- Hyponatremia refers to a low level of sodium in the blood.
- Hyponatremia 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.
- Symptoms are nonspecific and can include:
- Severe hyponatremia can lead to coma and can be fatal.
- Treatment of hyponatremia 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.
Dehydration & Exercise = Hyponatremia (Low Blood Sodium)
Viewer Question: My trainer is always telling me to stay hydrated. How much water should I drink when exercising? What will happen if I drink too much?
Fitness Expert's Response: The National Athletic Trainers' Association recommends the following hydration guidelines for exercise:
- Two to three hours pre-exercise: 17 to 20 fluid ounces of water or sports drink.
- Ten to 20 minutes pre-exercise: 7 to 10 ounces of water or sports drink.
- During exercise: Fluid replacement should approximate sweat and urine losses and at least maintain hydration at less than 2% body weight reduction. This generally requires 7 to 10 ounces of water or sports drink every 10 to 20 minutes.
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 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, a number of medications can lower blood sodium levels. Examples of these include diuretics, vasopressin, and the sulfonylurea drugs.
What are the symptoms of hyponatremia (low blood sodium)?
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:
Other possible symptoms include:
- muscle spasms or cramps,
- weakness, and tiredness.
Nausea and vomiting may accompany any of the symptoms.
How is hyponatremia (low blood sodium) diagnosed?
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 in order to determine the exact cause of the hyponatremia.
How is hyponatremia (low blood sodium) treated?
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 this case medications are often needed that treat the underlying cause of the hyponatremia as well as medications to manage the accompanying symptoms.
Medically Reviewed on 8/5/2016
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care
Harrison''s Principles on Internal Medicine, 14th edition, 2006.