What is the Anti-Diuretic Hormone
Diuresis is the process of ‘urination’, and ‘anti-diuretic’ refers to any substance that prevents this process. Naturally, our body has a process that controls the production of urine. The antidiuretic hormone (ADH), also known as vasopressin, is a hormone produced by the pituitary gland in the brain. As the name suggests, the primary function of this hormone is to control the amount of urine passed out from the body by controlling the amount of water reabsorbed by the kidneys. Water is taken in the body in form of food and drink and lost in urine, sweat, feces and breath. The anti-diuretic hormone thus maintains the optimum amount of water in the body.
Apart from this, there are certain other factors, namely physical stress, surgery, high levels of anxiety and consumption of excess alcohol, which also stimulate ADH. The secretion of ADH is stimulated by increase in plasma osmolality via specific receptors known as osmo-receptors and by a decrease in plasma volume via volume receptors.
What is the Anti-Diuretic Hormone Blood Test?
The anti-diuretic hormone blood test is used to measure the level of the anti-diuretic hormone in blood. It is used in diagnosis of conditions causing water imbalance in the body. It is also known as arginine vasopressin measurement. This is a very rarely prescribed test and in most situations, measurement of serum or urine osmolality is sufficient.
How is this Test Performed?
The anti-diuretic hormone blood test requires collection of a blood sample. The patient must be fasting for 12 hours, be adequately hydrated and limit physical activity for 10-12 hours before the test. Blood is collected from a vein either on the inside of the elbow or on the back of the hand. The area is cleaned with an antiseptic. An elastic band is tied on the upper arm of the patient. A needle is inserted into the vein and blood is collected in an airtight tube or vial. The elastic band is removed and a cotton gauze is applied over the wound. It has minimal risks like occasionally bleeding from the blood drawing site, fainting or haematoma (collection of blood under the skin).
The anti-diuretic hormone blood test is of two types:
- Water deprivation ADH stimulation test – It is used to differentiate the two types of diabetes insipidus. In central diabetes insipidus, ADH production from the brain is low and hence there is an inability to concentrate urine. In nephrogenic diabetes insipidus, the kidney is unable to respond to ADH due to damage to renal tubules..
- Water loading ADH suppression test – It is used to diagnose a specific condition known as Syndrome of inappropriate ADH secretion or SIADH. In SIADH, there is decreased blood sodium and osmolality but normal urine sodium. ADH levels are increased.
Why is the ADH Blood Test Ordered?
The anti-diuretic hormone blood test is usually ordered in the following cases:
- Diabetes insipidus - In diabetes insipidus, there is ADH deficiency and hence excretion of large quantities of dilute urine. In this case the person has excessive thirst and frequent urination.
- Syndrome of inappropriate ADH secretion (SIADH) - In SIADH, there is excess production of ADH. ADH test is done when the person has low levels of sodium in the blood and symptoms like headache, nausea, vomiting, confusion and in severe cases, coma and convulsions.
What Do the Results Mean
ADH normal values are lab specific but can range from 1-5 pg/ml.
Values above the specified range may indicate any of the following conditions:
- An infection or tumor of the CNS
- An infection or tumor of the lungs
- Post-surgical fluid imbalance
Values below the specified range may indicate :
- Diabetes insipidus
- Primary polydipsia
- Malfunction or damage to the pituitary gland
Care must be taken while diagnosing, as certain drugs are also known to affect ADH levels. Paracetamol, oestrogen, nicotine and barbiturates, narcotics and tricyclic anti-depressants, oral hypoglycemics and thiazide diuretics are known to increase ADH levels whereas alcohol, phenytoin and beta-adrenergic agonists are known to decrease ADH levels in blood.