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Hyponatremia (Low Level of Sodium in the Blood)

By James L. Lewis, III, MD

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.

  • At first, people become sluggish and confused, and if hyponatremia worsens, they may have muscle twitches and seizures and become progressively unresponsive.

  • The diagnosis is based on blood tests to measure the sodium level.

  • Restricting fluids and stopping use of diuretics can help, but severe hyponatremia is an emergency requiring use of drugs, intravenous fluids, or both.


Hyponatremia occurs when the body contains too little sodium for the amount of fluid it contains (see Overview of Sodium). The body may have too much, too little, or about a normal amount of fluid. In all cases, however, sodium is diluted. For example, people with severe vomiting or diarrhea lose sodium. If they replace their fluid losses with water, sodium is diluted. Disorders, such as cirrhosis and heart failure, can cause the body to retain sodium and fluid. Often the body retains more fluid than sodium, which means the sodium is diluted.


The brain is particularly sensitive to changes in the sodium level in blood. Therefore, symptoms of brain dysfunction, such as sluggishness (lethargy) and confusion, occur first. If the sodium level in blood falls quickly, symptoms tend to develop rapidly and be more severe. Older people are more likely to have severe symptoms.

As hyponatremia becomes more severe, muscle twitching and seizures may occur. People may become unresponsive, aroused only by vigorous stimulation (stupor), and eventually cannot be aroused (coma). Death may follow.

Diagnosis and Treatment

Hyponatremia is diagnosed by measuring the sodium level in blood. Determining the cause is more complex. Doctors consider the person’s circumstances, including other disorders present and drugs taken. Blood and urine tests are done to evaluate the amount of fluid in the body, the concentration of blood, and content of urine.

Mild hyponatremia can be treated by restricting fluid intake to less than 1 quart (about 1 liter) per day. If a diuretic is the cause, it is reduced or stopped. If the cause is a disorder, it is treated. Occasionally, people are given a sodium solution intravenously, a diuretic to increase excretion of fluid, or both, usually slowly, over several days. These treatments can correct the sodium level.

Severe hyponatremia is an emergency. To treat it, doctors slowly increase the level of sodium in the blood with drugs, intravenous fluids, or sometimes both. Increasing the level too rapidly can result in severe and often permanent brain damage.

Syndrome of Inappropriate Secretion of Antidiuretic Hormone

Syndrome of inappropriate secretion of antidiuretic hormone (SIADH) develops when too much antidiuretic hormone is released by the pituitary gland, causing the body to retain fluid and lower the sodium level by dilution.

Antidiuretic hormone (also called vasopressin) helps regulate the amount of water in the body by controlling how much water is excreted by the kidneys. High levels of antidiuretic hormone decrease water excretion by the kidneys. The pituitary gland produces and releases antidiuretic hormone 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.

Pain, stress, exercise, a low blood sugar level, and certain disorders of the heart, thyroid gland, kidneys, or adrenal glands can stimulate the release of antidiuretic hormone from the pituitary gland, as can the following drugs:

  • Chlorpropamide (which lowers the blood sugar level)

  • Carbamazepine (an anticonvulsant)

  • Vincristine (a chemotherapy drug)

  • Clofibrate (which lowers cholesterol levels)

  • Antipsychotic and antidepressant drugs

  • Aspirin, ibuprofen, and many other nonprescription pain relievers (analgesics)

  • Vasopressin (synthetic antidiuretic hormone) and oxytocin (both drugs help the body conserve fluids)

Secretion of antidiuretic hormone is termed inappropriate if it occurs even though blood volume and blood pressure are normal or high, electrolyte concentrations are low, and other triggers of antidiuretic hormone release are not present. When antidiuretic hormone is released in these situations, the sodium level in blood decreases, and the body retains too much fluid.

What Causes SIADH?

Type of Disorder


Brain or nervous system

Abscesses in the brain

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

Encephalitis (inflammation of the brain)

Guillain-Barré syndrome

Head injury

Hypothalamus disorders, including tumors (rare)





Acute respiratory failure




Brain cancer

Lung cancer


Pancreatic cancer

Cancer of the small intestine




SIADH = syndrome of inappropriate secretion of antidiuretic hormone.

SIADH is common among older people and is fairly common among people who are hospitalized.

Many conditions increase the risk of developing SIADH. SIADH may result when antidiuretic hormone is produced outside the pituitary gland, as occurs in some lung and other cancers.

Symptoms of SIADH tend to be those of the low sodium level in blood (hyponatremia) that accompanies it ( Hyponatremia (Low Level of Sodium in the Blood)).

Diagnosis and Treatment

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 antidiuretic hormone (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..

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, drugs such as demeclocycline and lithium that decrease the effect of antidiuretic hormone on the kidneys or newer drugs such as conivaptan and tolvaptan that block antidiuretic hormone receptors and prevent the kidneys from responding to antidiuretic hormone may be used.

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