Sodium is found mostly in the body fluids outside the cells. Sodium is an electrolyte (mineral). It is very important for maintaining blood pressure. Sodium is also needed for nerves, muscles, and other body tissues to work properly.

When the amount of sodium in fluids outside cells drops below normal, water moves into the cells. This causes the cells to swell with too much water. Brain cells are especially sensitive to swelling, and this causes many of the symptoms of low blood sodium.

With hyponatremia, the imbalance of water to sodium is caused by one of three conditions:

• Euvolemic hyponatremia -- total body water increases, but the body's sodium content stays the same
• Hypervolemic hyponatremia -- both sodium and water content in the body increase, but the water gain is greater
• Hypovolemic hyponatremia -- water and sodium are both lost from the body, but the sodium loss is greater

Low blood sodium can be caused by:

• Burns that affect a large area of the body
• Diarrhea
• Diuretic medicines (water pills), which increase urine output and loss of sodium through the urine
• Heart failure
• Kidney diseases
• Cirrhosis (advanced liver disease)
• Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
• Sweating
• Vomiting

Common symptoms include:

• Confusion, irritability, restlessness
• Convulsions
• Fatigue
• Headache
• Loss of appetite
• Muscle weakness, spasms, or cramps
• Nausea, vomiting

Your health care provider will perform a complete physical examination and ask about your symptoms. Blood and urine tests will be done.

Lab tests that can confirm and help diagnose low sodium include:

• Comprehensive metabolic panel (includes blood sodium, normal range is 135 to 145 mEq/L, or 135 to 145 mmol/L)
• Blood osmolality
• Urine osmolality
• Urine sodium (normal level is 20 mEq/L in a random urine sample, and 40 to 220 mEq per day for a 24-hour urine test)

The cause of low blood sodium must be diagnosed and treated. If cancer is the cause of the condition, then radiation, chemotherapy, or surgery to remove the tumor may correct the sodium imbalance.

Other treatments depend on the specific type of hyponatremia.

Treatments may include:

• Fluids through a vein (IV)
• Medicines to relieve symptoms
• Limiting water intake

Outcome depends on the condition that is causing the problem. Hyponatremia that occurs in less than 48 hours (acute hyponatremia), is more dangerous than hyponatremia that develops slowly over time. When blood sodium level falls slowly over days or weeks (chronic hyponatremia), the brain cells have time to adjust and swelling may be minimal.

In severe cases, low blood sodium can lead to:

• Decreased consciousness, hallucinations or coma
• Brain herniation
• Death

When your body's sodium level drops too much, it can be a life-threatening emergency. Call your provider right away if you have symptoms of this condition.

Treating the condition that is causing hyponatremia can help.

If you play sports or do other vigorous activity, drink fluids such as sports drinks that contain electrolytes to keep your body's sodium level in a healthy range.

Hyponatremia; Dilutional hyponatremia; Euvolemic hyponatremia; Hypervolemic hyponatremia; Hypovolemic hyponatremia

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Pasco J. Electrolyte disturbances. In: Cameron P, Little M, Mitra B, Deasy C, eds. Textbook of Adult Emergency Medicine. 5th ed. Philadelphia, PA: Elsevier; 2020:chap 12.2

Verbalis JG. Disorders of water balance. In: Yu ASL, Chertow GM, Luyckx VA, Marsden PA, Skorecki K, Taal MW, eds. Brenner and Rector's The Kidney. 11th ed. Philadelphia, PA: Elsevier; 2020:chap 15.

Updated by: Jacob Berman, MD, MPH, Clinical Assistant Professor of Medicine, Division of General Internal Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.