ADH

The antidiuretic hormone (ADH) test may be requested by itself or along with or following other tests to help detect, diagnose, and determine the cause of antidiuretic hormone deficiencies and excesses. However, this test is not widely used; diagnoses of these conditions are often based on clinical history and other laboratory tests, such as blood and urine osmolality as well as electrolytes.

Symptoms of ADH deficiency are often seen with one of two types of diabetes insipidus. Central diabetes insipidus is a decrease in the production of ADH by the hypothalamus or in the release of ADH from the pituitary; nephrogenic diabetes insipidus is characterised by a decrease in the kidney's response to ADH. A water deprivation test is the preferred means to diagnose diabetes insipidus and distinguish between the central and nephrogenic forms. The test involves collection of several timed urine and blood samples for osmolality analysis during strictly controlled fluid restriction and after administration of synthetic ADH (ddAVP). This procedure must be performed under close medical supervision as it can sometimes lead to severe dehydration and can pose a risk to some people with underlying diseases. If the results of the initial water deprivation test are equivocal, then the test may be repeated and additional samples obtained for ADH analysis at defined intervals. The levels of ADH obtained are used to determine if there is an appropriate ADH response to water deprivation.

Symptoms of increased levels of ADH are often seen with "Syndromes of Inappropriate ADH production" or SIADH. Testing for SIADH may include blood and urine osmolality, blood and urine sodium, and tests to exclude poor renal or adrenal function. Measurement of ADH for the diagnosis of SIADH is only required in exceptional circumstances.

An ADH test is not requested routinely. The ADH test may sometimes be required to help investigate hyponatraemia and its associated symptoms, and to identify SIADH If a diagnosis of diabetes insipidus is suspected then ADH may be requested as part of a water deprivation test

ADH test results alone are not diagnostic of a specific condition. The results are usually evaluated in conjunction with a person's medical history, physical examination, and results of other tests. Excesses and deficiencies of ADH may be temporary or persistent, acute or chronic, and may be due to an underlying disease, an infection, an inherited condition, due to a surgery or trauma, or even due to a psychological problem that leads to excess water drinking.

Increased ADH concentrations are often associated with SIADH and may be seen with a variety of cancers, including leukaemia, lymphoma, lung cancer, pancreatic, bladder, and brain cancer. Greatly increased concentrations of ADH may be seen with systemic cancers that produce ADH. Moderate increases in ADH may be seen with nervous system disorders such as Guillain-Barré syndrome, multiple sclerosis, epilepsy, acute intermittent porphyria, with pulmonary disorders such as cystic fibrosis, emphysema, and tuberculosis, and in those with HIV/AIDS. The ADH test may sometimes be requested to help investigate hyponatraemia and its associated symptoms, and to identify SIADH, but it is not generally required to diagnose or monitor any of the diseases or conditions that may cause it.

A low ADH result may be seen with central diabetes insipidus, compulsive water drinking, and with low serum osmolality. An increased ADH concentration may be seen in nephrogenic diabetes insipidus, with dehydration, trauma, and surgery.

With ADH stimulation tests, such as the water deprivation test, a doctor is looking for appropriate concentrations of ADH, osmolality, and kidney water retention responses.

• A water deprivation ADH stimulation test may be used to differentiate between the two types of diabetes insipidus.
  • Central diabetes insipidus is characterised by abnormally low production of ADH and the inability to concentrate urine that is reflected as an increase in urine osmolality after ADH administration but not an increase due to water deprivation alone.
  • Nephrogenic diabetes insipidus is the kidney's inability to respond to ADH that is reflected as no change in urine osmolality before or after ADH administration.

In general, the ability to concentrate urine decreases with age.

ADH secretion increases when a person is standing, at night, and with pain, stress and exercise. Secretion decreases with hypertension and when someone is lying down.

Many drugs can affect ADH levels. They include:

• Drugs that stimulate ADH release, such as: barbiturates, desipramine, histamine, morphine, nicotine, amitriptyline and carbamazepine.
• Drugs that promote ADH action, such as: acetaminophen, metformin, tolbutamide, aspirin, theophylline, and non-steroidal anti-inflammatory drugs.
• Drugs that decrease ADH or its effects, such as: ethanol, lithium, and phenytoin.

No, for most people, ADH is appropriately produced and utilised by the body to maintain water balance. The ADH test is not used as a general screening test and most people will never have one done.