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Syndrome of Inappropriate Secretion of Antidiuretic Hormone

By James L. Lewis, III, MD, Attending Physician, Brookwood Baptist Health and Saint Vincent’s Ascension Health, Birmingham

The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) develops when too much antidiuretic hormone (vasopressin) is released by the pituitary gland under certain inappropriate conditions, causing the body to retain fluid and lower the blood sodium level by dilution.

Vasopressin (also called antidiuretic hormone) helps regulate the amount of water in the body by controlling how much water is excreted by the kidneys. Vasopressin decreases water excretion by the kidneys. As a result, more water is retained in the body, which dilutes the level of sodium in the body. A low level of sodium is called hyponatremia.

The pituitary gland appropriately produces and releases vasopressin when the blood volume (amount of fluid in the blood vessels) or blood pressure goes down or when levels of electrolytes (such as sodium) become too high.

Secretion of vasopressin is termed inappropriate if it occurs when

When vasopressin is released in these situations, the body retains too much fluid, and the sodium level in blood decreases.

Causes of SIADH

Many conditions increase the risk of developing SIADH. SIADH may result when vasopressin is produced outside the pituitary gland, as occurs in some lung and other cancers. SIADH is common among older people and is fairly common among people who are hospitalized.

SIADH has a long list of possible causes that typically require additional tests to uncover.

What Causes SIADH?

Type of Disorder

Examples

Brain or nervous system

Bleeding (hemorrhage) within the layers of tissue covering the brain

Encephalitis (inflammation of the brain)

Head injury

Hypothalamus disorders, including tumors (rare)

Tumors

Lung

Acute respiratory failure

Cancers

Brain cancer

Cancer of the small intestine

Other

Surgery

Undernutrition

SIADH = syndrome of inappropriate secretion of antidiuretic hormone.

Symptoms of SIADH

Symptoms of SIADH tend to be those of the low sodium level in blood (hyponatremia) that accompanies it. Symptoms include sluggishness and confusion.

Diagnosis of SIADH

  • Blood and urine tests

Doctors suspect SIADH based on a person’s circumstances and symptoms.

Blood and urine tests are done to measure the sodium and potassium levels and to determine how concentrated the blood and urine are (osmolality). Doctors also rule out other possible causes of excess vasopressin (such as pain, stress, drugs, or cancer).

Once SIADH is diagnosed, doctors try to identify the cause and address it so the sodium level slowly returns to normal.

Treatment of SIADH

  • Restriction of fluid intake

Doctors restrict fluid intake and treat the cause if possible.

Intravenous fluids including fluids containing very high concentrations of sodium (hypertonic saline) are sometimes given. Such treatments must be given carefully to avoid rapid increases in the sodium level.

If the sodium level in blood continues to decrease or does not increase despite restriction of fluid intake, doctors may prescribe drugs such as demeclocycline or lithium, which decrease the effect of vasopressin on the kidneys, or newer drugs such as conivaptan and tolvaptan, which block vasopressin receptors and prevent the kidneys from responding to vasopressin.

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