Hyponatraemia

Hyponatraemia occurs when the level of sodium in your blood drops too low, creating an imbalance between water and sodium in your body. This common condition can range from mild with few symptoms to severe and life-threatening, affecting your brain and other vital organs.

Table of contents

• What is hyponatraemia?
• Symptoms and warning signs
• What causes hyponatraemia?
• Who is at risk?
• How is it diagnosed?
• Treatment approaches
• Possible complications
• How to prevent hyponatraemia

What is hyponatraemia?

Hyponatraemia is a condition that happens when the level of sodium (a type of salt) in the blood is lower than the normal range. Healthcare providers generally define it as a blood sodium level below 135 milliequivalents per liter (mEq/L), though the exact cut-off may vary slightly between laboratories.^[1][2]

Sodium is an electrolyte — a mineral that carries an electrical charge and helps regulate the amount of water in and around your cells. It plays a vital role in proper muscle and nerve function and helps maintain stable blood pressure levels.^[1][6] The normal sodium level in blood typically ranges from 135 to 145 mEq/L.^[4]

In hyponatraemia, one or more factors cause the sodium in your body to become diluted. This can happen when your body contains too much water relative to the amount of sodium, or when sodium is lost in greater amounts than water. When this imbalance occurs, your body’s water levels rise and cells begin to swell. This swelling can cause many health problems, ranging from mild to life-threatening, especially in the brain.^[1][10]

Hyponatraemia is the most common electrolyte disorder encountered in clinical practice. Studies show it affects between 4% and 7% of people in outpatient settings, with rates as high as 18.8% in nursing homes and 22.2% in hospital wards for elderly patients.^[7][8]

The condition can develop suddenly (acute hyponatraemia, occurring in less than 48 hours) or gradually over time (chronic hyponatraemia, developing over 48 hours or more).^[5] Acute hyponatraemia tends to cause more severe symptoms because your body doesn’t have time to adjust to the changing sodium levels.^[10]

Symptoms and warning signs

The symptoms of hyponatraemia depend on how severe the condition is and how quickly the sodium level drops. When sodium decreases gradually, you may experience minimal or no symptoms at first. However, rapid decreases can result in severe symptoms that require immediate medical attention.^[7]

Common symptoms of mild to moderate hyponatraemia include:^[1][10]

• Nausea and vomiting
• Headache
• Confusion or difficulty focusing
• Loss of energy, drowsiness and fatigue
• Restlessness and irritability
• Muscle weakness, spasms or cramps
• Loss of appetite

Most people with hyponatraemia have few or no obvious symptoms. Often, symptoms don’t appear until the sodium level drops below 120 mEq/L, and these symptoms are usually nonspecific, making the condition difficult to recognize without laboratory testing.^[8]

Severe hyponatraemia can cause serious neurological symptoms, particularly when sodium levels drop very low (usually below 115 mEq/L). These symptoms occur because of swelling in the brain and include:^[4][10]

• Seizures
• Loss of consciousness or coma
• Confusion or altered mental state
• Difficulty walking (ataxia)
• Respiratory failure

In addition to these immediate symptoms, research has shown that even mild chronic hyponatraemia can lead to problems with balance and thinking, resulting in an increased risk of falls and bone fractures, particularly in older adults.^[11]

Seek emergency care immediately for anyone who develops serious signs and symptoms such as seizures, severe confusion, or loss of consciousness. If you know you are at risk of hyponatraemia and experience nausea, headaches, cramping or weakness, contact your healthcare professional promptly.^[1][12]

What causes hyponatraemia?

Hyponatraemia occurs when the balance between water and sodium in your body becomes disrupted. In most cases, this happens because your body contains too much water relative to sodium, effectively diluting the sodium in your blood. Less commonly, it results from losing large amounts of sodium from your body.^[10][17]

Healthcare professionals classify hyponatraemia based on the body’s fluid status into three main categories:^[2][5]

Hypovolemic hyponatraemia occurs when both total body water and sodium decrease, but sodium decreases more. This can be caused by:^[2]

• Severe vomiting or diarrhea
• Excessive sweating or burns
• Use of diuretic medications (water pills)
• Kidney diseases that cause salt loss
• Bleeding or fluid loss into body spaces

Euvolemic hyponatraemia happens when total body water increases while sodium levels stay about the same. Common causes include:^[2][5]

• Syndrome of inappropriate antidiuretic hormone secretion (SIADH) — a condition where the body retains too much water
• Hypothyroidism (underactive thyroid gland)
• Drinking too much water (primary polydipsia)
• Certain medications
• Severe hypothyroidism or adrenal gland problems

Hypervolemic hyponatraemia develops when both water and sodium increase in the body, but water increases more. This occurs in:^[2][5]

• Heart failure
• Liver cirrhosis
• Kidney failure
• Nephrotic syndrome (severe kidney disease with protein loss)

