Most of the body's sodium is located in blood and in the fluid around cells. Sodium helps the body keep fluids in a normal balance (see Water Balance: About Body Water). Sodium plays a key role in normal nerve and muscle function.

The body obtains sodium through food and drink and loses it primarily in sweat and urine. Healthy kidneys maintain a consistent level of sodium in the body by adjusting the amount excreted in the urine. When sodium consumption and loss are not in balance, the total amount of sodium in the body is affected.

Controlling Blood Volume: The total amount of sodium affects the amount of fluid in blood and around cells. The body continually monitors blood volume and sodium (and other electrolyte) concentrations. When either becomes too high, sensors in the heart, blood vessels, and kidneys detect the increases and stimulate the kidneys to increase sodium excretion, thus returning blood volume to normal. When blood volume or sodium concentration becomes too low, those sensors trigger mechanisms to increase blood volume. These mechanisms include the following:

• The kidneys stimulate the adrenal glands to secrete the hormone aldosterone. Aldosterone causes the kidneys to retain sodium and to excrete potassium. When sodium is retained, less urine is produced, eventually causing blood volume to increase.
• The pituitary gland secretes antidiuretic hormone. Antidiuretic hormone causes the kidneys to conserve fluid. Then blood volume increases.

Hyponatremia

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.

Causes

Hyponatremia occurs when the body contains too little sodium for the amount of fluid it contains. 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.

Causes of Hyponatremia
Addison's disease (underactive adrenal glands) Blockage of the small intestine Burns if severe Cirrhosis (formation of scar tissue in the liver) Consumption of too much water, as occurs in some psychiatric disorders Diarrhea Drugs such as barbiturates, carbamazepine, chlorpropamide, clofibrate, diuretics (most common), opioids, tolbutamide, and vincristine Heart failure Hypothyroidism Kidney disorders Pancreatitis Peritonitis (inflammation of the abdominal cavity) Syndrome of inappropriate secretion of antidiuretic hormone (SIADH) Vomiting

Symptoms

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 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 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 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 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.

PrintOpen table
What Causes SIADH? Type of Disorder Examples 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) Meningitis Strokes Tumors Lung Acute respiratory failure Pneumonia Tuberculosis Cancers Brain cancer Lung cancer Lymphoma Pancreatic cancer Cancer of the small intestine Other Surgery Undernutrition 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 (see see Electrolyte Balance: Hyponatremia).

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 determine how well the pituitary gland is functioning.

Doctors restrict fluid intake and treat the cause if possible. If the sodium level in blood continues to decrease or does not increase despite restriction of fluid intake, drugs that decrease the effect of antidiuretic hormone on the kidneys (such as demeclocycline or thiazide diuretics) may be used.

Spotlight on Aging
As people age, the body is less able to balance fluid and sodium for several reasons: Decreased thirst: As people age, they sense thirst less quickly or less intensely and thus may not drink fluids when needed. Changes in the kidneys: The kidneys may function less well. As a result, more fluid may be excreted in urine, and the kidneys may become less able to concentrate urine. Less fluid in the body: The older body contains less fluid. Only 45% of body weight is fluid in healthy older people, compared with 60% in younger people. This change means that a slight loss of fluid and sodium, as can result from a fever or even breathing rapidly, can have more serious consequences. Inability to obtain water: Some older people have physical problems that prevent them from getting something to drink when they want or need it. Others may have dementia, which may prevent them from realizing they are thirsty or from saying so. These people may have to depend on other people to provide them with water. In older people, a low sodium level in blood (hyponatremia) usually results from retaining too much water, as occurs in heart failure. Older people who are given fluids intravenously during hospitalization or before surgery are at high risk of developing hyponatremia. Using liquid nutritional supplements, which are often low in sodium, is another cause of hyponatremia in older people. A high sodium level in blood (hypernatremia), which is common among older people, usually results from dehydration, which is typically caused by loss of fluid, not consuming enough fluid, or both. Disorders that increase the risk of sodium-fluid imbalance, such as heart failure and kidney disorders, are common among older people. Taking a diuretic (which forces the kidneys to excrete more water) increases the risk of hypernatremia further—especially when the weather is hot or when older people become ill and do not drink enough water. Taking a certain type of diuretic (thiazide diuretics, such as hydrochlorothiazide), particularly if the kidneys are not functioning normally, can result in a dangerously low sodium level. Severe symptoms can develop within a few weeks after the drug is started. Symptoms of hyponatremia or hypernatremia are usually more severe in older people. For example, hyponatremia can result in delirium, causing confusion, agitation, or lethargy.

Hypernatremia

In hypernatremia, the level of sodium in blood is too high.

• Hypernatremia has many causes, but dehydration is most common, including not drinking enough fluids, diarrhea, kidney dysfunction, and diuretics.
• Mainly, people are thirsty, and they may become confused or have muscles twitches and seizures.
• Blood tests are done to measure the sodium level.
• Usually, fluids are given intravenously to slowly reduce the sodium level in the blood.

In hypernatremia, the body contains too little water for the amount of sodium. The sodium level in blood becomes abnormally high when water loss exceeds sodium loss, as typically occurs in dehydration.

Usually, hypernatremia results from dehydration (see Water Balance: Dehydration). For example, people may lose body fluids and become dehydrated from drinking too little, vomiting, diarrhea, diuretic use, or excessive sweating. People with diabetes mellitus and high blood sugar may have excessive urine volumes, causing dehydration. Diabetes insipidus (which causes excessive urine volume without high blood sugar—see Tubular and Cystic Kidney Disorders: Nephrogenic Diabetes Insipidus) and kidney disorders can also cause dehydration. Rarely adrenal gland disorders can cause hyponatremia without dehydration. Hypernatremia is most common among older people.

Hypernatremia typically causes thirst. The most serious symptoms of hypernatremia result from brain dysfunction. Severe hypernatremia can lead to confusion, muscle twitching, seizures, coma, and death.

Diagnosis and Treatment

The diagnosis is based on blood tests indicating that the sodium level is high.

Hypernatremia is treated by replacing fluids. In all but the mildest cases, dilute fluids (containing water and a small amount of sodium in carefully adjusted concentrations) are given intravenously. The sodium level in blood is reduced very slowly because reducing the level too rapidly can cause permanent brain damage.

Last full review/revision August 2008 by Larry E. Johnson, MD, PhD