Sodium
Sodium is the main electrolyte controlling fluid balance — where sodium goes, water follows. Importantly, abnormal sodium reflects WATER balance problems, not salt problems. Low sodium (hyponatremia) usually means too much water (dilution) from SIADH, heart failure, cirrhosis, or diuretics. High sodium (hypernatremia) means water deficit (dehydration). Brain is exquisitely sensitive — correction must be SLOW to avoid osmotic demyelination syndrome (ODS).
Sodium is the most abundant electrolyte outside your cells and the primary driver of fluid balance in your body. Where sodium goes, water follows — this simple principle controls blood volume, blood pressure, and the water distribution between your blood, tissues, and cells. Sodium is also essential for nerve impulse transmission and muscle contraction.
Why does this matter? Sodium abnormalities primarily reflect water balance problems, not salt problems. Low sodium (hyponatremia) usually means too much water relative to sodium — it’s the most common electrolyte disorder and can cause serious neurological symptoms from brain swelling. High sodium (hypernatremia) usually means too little water — indicating dehydration that also affects brain function.
Sodium is included in every basic and comprehensive metabolic panel. Its critical role in brain function means significant abnormalities require prompt evaluation and careful correction.
Key Benefits of Testing
Sodium testing reveals fluid balance status — whether you’re dehydrated, overhydrated, or have conditions disrupting normal water regulation. Because sodium directly affects brain function, detecting abnormalities helps prevent or explain neurological symptoms.
This test also helps diagnose underlying conditions. Abnormal sodium often points to kidney problems, hormonal disorders (like SIADH or adrenal insufficiency), heart failure, or liver disease — conditions that disrupt normal fluid regulation.
What Does This Test Measure?
Sodium measures the concentration of sodium ions in your blood serum. Your lab provides results alongside their reference range.
Sodium’s Role in the Body
Fluid balance: Sodium is the main determinant of extracellular fluid volume. The body adjusts water retention and excretion to maintain sodium concentration — thirst makes you drink when sodium is high; kidneys retain or excrete water based on sodium status.
Blood pressure: By controlling fluid volume, sodium influences blood pressure. This is why high-sodium diets can raise blood pressure in salt-sensitive individuals.
Nerve and muscle function: Sodium gradients across cell membranes enable nerve impulse transmission and muscle contraction.
Sodium Concentration vs. Total Body Sodium
A critical concept: blood sodium concentration reflects the ratio of sodium to water, not total body sodium. You can have:
Low sodium with high total body sodium: In heart failure or cirrhosis, total body sodium and water are both increased, but water increases more — concentration is low.
Low sodium with low total body sodium: In true sodium depletion from vomiting, diarrhea, or diuretics.
Low sodium with normal total body sodium: In SIADH, excess water is retained while sodium is normal — diluting the concentration.
Understanding this helps determine the correct treatment.
Regulation of Sodium
ADH (antidiuretic hormone): Controls water retention by kidneys. High ADH = retain water = dilute sodium. Low ADH = excrete water = concentrate sodium.
Aldosterone: Controls sodium retention by kidneys. High aldosterone = retain sodium. Low aldosterone = lose sodium.
Thirst: Triggers water intake when sodium concentration rises.
Why This Test Matters
Protects Brain Function
The brain is exquisitely sensitive to sodium changes. Low sodium causes brain swelling (water moves into brain cells) — leading to headache, confusion, seizures, and even coma. High sodium causes brain shrinkage — also causing neurological symptoms. Proper sodium is critical for brain safety.
Evaluates Hydration Status
Sodium helps assess whether you’re dehydrated (high sodium suggests water deficit) or overhydrated (low sodium may suggest water excess). This guides fluid management in many clinical situations.
Diagnoses Underlying Conditions
Abnormal sodium prompts investigation into why fluid balance is disrupted. Common underlying causes include:
- SIADH (excess ADH causing water retention)
- Heart failure (fluid overload)
- Liver cirrhosis (fluid retention)
- Kidney disease
- Adrenal insufficiency
- Diabetes insipidus (inability to concentrate urine)
Monitors Medication Effects
Many medications affect sodium — diuretics, certain psychiatric medications, and others. Monitoring prevents dangerous abnormalities.
Guides Treatment
The rate and method of sodium correction depends on the cause and severity. Correcting too quickly can cause serious brain damage. Sodium levels guide this careful process.
