Hyponatremia is a condition in which the sodium level in the blood is abnormally low. Sodium is a critical electrolyte that helps regulate fluid balance, nerve function, and muscle contraction. When sodium levels fall, water shifts into cells, which can disrupt normal cellular function—particularly in the brain.
Hyponatremia is one of the most common electrolyte disorders encountered in clinical practice. Its severity ranges from mild and asymptomatic to life-threatening, depending on how low the sodium level is and how quickly it falls.
What Is Considered Low Sodium?
In adults, normal blood sodium levels typically range from 135 to 145 mmol/L.
Hyponatremia is generally defined as:
- Mild: 130–134 mmol/L
- Moderate: 125–129 mmol/L
- Severe: <125 mmol/L
Clinical significance depends not only on the number, but also on the rate of decline and the patient’s underlying health.
Why Sodium Balance Matters
Sodium plays a central role in maintaining the balance between water inside and outside cells. When sodium levels drop, excess water enters cells, causing them to swell.
Brain cells are especially sensitive to this shift. Rapid or severe hyponatremia can lead to brain swelling, resulting in confusion, seizures, coma, or death if not treated promptly.
Causes of Hyponatremia
Hyponatremia does not result from sodium deficiency alone—it is most often caused by excess water relative to sodium. Understanding the underlying mechanism is essential for proper treatment.
Excess Water Retention (Dilutional Hyponatremia)
One of the most common mechanisms is retention of free water, which dilutes sodium concentration.
This occurs in conditions where the hormone responsible for water regulation is inappropriately elevated. These include:
- Heart failure
- Liver cirrhosis
- Certain kidney diseases
In these conditions, the body retains water despite already having adequate or excess fluid, leading to dilution of sodium.
Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH)
SIADH is a frequent and important cause of hyponatremia. In this condition, the body releases antidiuretic hormone even when it is not needed, causing excessive water reabsorption by the kidneys.
SIADH may be triggered by:
- Lung diseases or infections
- Certain cancers
- Brain disorders or injury
- Medications that affect hormone regulation
Sodium levels fall because water is retained, not because sodium is lost.
Medication-Related Causes
Many commonly used medications can contribute to hyponatremia.
These include:
- Diuretics, especially thiazide-type diuretics
- Antidepressants and antipsychotic medications
- Anti-seizure medications
- Certain pain medications
Medication-induced hyponatremia is particularly common in older adults and often develops gradually.
Sodium Loss Through the Gastrointestinal Tract
Loss of sodium-rich fluids can lead to hyponatremia when losses are replaced with water alone.
This may occur with:
- Prolonged vomiting
- Diarrhea
- Excessive sweating without adequate electrolyte replacement
In these situations, both sodium and water are lost, but water replacement without sodium worsens the imbalance.
Kidney and Hormonal Disorders
Kidney disease can impair the body’s ability to excrete excess water. Hormonal deficiencies, such as adrenal insufficiency or severe hypothyroidism, can also interfere with sodium and water regulation.
These causes are less common but important to identify because targeted treatment can correct the imbalance.
Excessive Water Intake
In rare cases, hyponatremia results from consuming large volumes of water in a short period, overwhelming the kidneys’ ability to excrete free water. This may occur during endurance activities or certain psychiatric conditions.
Symptoms of Hyponatremia
Symptoms depend on severity and speed of onset.
Mild hyponatremia may cause no symptoms or subtle issues such as fatigue or headache. As sodium levels fall further, symptoms may include nausea, confusion, muscle cramps, imbalance, seizures, or loss of consciousness.
Rapid-onset hyponatremia is particularly dangerous and requires urgent medical care.
How Hyponatremia Is Diagnosed
Diagnosis is made through blood tests measuring sodium concentration. Additional laboratory studies help determine fluid status, kidney function, and hormonal balance.
Identifying the underlying cause is critical, as treatment strategies differ substantially depending on the mechanism.
Treatment Options for Hyponatremia
Treatment depends on severity, symptoms, and underlying cause.
Mild cases may require only fluid restriction or medication adjustment. More severe or symptomatic cases may require intravenous therapy and close monitoring.
Importantly, sodium correction must be done carefully and gradually to avoid serious neurological complications.
Fluid Restriction
In many cases—particularly when hyponatremia is caused by excess water retention—the cornerstone of treatment is fluid restriction.
Limiting fluid intake reduces further dilution of sodium and allows levels to gradually normalize. This approach is commonly used in conditions such as SIADH, heart failure, and liver disease.
Fluid restriction may be temporary or longer-term, depending on the underlying condition.
Treating the Underlying Cause
Correcting the underlying problem is essential for sustained improvement.
This may include:
- Adjusting or discontinuing medications that contribute to hyponatremia
- Treating infections, lung disease, or neurological conditions triggering hormone imbalance
- Optimizing treatment of heart failure, kidney disease, or liver disease
- Addressing hormonal deficiencies such as adrenal or thyroid disorders
Without addressing the root cause, sodium levels often recur despite temporary correction.
Intravenous Fluid Therapy
In some patients, hyponatremia results from true sodium loss, such as with vomiting, diarrhea, or excessive diuretic use. In these cases, intravenous fluids containing sodium may be required to restore both volume and electrolyte balance.
The type and rate of fluid administration are carefully selected based on clinical assessment and laboratory findings.
Hypertonic Saline for Severe or Symptomatic Hyponatremia
Severe hyponatremia accompanied by neurological symptoms—such as confusion, seizures, or reduced consciousness—is a medical emergency.
In these situations, hypertonic saline may be administered under close monitoring to raise sodium levels in a controlled manner. The goal is rapid symptom relief, not immediate normalization of sodium.
Correction is done cautiously to avoid serious neurological complications from overly rapid sodium correction.
Medications That Promote Free Water Excretion
In selected patients with persistent dilutional hyponatremia, medications that help the kidneys excrete excess water may be considered. These treatments are typically reserved for specific situations and require specialist supervision.
They are not appropriate for all forms of hyponatremia and are used only when standard measures are insufficient.
Duration of Treatment and Monitoring
The time required for correction varies widely. Mild cases may resolve within days, while chronic or recurrent hyponatremia may require longer-term management.
Frequent monitoring of sodium levels is essential during treatment to ensure safe correction and prevent complications. Follow-up focuses on maintaining stable sodium levels and preventing recurrence.
When to Seek Medical Attention
Confusion, seizures, severe headache, vomiting, or sudden changes in mental status should prompt immediate medical evaluation, especially in individuals with known risk factors.
In Summary
Hyponatremia is a common electrolyte disorder characterized by low blood sodium levels, most often caused by excess water relative to sodium rather than true sodium depletion. Causes include fluid-retaining conditions, hormonal disturbances, medications, and inappropriate hormone secretion. Accurate diagnosis of the underlying mechanism is essential, as treatment strategies differ and rapid correction can be dangerous.
Reference: Hyponatremia
