have symptoms of headache, nausea, vomiting, and in severe cases, coma and convulsions.
Increased ADH is associated with a variety of cancers, nervous system disorders, pulmonary diseases, and HIV/AIDS. These illnesses include:
- lung, pancreatic, bladder, and brain cancers
- systemic cancers that produce ADH
- Guillain-Barrè syndrome
- multiple sclerosis
- acute intermittent porphyria (a genetic disorder that affects heme production, an important part of blood)
- cystic fibrosis
Dehydration, brain trauma, and surgery can also cause excess ADH.
Nephrogenic diabetes insipidus is a very rare disorder that results in excessive ADH. It is a kidney malfunction that affects the kidney’s ability to respond to ADH signals to reabsorb water. With this condition, there is ample ADH in the blood, but the kidney can’t respond to it. Signs and symptoms are similar to central diabetes insipidus: excessive urination (polyuria), followed by extreme thirst (polydipsia).
Nephrogenic diabetes insipidus is not related to the more common diabetes mellitus, which affects the level of insulin hormone in the blood.
A doctor or nurse draws blood from a vein, usually on the underside of the elbow. The site is first cleaned with a germ-killing antiseptic. Then an elastic band is wrapped around the arm above the vein. This causes the vein to swell with blood.
The healthcare provider gently inserts a needle syringe into the vein. Blood collects in the syringe tube. When the tube is full, they remove the needle.
The elastic band is then removed, and the needle puncture site is covered with sterile gauze to stop bleeding.
Many medications and other substances can affect the levels of ADH in your blood. Before the test, your doctor may ask you to avoid:
- clonidine (a blood pressure medication)
- haloperidol (a drug to treat psychotic disorders and behavioral disorders)
Uncommon risks of blood tests are excessive bleeding, fainting (or feeling light headed), hematoma (blood pooling under the skin), and infection at the puncture site.
Abnormally high levels of ADH may mean:
- brain injury or trauma
- brain tumor
- brain infection
- central nervous system infection or tumor
- lung infection
- small cell carcinoma lung cancer
- fluid imbalance after surgery
- syndrome of inappropriate ADH (SIADH)
- nephrogenic diabetes insipidus (very rare)
- acute porphyria (very rare)
Abnormally low levels of ADH may mean:
- pituitary damage
- primary polydipsia
- central diabetes insipidus (rare)
The ADH test along is usually not enough to make a diagnosis. Your doctor will probably need to perform combination of tests. Some test that may be performed in conjunction with an ADH test include:
a blood or urine test that measures the concentration of dissolved particles in the blood serum and the urine
a blood test that is used to measure the amount of electrolytes, usually sodium or potassium, in the body, or to measure the pH (acidity) of the blood
Water Deprivation Test
Basically, this is testing for osmolality and electrolytes over a period of time (about six-and-a-half hours) in which you are not allowed to drink anything.