Hyponatremia (Low Blood Sodium)

Medically Reviewed on 3/3/2023

Hyponatremia (low blood sodium) definition and facts

Illustration of blood cells
Hyponatremia is low blood sodium or salt deficiency.
  • Hyponatremia refers to a low level of sodium in the blood.
  • Hyponatremia or low sodium blood levels may result from excess fluid in the body relative to a normal amount of sodium, or it may be due to a loss of sodium and body fluid caused by chronic conditions like kidney or congestive heart failure.
  • Symptoms of low blood sodium are nonspecific, and can include:
  • Severe hyponatremia can lead to coma and can be fatal.
  • Drinking excess water while exercising strenuously can cause low sodium levels in the blood if adequate sodium is not replaced.
  • Treatment of low blood sodium involves intravenous fluid and electrolyte replacement, medications to manage the symptoms of hyponatremia, as well as any treatments for the underlying cause.

What is hyponatremia (low blood sodium)?

Hyponatremia refers to a lower-than-normal level of sodium in the blood. Sodium is essential for many body functions including the maintenance of fluid balance, regulation of blood pressure, and normal function of the nervous system. Hyponatremia has sometimes been referred to as "water intoxication," especially when it is due to the consumption of excess water, for example during strenuous exercise, without adequate replacement of sodium.

Sodium is the major positively charged ion (cation) in the fluid outside of cells of the body. The chemical notation for sodium is Na. When combined with chloride (Cl), the resulting substance is table salt (NaCl).

The normal blood sodium level is 135 - 145 milliEquivalents/liter (mEq/L), or in international units, 135 - 145 millimoles/liter (mmol/L). Results may vary slightly among different laboratories.

What are 3 types of hyponatremia?

The three most common ways to classify hyponatremia include these types:

  1. Hypovolemic hyponatremia: Occurs when the body loses sodium and water; however, the loss of water is greater, which leads to a decrease in sodium concentration and blood volume. This can result from excessive sweating, diarrhea, vomiting, or diuretics.
  2. Euvolmeic hyponatremia: Occurs from an excess of water in the body with a normal sodium concentration. This can result from conditions including the syndrome of inappropriate antidiuretic hormone secretion (SIADH) and hypothyroidism.
  3. Hypervolemic hyponatremia: Occurs due to an excess of both sodium and water in the body; however, the water retention is greater, which leads to a dilution of sodium concentration. This can result from conditions including liver cirrhosis, kidney disease, and heart failure.

What happens if your sodium level is 120?

A sodium level of 120 mmol/L (millimoles per liter) is known as a severe case of hyponatremia. This sodium level can significantly affect your body's normal functioning, leading to potentially life-threatening symptoms. These symptoms may include:

If you experience any of the symptoms above, it is important that you seek medical attention immediately.


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What are the warning signs of hyponatremia?

When sodium levels in the body are low, water tends to enter cells, causing them to swell. When this occurs in the brain, it is referred to as cerebral edema. Cerebral edema is particularly dangerous because the brain is confined in the skull without room for expansion, and the swelling can lead to brain damage as the pressure increases within the skull. Cerebral edema occurs only in severe cases of hyponatremia.

In chronic hyponatremia, in which the blood sodium levels drop gradually over time, symptoms are typically less severe than with acute hyponatremia (a sudden drop in blood sodium level).

Symptoms can be very nonspecific and can include:

  1. headache,
  2. confusion or altered mental state,
  3. seizures, and
  4. decreased consciousness, which can proceed to coma and death.

Other possible symptoms include:

  1. restlessness,
  2. muscle spasms or cramps,
  3. weakness, and
  4. tiredness.

Nausea and vomiting may accompany any of the symptoms.

What causes hyponatremia (low blood sodium)?

A low sodium level in the blood may result from excess water or fluid in the body, diluting the normal amount of sodium so that the concentration appears low. This type of hyponatremia can be the result of chronic conditions such as kidney failure (when excess fluid cannot be efficiently excreted) and congestive heart failure, in which excess fluid accumulates in the body. SIADH (syndrome of inappropriate anti-diuretic hormone) is a disease whereby the body produces too much anti-diuretic hormone (ADH), resulting in the retention of water in the body. Consuming excess water, for example during strenuous exercise, without adequate replacement of sodium, can also result in hyponatremia.

Hyponatremia can also result when sodium is lost from the body or when both sodium and fluid are lost from the body, for example, during prolonged sweating and severe vomiting or diarrhea.

Medical conditions that can sometimes be associated with hyponatremia are adrenal insufficiency, hypothyroidism, and cirrhosis of the liver.

Finally, several medications can lower blood sodium levels. Examples of these include diuretics, vasopressin, and sulfonylurea drugs.

What tests diagnose hyponatremia (low blood sodium)?

The symptoms of hyponatremia are nonspecific, so a blood test measuring the sodium level is required to confirm the diagnosis of hyponatremia. Sometimes the medical history (such as prolonged vomiting or excessive sweating) will suggest the diagnosis. In other cases, further blood tests, urine tests, and imaging studies may be needed to determine the exact cause of the hyponatremia.

What diets and treatment guidelines are used for hyponatremia (low blood sodium)?

Mild chronic hyponatremia may not require treatment other than adjustments in diet, lifestyle, or medications. For severe or acute hyponatremia, treatment typically involves the intravenous administration of fluids and electrolytes. In these situations, medications are often needed that treat the underlying cause of the hyponatremia as well as medications to manage the accompanying symptoms.

How do you fix hyponatremia?

Treatment of hyponatremia depends on the underlying cause and severity of the condition; however, here are some common ways to fix hyponatremia:

  • Fluid restriction
  • Sodium replacement
  • Medications
  • Intravenous fluids
  • Treating underlying conditions
Medically Reviewed on 3/3/2023
Jameson, JL, et al. Harrison's Principles of Internal Medicine, 20th ed. (Vol.1 & Vol.2). McGraw-Hill Education 2018.