Medications are among the most common causes of hyponatraemia, particularly thiazide diuretics, which are frequently prescribed for high blood pressure. Other medications that can cause hyponatraemia include certain antidepressants, antipsychotic drugs, pain relievers like aspirin and ibuprofen, and some anti-seizure medications.^[6][8]

SIADH and thiazide diuretic use are the most common causes of hyponatraemia seen in emergency departments.^[5] In SIADH, the body inappropriately secretes vasopressin (also called antidiuretic hormone), causing the kidneys to retain too much water. This can occur with certain cancers, lung infections, brain disorders, and various other conditions.^[6]

Other less common causes include excessive consumption of beer or other alcohol (called beer potomania), exercising intensely for long periods while drinking too much water (exercise-associated hyponatraemia), and certain hormonal disorders affecting the thyroid or adrenal glands.^[5][6]

Who is at risk?

Certain people are more likely to develop hyponatraemia than others. You have a higher risk if you have any of the ongoing medical conditions that cause hyponatraemia, such as heart failure, liver disease, kidney problems, or brain disorders.^[10][17]

Older adults are particularly vulnerable to developing hyponatraemia. Age-related changes in kidney function, increased likelihood of taking medications that affect sodium levels, and higher rates of chronic illnesses all contribute to this increased risk.^[7]

People who take certain medications, especially thiazide diuretics, are at significant risk. These commonly used and generally safe medications increase sodium excretion more than water excretion, particularly in older adults who may be more prone to low sodium. Those who develop moderate or severe hyponatraemia while taking thiazides may need to discuss substituting another medication with their doctor.^[6]

Athletes who exercise for long periods, especially in hot weather, can develop hyponatraemia if they drink excessive amounts of water or low-sodium beverages without replacing lost sodium through sweating. This condition, called exercise-associated hyponatraemia, occurs during or up to 24 hours after prolonged, intense physical exercise.^[5]

People who have recently undergone surgery also face increased risk of developing hyponatraemia, as do those with mental health conditions that cause excessive water drinking.^[10][17]

Hospitalized patients are at higher risk, with studies showing that those who develop hyponatraemia during their hospital stay have worse outcomes than those who are admitted with already-low sodium levels.^[7]

How is it diagnosed?

Because symptoms of hyponatraemia can be vague or absent, laboratory testing is essential for diagnosis. Your healthcare professional will start with a medical history and physical examination, including questions about medications you take, recent illnesses, and symptoms you’ve experienced.^[12][21]

However, it’s impossible to diagnose hyponatraemia based on physical examination alone. Blood tests are needed to confirm low blood sodium levels and determine the severity of the condition.^[1][12]

Three laboratory tests are essential in evaluating hyponatraemia:^[4]

• Serum sodium level — confirms the diagnosis and determines severity
• Serum osmolality — measures the concentration of particles in your blood to help identify the type of hyponatraemia
• Urine tests — including urine osmolality and urine sodium concentration, which help determine the underlying cause

Healthcare providers also assess your volume status by examining whether you appear dehydrated, normally hydrated, or have excess fluid in your body. This assessment, combined with laboratory results, helps establish the primary underlying cause in a systematic way.^[4]

Additional tests may include measuring blood glucose levels, kidney function tests, and hormone levels to check thyroid and adrenal gland function. These help identify specific conditions causing the sodium imbalance.^[10][17]

The speed at which hyponatraemia developed is also important information. Your doctor will try to determine whether your condition is acute (developed in less than 48 hours) or chronic (developed over 48 hours or longer), as this affects treatment decisions.^[9]

Treatment approaches

Treatment of hyponatraemia focuses on addressing the underlying cause while carefully correcting the sodium imbalance. The approach depends on several factors: the severity of symptoms, how quickly the condition developed, and your body’s fluid status.^[9][12]

For mild, chronic hyponatraemia with few or no symptoms, treatment often involves conservative measures. If the condition is due to drinking too much water, your healthcare professional may recommend limiting fluid intake. If medications are the cause, adjusting or stopping the problematic medication (under medical supervision) may resolve the issue.^[12][21]

Fluid restriction is the main treatment for many cases of chronic hyponatraemia, particularly in people with normal or increased body fluid levels. This means limiting your daily water and fluid intake to allow your body to restore the proper balance between water and sodium.^[7][8]

Treatment based on fluid status includes:^[7]

• For hyponatraemia with dehydration: giving fluids containing sodium, such as normal saline through an IV
• For hyponatraemia with normal fluid levels: restricting fluid intake
• For hyponatraemia with excess fluid: using diuretics to remove excess water while treating the underlying condition (such as heart failure or liver disease)

If your healthcare professional recommends it, you should drink fluids that contain sodium, such as sports drinks, or eat salty foods. On the other hand, if advised to limit fluids, you should restrict water and beverages that are mostly water, including tea, coffee, and juice.^[15][19]

