Doctors use an ADH test to look for possible underlying causes of fluid and electrolyte imbalances.
An ADH test can help diagnose the syndrome of inappropriate diuretic hormone, or SIADH, a condition that occurs when the body makes too much ADH and retains excess water as a result. It can also help a doctor identify other conditions.
This article will explain how the test works and what the results could mean.
How the test works
The ADH test measures how much antidiuretic hormone is in the blood.
A doctor will draw a blood sample to use for the ADH test.
Before the test, they will give the individual specific instructions on how to prepare.
It may be necessary to refrain from drinking water for 4–6 hours before taking the test.
Alternatively, a doctor might want the individual to drink lots of water to see how the body responds.
Radioactive isotopes, which have specific medical uses, can cause inaccurate results. Therefore, anyone having an ADH test must not receive radioactive isotopes during the 24-hour period leading up to the test.
ADH is not a standard blood test, so many hospitals and doctors' offices may have to send the blood sample to a more extensive laboratory. As a result, it may take several days to get the results.
A doctor will typically order an ADH blood test along with a physical examination, electrolyte tests, and urine tests. They may also ask the laboratory to test for plasma osmolality and sodium levels.
What do the results mean?
High, low, or normal ADH test results can sometimes help a doctor make a diagnosis. However, they do not always indicate a medical condition.
Normal values may vary between laboratories, but a typical amount of ADH is less than 4.3 picograms/milliliter (pg/mL) for adults.
Moderately high levels of ADH can be an indication of tuberculosis.
High levels of ADH often indicate SIADH. However, they may also be due to certain types of cancer, including:
The hypothalamus produces ADH, and the pituitary gland releases it. The hypothalamus and pituitary gland are both in the brain, so a brain tumor or head injury can also cause high ADH levels.
Very high ADH levels may be dangerous because they can cause fluid imbalances that lead to seizures or cerebral edema.
A person may also have high ADH levels if they have heart failure. This can result in excess fluid building up in the body.
Moderately high levels of ADH can indicate a range of medical conditions, including:
- acute intermittent porphyria (AIP)
- cystic fibrosis (CF)
- Guillain-Barré syndrome
- multiple sclerosis (MS)
People with the following conditions may have low ADH levels:
- Diabetes insipidus: This condition causes the kidneys to release large amounts of water. This makes a person feel very thirsty, almost regardless of how much they drink.
- Excessive water intake.
- Low serum osmolality: This condition occurs when a person has very low sodium levels in their body.
Some medications can reduce the amount of ADH in the body. These include lithium, phenytoin, and ethanol.
An ADH test is not a common blood test, but it can provide doctors with key information relating to specific conditions, such as SIADH and diabetes insipidus.
If a person is concerned about costs and insurance coverage, they should contact their insurance company and doctor for more information.
The test is usually quick and requires only a small blood sample. A doctor will explain any high or low ADH levels when they receive the results.