What Can Affect Your Sodium?
Causes of Low Sodium (Hyponatremia)
Excess water relative to sodium:
- SIADH (syndrome of inappropriate ADH) — from medications, lung disease, brain disorders, pain, nausea
- Excessive water intake (psychogenic polydipsia, marathon runners)
- Hypothyroidism (severe)
- Adrenal insufficiency
Sodium and water loss (hypovolemic):
- Diuretics — especially thiazides
- Vomiting and diarrhea
- Excessive sweating
- Burns
- Kidney disease with salt wasting
Fluid overload states (hypervolemic):
- Heart failure — body retains sodium and water, but more water
- Liver cirrhosis with ascites
- Nephrotic syndrome
Medications:
- Thiazide diuretics (common cause)
- SSRIs and other antidepressants
- Carbamazepine
- NSAIDs
- Ecstasy (MDMA)
Pseudohyponatremia (falsely low):
- Very high lipids or proteins can artifactually lower measured sodium
Causes of High Sodium (Hypernatremia)
Water deficit:
- Inadequate water intake — elderly, infants, altered mental status
- Diabetes insipidus — can’t concentrate urine, lose excessive water
- Fever, burns, respiratory losses
Excessive water loss:
- Diarrhea (especially in children)
- Osmotic diuresis (high blood sugar, mannitol)
- Excessive sweating without water replacement
Excessive sodium intake (rare):
- Hypertonic saline administration
- Sodium bicarbonate treatment
- Salt poisoning
Testing Considerations
No fasting required. Sample should be processed promptly. Severely elevated lipids or proteins can cause measurement artifacts. Results should be interpreted with other electrolytes, kidney function, and clinical context.
When Should You Get Tested?
Neurological Symptoms
Confusion, headache, nausea, lethargy, seizures, or altered consciousness may indicate sodium abnormality affecting the brain.
Conditions Affecting Fluid Balance
Heart failure, liver disease, kidney disease, and hormonal disorders require sodium monitoring.
Taking Medications That Affect Sodium
Diuretics, certain psychiatric medications, and others warrant periodic sodium checks.
Symptoms of Dehydration
Excessive thirst, decreased urination, dizziness, or conditions causing fluid loss (vomiting, diarrhea, excessive sweating).
Symptoms of Fluid Overload
Swelling (edema), shortness of breath, or weight gain from fluid retention.
Elderly Patients
Older adults are at increased risk for both hyponatremia and hypernatremia due to impaired thirst and kidney function.
Routine Health Screening
Sodium is included in basic and comprehensive metabolic panels during routine checkups.
Understanding Your Results
Your lab provides reference ranges. Sodium interpretation requires clinical context:
Within reference range: Fluid balance appears normal. Brain function should not be affected by sodium level.
Below reference range (hyponatremia): Most common electrolyte abnormality. Severity and symptoms determine urgency. Mild chronic hyponatremia may be asymptomatic; acute or severe hyponatremia can cause seizures and coma. Must determine the cause (volume status, medications, underlying disease) to treat appropriately.
Above reference range (hypernatremia): Usually indicates water deficit (dehydration). Almost always indicates thirst mechanism impairment or inability to access water — alert patients don’t become hypernatremic because they drink. Requires careful water replacement.
Severity and Symptoms
Mild abnormalities: May be asymptomatic, especially if chronic. Still require evaluation.
Moderate abnormalities: Can cause nausea, headache, confusion, fatigue.
Severe abnormalities: Can cause seizures, coma, and death. Require urgent treatment.
The Importance of Correction Rate
Sodium must be corrected carefully:
Chronic hyponatremia corrected too fast: Can cause osmotic demyelination syndrome (ODS) — devastating brain damage.
Chronic hypernatremia corrected too fast: Can cause cerebral edema.
This is why treatment is done gradually under careful monitoring.
What to Do About Abnormal Results
For Low Sodium (Hyponatremia)
Assess symptoms: Severe symptoms (seizures, severe confusion) require urgent treatment.
Determine volume status:
- Hypovolemic (dehydrated): Replace sodium and water with normal saline
- Euvolemic (SIADH, hypothyroidism): Fluid restriction, treat underlying cause
- Hypervolemic (heart failure, cirrhosis): Fluid and sodium restriction, treat underlying condition
Review medications: Stop or change offending drugs if possible (especially thiazides).