Severe, symptomatic hyponatraemia requires urgent treatment, typically in a hospital setting. For people with serious symptoms like seizures, severe confusion, or suspected brain swelling, doctors may use hypertonic saline (a concentrated salt solution given through an IV). The goal is to increase the blood sodium level by about 4 to 6 mEq/L in the first few hours to prevent brain damage and reduce symptoms.^[9][13]

Treatment guidelines recommend giving 100 to 150 milliliters of 3% saline solution as needed for severe symptomatic cases, with frequent monitoring of sodium levels after each dose.^[7][9]

For chronic hyponatraemia that persists despite fluid restriction, doctors may prescribe medications. Newer drugs called vaptans (vasopressin receptor antagonists) work by blocking the effects of vasopressin, allowing the kidneys to excrete more water while retaining sodium. These medications appear to be safe for treating severe euvolemic and hypervolemic hyponatraemia but are not recommended for routine use.^[7][14]

Loop diuretics can be useful in managing hyponatraemia in people with excess body fluid and in some cases of chronic SIADH.^[8]

A critical aspect of treatment is avoiding correction that is too rapid. Correcting sodium levels too quickly can lead to a serious complication called osmotic demyelination syndrome (also known as central pontine myelinolysis), which can cause permanent brain damage. The recommended rate of sodium correction should generally be no more than 6 to 8 mEq/L in the first 24 hours and 18 mEq/L or less in 48 hours.^[7][9]

During treatment, your healthcare team will monitor your sodium levels frequently and watch for signs of excessive fluid loss through urination. If sodium levels rise too quickly, treatment may include giving fluids to slow the correction rate.^[9]

Always take your medicines exactly as prescribed and contact your doctor if you have any problems with your medication or if you don’t improve as expected.^[15][19]

Possible complications

Hyponatraemia is associated with significant health risks and can lead to serious complications, particularly when severe or left untreated. The condition is linked to increased rates of illness, death, longer hospital stays, and higher healthcare costs.^[7][11]

Severe hyponatraemia can cause dangerous swelling in your tissues, especially in your brain. This brain swelling (cerebral edema) can lead to seizures, coma, permanent brain damage, or death if not treated promptly.^[1][10][17]

Studies show that mild hyponatraemia in outpatient settings is associated with increased mortality compared to people with normal sodium levels. In hospitalized patients, particularly those undergoing surgery or being treated for heart failure, hyponatraemia increases the risk of complications, extends hospital stays, and raises mortality rates.^[7]

The presence of hyponatraemia suggests a worse outlook for people with various serious conditions, including liver cirrhosis, pulmonary hypertension, heart attack, chronic kidney disease, hip fractures, and pulmonary embolism. However, it remains unclear whether hyponatraemia directly causes these poor outcomes or simply reflects the severity of the underlying disease.^[7]

Even mild chronic hyponatraemia that causes few obvious symptoms can have significant effects. Research has shown it is associated with problems with balance and walking, as well as thinking difficulties. These issues lead to an increased frequency of falls and resulting bone fractures, particularly in older adults.^[11]

A serious complication can also arise from treatment itself. Osmotic demyelination syndrome occurs when sodium levels are corrected too rapidly. This condition damages the protective covering (myelin) of nerve cells in the brain, particularly in an area called the pons. People at higher risk for this complication include those with very low starting sodium levels (below 105 mEq/L), significant potassium deficiency, alcohol use disorder, malnutrition, and advanced liver disease.^[9]

How to prevent hyponatraemia

While not all cases of hyponatraemia can be prevented, several measures can help reduce your risk of developing this condition.

If you take medications known to cause hyponatraemia, such as diuretics or certain antidepressants, work closely with your healthcare professional to monitor your sodium levels regularly. Never stop or adjust your medications without medical guidance.^[12][21]

Be mindful of your fluid intake, especially if you have conditions that put you at risk. While staying hydrated is important, drinking excessive amounts of water can dilute sodium levels. This is particularly important during prolonged exercise, where you should replace both fluids and electrolytes rather than drinking water alone.^[5]

If you exercise for extended periods, especially in hot weather, consider sports drinks that contain sodium and other electrolytes rather than drinking only water. Athletes should be aware of the signs of exercise-associated hyponatraemia and avoid drinking more fluid than they lose through sweating.^[5]

Manage underlying health conditions that can lead to hyponatraemia. If you have heart failure, liver disease, kidney problems, or thyroid disorders, follow your treatment plan carefully and attend regular follow-up appointments for monitoring.^[10][17]

If you have had hyponatraemia before, make sure your healthcare providers are aware of this history, especially before surgeries or when starting new medications. Get your sodium levels tested when your doctor recommends it.^[15][19]

Seek prompt medical attention for conditions that can lead to significant fluid and sodium loss, such as severe vomiting, diarrhea, or excessive sweating. When replacing lost fluids in these situations, include sources of sodium rather than drinking only water.^[18]

If you are at risk of hyponatraemia and experience symptoms such as nausea, headaches, cramping, or weakness, contact your healthcare professional promptly. Early detection and treatment can prevent the condition from becoming severe.^[1][12]