The syndrome of inappropriate secretion of antidiuretic hormone 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.
(See also Overview of Electrolytes and Overview of Sodium's Role in the Body.)
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 in the blood 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
Blood volume is normal or high
Blood pressure is normal or high
Other appropriate reasons for vasopressin release are not present
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 adults and is fairly common among people who are hospitalized.
SIADH has a long list of possible causes that typically require additional tests to uncover.
Symptoms of SIADH
Symptoms of SIADH tend to be those related to 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 in people with hyponatremia that cannot be explained by other factors, such as pain, stress, strenuous exercise, a high blood sugar level, and certain disorders of the heart, thyroid gland, kidneys, or adrenal glands that can decrease blood volume and appropriately stimulate the release of vasopressin from the pituitary.
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, medications, 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. People with SIADH need treatment of hyponatremia for the long term.
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.
vasopressinvasopressin receptors and prevent the kidneys from responding tovasopressin.
