Antidiuretic hormone (ADH) is a chemical messenger that assists the kidneys in regulating the amount of water in the body. The ADH test measures the quantity of ADH present in the blood. This test is usually carried out in consonance with other tests all for the aim of detecting the cause of its increased or decreased levels in the blood.
What is an antidiuretic hormone?
Antidiuretic hormone otherwise called arginine vasopressin is a chemical messenger secreted by the posterior hypothalamus in the brain and stored in the posterior pituitary gland. It is responsible for sending signals to the kidney on water conservation.
It continuously regulates and maintains the water quantity in the blood. A high water concentration elevates the blood volume and subsequently blood pressure. ADH works with osmotic sensors and baroreceptors to regulate water metabolism. These osmotic sensors are resident in the hypothalamus and respond to particulate concentrations within the blood. The particles include sodium molecules, potassium molecules, chloride molecules, and carbon dioxide. When the concentration of these particles isn’t balanced or the blood’s pressure is low, the osmotic sensors and baroreceptors send signals to the kidneys to either store water or release water thus maintaining a homeostatic range of these substances. They also regulate the thirst process.
The essence of the antidiuretic hormone test
Antidiuretic hormone level is measured in pictogram (pg.) and the normal range is between 1-5picrograms per millimeter (pg/ml). There may be a variation even for this normal range based on the laboratory the test is performed. Abnormal levels of antidiuretic hormone either too high or too low is caused by a variety of conditions.
Antidiuretic hormone deficiency
Reduced levels of blood ADH may be due to a low blood serum osmolality or having the compulsive urge to drink too much water.
An unusual condition known as central diabetes insipidus which is a disorder of water metabolism may also cause an ADH deficiency. Central diabetes insipidus is brought about by a rise either in ADH production by the hypothalamus or an increase in ADH release from the pituitary gland.
Symptoms commonly encountered include polyuria which means an increase in the urine quantity, polydipsia which is an increased level of thirst.
People that have central diabetes insipidus often feel very tired mainly because their sleep pattern is irregular as it is frequently disrupted by the urge to urinate. Their urine is macroscopically clear, is odorless by smell and the specific gravity is low as the particulate concentration is abnormally low.
Central diabetes insipidus when it is left untreated can result in a severe form of dehydration leaving the body with inadequate water supply to function.
Antidiuretic hormone excess
When the antidiuretic hormone concentration in the blood is too high, the cause may be a
Increased ADH level is linked with the following:
- Cancer of the lung
- Brain cancer
- Systemic cancers that cause ADH production
- Guillain-Barre syndrome
- Multiple sclerosis
- Cystic fibrosis
- Acute intermittent porphyria (AIP) which is a genetic condition that disturbs the production of heme which serves as an important blood component in the body
Excess antidiuretic hormone levels can also be caused by dehydration, trauma to the brain, and even a surgical operation.
The other form of diabetes insipidus, nephrogenic diabetes insipidus may also affect the level of ADH in the blood although this rarely occurs. In this condition, ADH is sufficient in the blood but despite this, the kidney does not respond to it thereby causing the urine formed to be very dilute. The symptoms and signs bear semblance to those of central diabetes insipidus. These features include frequent and excessive urination called polyuria, followed by severe thirst called polydipsia. Doing a test for antidiuretic hormone will detect if the levels are too high or too low or normal thereby distinguishing central diabetes insipidus from nephrogenic diabetes insipidus.
Nephrogenic diabetes insipidus is not linked to the well-known diabetes mellitus which is a disorder affecting the quantity of insulin in the blood.
Procedure for the antidiuretic hormone test
Like other tests, a blood sample is needed. This is usually collected by a health care provider and venous blood is used. The following steps are taken by the healthcare provider
- Clean the site with an antiseptic to prevent infection
- Wrap a tourniquet over the individual’s arm above the area where a chosen suitable vein is located. This act enables the vein to be more visible as it is more engorged
- Gently introduce a needle syringe into the vein. Blood is collected inside the syringe tube. Remove the needle when the tube is full
- Release the tourniquet and cover the site where the needle puncture occurred with sterile gauze and put some mild pressure to stop the bleeding.
Preparing for the antidiuretic hormone blood test
- An anti-hypertensive drug Clonidine
- Haloperidol, which is a drug used in the
treatmentof psychotic and behavioral disorders in mental health
Possible risks from an ADH test
The not so common risks that are usually associated with undergoing an antidiuretic hormone test include:
- Severe bleeding
- Blood accumulation under the skin to form a mass called a
- Infection at the puncture location
Interpretation of test result
When the blood levels of ADH are unusually high, it could depict any of the following:
- Presence of a brain injury or trauma
- A brain cancer
- A brain infection
- A central nervous system infection or cancer
- A lung infection
- A lung cancer, usually the small cell type
- Fluid imbalance following surgery
- Syndrome of inappropriate ADH secretion (SIADH)
- A cerebrovascular accident/stroke
- Very rarely, nephrogenic diabetes insipidus
- Acute intermittent porphyria (AIP) which is also very rare
When the levels are also very low, it could mean that any of the following is present:
- A pituitary injury or damage
- Primary polydipsia
- Central diabetes insipidus which is also rare
What next after testing?
An isolated antidiuretic hormone test isn’t sufficient for a diagnosis to be made. Several tests are further conducted to strengthen the diagnosis of a particular condition. These additional tests include:
- Osmolality test, this is done to measure the quantity of dissolved particles in the blood. It is either a blood or a urine test.
- An electrolyte screening which is a blood test done to measure the amounts of electrolytes especially sodium and potassium that are present in the blood
- A water deprivation test: is usually done to check how often one urinates if water intake is stopped for a few hours.