Correct slowly: Generally no more than 8-10 mEq/L per 24 hours for chronic hyponatremia to avoid ODS.
For High Sodium (Hypernatremia)
Identify water deficit: Calculate how much free water is needed to correct sodium.
Replace water: Oral water if possible; hypotonic IV fluids if not.
Find the cause: Why isn’t the patient drinking? Is there excessive water loss? Diabetes insipidus?
Correct gradually: Generally no more than 10-12 mEq/L per 24 hours to avoid cerebral edema.
Monitor Closely
Recheck sodium frequently during correction — typically every 4-6 hours in acute settings. Adjust treatment based on response.
Related Health Conditions
SIADH (Syndrome of Inappropriate ADH)
Most Common Cause of Euvolemic Hyponatremia: Excess ADH causes water retention, diluting sodium. Caused by medications, lung disease, brain disorders, and many other conditions. Treatment is fluid restriction and addressing the underlying cause.
Heart Failure
Hypervolemic Hyponatremia: Despite fluid overload, sodium is diluted because water is retained more than sodium. Indicates more severe heart failure and poor prognosis.
Liver Cirrhosis
Fluid Retention with Diluted Sodium: Advanced liver disease causes sodium and water retention with ascites. Hyponatremia indicates advanced disease.
Diabetes Insipidus
Inability to Concentrate Urine: Either the brain doesn’t make ADH (central) or kidneys don’t respond to it (nephrogenic). Causes massive water loss and hypernatremia if water isn’t replaced.
Adrenal Insufficiency
Aldosterone and Cortisol Deficiency: Loss of aldosterone causes sodium wasting. Loss of cortisol impairs water excretion. Can cause hyponatremia.
Why Regular Testing Matters
Sodium abnormalities can develop quickly, especially in hospitalized patients or those on diuretics. Because the brain is so sensitive to sodium changes, early detection allows gradual correction before serious neurological symptoms develop. For those with heart failure, liver disease, or on relevant medications, regular monitoring helps maintain sodium in the safe range.
Related Biomarkers Often Tested Together
Potassium — Often affected by the same conditions and medications. Critical for heart function.
Chloride — Moves with sodium; together they reflect fluid status.
Bicarbonate (CO2) — Completes the electrolyte panel; reflects acid-base status.
Creatinine and BUN — Kidney function affects sodium regulation.
Glucose — High glucose can cause pseudohyponatremia and osmotic diuresis.
Serum Osmolality — Helps classify hyponatremia.
Note: Information provided in this article is for educational purposes and doesn’t replace personalized medical advice.
Frequently Asked Questions
Sodium is the main electrolyte outside your cells, controlling fluid balance, blood pressure, and nerve/muscle function. Where sodium goes, water follows — making sodium the key determinant of your body’s fluid status.
Low sodium (hyponatremia) usually means too much water relative to sodium — the sodium is diluted. This can be from excess water intake, water retention (SIADH, heart failure, cirrhosis), or sodium loss (diuretics, vomiting). It’s about the water-to-sodium ratio, not just salt intake.
High sodium (hypernatremia) almost always means water deficit — you’ve lost more water than sodium. This happens when thirst is impaired (elderly, confused patients), water isn’t available, or excessive water is lost (diabetes insipidus, diarrhea). Alert patients with access to water rarely become hypernatremic.
The brain is surrounded by skull and can’t expand. When sodium drops rapidly, water moves into brain cells causing swelling — leading to confusion, seizures, and coma. When sodium rises rapidly, water leaves brain cells causing shrinkage — also causing neurological problems.
Not typically. Blood sodium is regulated by water balance, not salt intake. Eating less salt mainly affects blood pressure in salt-sensitive people. Hyponatremia is usually caused by water excess or retention, not salt deficiency (except in extreme cases).
No fasting required.
In chronic hyponatremia, brain cells adapt by losing solutes. If sodium is corrected too quickly, water rapidly leaves brain cells, causing potentially devastating damage called osmotic demyelination syndrome. Slow correction allows brain cells to readapt safely.
For routine screening: as part of annual metabolic panel. For those on diuretics or with heart/liver/kidney disease: periodically as recommended. During acute illness or hospitalization: frequently until stable.
References
Key Sources:
- Sterns RH. Disorders of plasma sodium. N Engl J Med. 2015;372(1):55-65.
- Verbalis JG, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1-42.
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493-1499.
