(See also Overview of Electrolytes Overview of Electrolytes More than half of a person's body weight is water. Doctors think about water in the body as being restricted to various spaces, called fluid compartments. The three main compartments are Fluid... read more and Overview of Sodium's Role in the Body Overview of Sodium's Role in the Body Sodium is one of the body's electrolytes, which are minerals that the body needs in relatively large amounts. Electrolytes carry an electric charge when dissolved in body fluids such as blood... read more .)
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 Hyponatremia (Low Level of Sodium in the Blood) In hyponatremia, the level of sodium in blood is too low. A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics... read more .
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 Hyponatremia (Low Level of Sodium in the Blood) In hyponatremia, the level of sodium in blood is too low. A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics... read more ) 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 What Causes SIADH? 
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 Treatment In hyponatremia, the level of sodium in blood is too low. A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics... read more 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.
If the sodium level in blood continues to decrease or does not increase despite restriction of fluid intake, doctors may prescribe medications such as demeclocycline or lithium, which decrease the effect of vasopressin on the kidneys, or medications such as conivaptan and tolvaptan, which block vasopressin receptors and prevent the kidneys from responding tovasopressin.
