What sodium level is dangerously low?

A serum sodium below 120 mEq/L is considered dangerously low and can cause seizures, coma, and death. Severe hyponatremia (<125 mEq/L) warrants urgent evaluation and treatment. However, the danger depends on how quickly sodium fell — acute hyponatremia (developing in <48 hours) is far more dangerous than chronic hyponatremia at the same level, because the brain has not had time to adapt. Symptoms can also occur at levels of 125–130 mEq/L, especially in the elderly.

What causes low sodium levels?

The most common causes of low sodium (hyponatremia) include: SIADH (syndrome of inappropriate antidiuretic hormone), which is the most frequent cause in hospitalized patients; thiazide diuretics; heart failure; cirrhosis; adrenal insufficiency; hypothyroidism (severe); excessive water intake (psychogenic polydipsia); and medications such as SSRIs, carbamazepine, and oxcarbazepine. The underlying mechanism is usually excess water relative to sodium, rather than true sodium depletion.

What are symptoms of low sodium?

Symptoms of hyponatremia depend on severity and rate of onset. Mild hyponatremia (130–135 mEq/L) may cause subtle symptoms: fatigue, difficulty concentrating, and unsteadiness. Moderate hyponatremia (125–129 mEq/L) can cause nausea, headache, confusion, and increased fall risk. Severe hyponatremia (<125 mEq/L) may lead to vomiting, seizures, respiratory arrest, and coma. Chronic hyponatremia may be relatively asymptomatic even at low levels, but still increases fracture risk and gait instability.

What sodium level requires hospitalization?

There is no universal cutoff, but sodium below 125 mEq/L generally requires hospitalization for monitoring and treatment. Hospitalization is also indicated for any sodium level if the patient has severe symptoms (seizures, altered consciousness, respiratory distress), if the sodium dropped rapidly, or if the underlying cause requires inpatient management. Acute symptomatic hyponatremia is a medical emergency regardless of the exact number.

How do you correct low sodium?

Treatment depends on the cause, severity, and chronicity. For acute symptomatic hyponatremia: give 100–150 mL boluses of 3% hypertonic saline to raise sodium by 4–6 mEq/L in the first few hours. For chronic hyponatremia: correct slowly — no more than 8–10 mEq/L in 24 hours to avoid osmotic demyelination syndrome (ODS). Treatment of the underlying cause is essential: fluid restriction for SIADH, diuretic adjustment, hormone replacement for adrenal insufficiency. Monitoring sodium every 2–4 hours during active correction is critical.

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Sodium Levels: Normal Range, Causes & Symptoms

Complete reference for normal sodium levels, hyponatremia (low sodium), and hypernatremia (high sodium) — including causes, symptoms, severity classifications, and when to seek treatment.

Normal Sodium Range

136–145

mEq/L (mmol/L)

Mild low: 130–135

Moderate low: 125–129

Severe low: <125

High: >145

What Is Serum Sodium?

Sodium is the primary extracellular cation and the most important determinant of serum osmolality. It plays a critical role in maintaining fluid balance, nerve impulse transmission, and muscle contraction. Serum sodium concentration is tightly regulated between 136–145 mEq/L (mmol/L) through a complex interplay of antidiuretic hormone (ADH/vasopressin), the renin-angiotensin-aldosterone system (RAAS), thirst mechanisms, and renal water handling.

Importantly, serum sodium reflects the ratio of total body sodium to total body water — not the absolute amount of sodium. Hyponatremia (low sodium) is most often a disorder of water excess rather than sodium depletion, while hypernatremia (high sodium) almost always reflects a water deficit relative to sodium.

Sodium abnormalities are among the most common electrolyte disorders encountered in clinical practice. Hyponatremia is present in up to 15–30% of hospitalized patients and is independently associated with increased morbidity and mortality. Accurate diagnosis requires integrating serum sodium with volume status assessment, urine studies, and clinical context.

Normal Sodium Levels & Severity Classification

Classification	Sodium (mEq/L)	SI Units (mmol/L)	Clinical Significance
Severe hyponatremia	<125	<125	Risk of seizures, coma, cerebral edema; medical emergency
Moderate hyponatremia	125–129	125–129	Nausea, headache, confusion; warrants investigation and monitoring
Mild hyponatremia	130–135	130–135	Often asymptomatic; associated with increased fall risk in elderly
Normal	136–145	136–145	Normal range
Mild hypernatremia	146–150	146–150	Thirst; mild neurological symptoms possible
Moderate hypernatremia	151–159	151–159	Lethargy, irritability, weakness
Severe hypernatremia	≥160	≥160	Seizures, coma, cerebral hemorrhage; high mortality

Note: For sodium, conventional units (mEq/L) and SI units (mmol/L) are numerically identical. Always assess the rate of change and chronicity alongside the absolute value.

What Does a High Sodium Level Mean? (Hypernatremia)

Hypernatremia (sodium >145 mEq/L) almost always reflects a deficit of water relative to sodium. It occurs when water loss exceeds sodium loss, or when sodium intake exceeds the body's ability to excrete it. In almost all cases, it indicates either impaired thirst (altered mental status, infancy, elderly) or restricted access to water.

Common Causes of Hypernatremia

Dehydration / inadequate water intake — the most common cause, especially in hospitalized, elderly, or cognitively impaired patients who cannot access or request water
Diabetes insipidus (DI) — central DI (deficient ADH production) or nephrogenic DI (renal resistance to ADH) causes massive dilute urine output with rising serum sodium
Excessive salt intake — hypertonic saline administration, sodium bicarbonate infusions, salt ingestion (rare)
Cushing syndrome — cortisol excess has mild mineralocorticoid effect promoting sodium retention
Osmotic diuresis — uncontrolled diabetes (glucosuria), mannitol, or high urea can cause water loss exceeding sodium loss
Gastrointestinal losses — diarrhea (especially osmotic/viral) can cause hypotonic fluid losses

Symptoms of Hypernatremia

Intense thirst (if alert and thirst mechanism intact)
Lethargy, weakness, irritability
Muscle twitching and hyperreflexia
Confusion, seizures, coma (severe cases)
Intracranial hemorrhage (in severe, acute hypernatremia — brain shrinkage tears bridging veins)

What Does a Low Sodium Level Mean? (Hyponatremia)

Hyponatremia (sodium <136 mEq/L) is the most common electrolyte abnormality in hospitalized patients. It represents a relative excess of water compared to sodium. The diagnostic approach requires assessing serum osmolality, volume status, urine osmolality, and urine sodium to determine the cause.

Common Causes of Hyponatremia

SIADH (Syndrome of Inappropriate ADH) — the most frequent cause in hospitalized patients; excess ADH causes water retention with dilute sodium. Triggered by medications (SSRIs, carbamazepine), CNS disorders, pulmonary disease, pain, and nausea
Thiazide diuretics — impair the kidney's diluting ability in the distal tubule; one of the most common outpatient causes
Heart failure — reduced effective circulating volume triggers ADH release and water retention, leading to dilutional hyponatremia
Cirrhosis — splanchnic vasodilation reduces effective circulating volume, stimulating ADH and water retention
Adrenal insufficiency — cortisol deficiency causes impaired free water excretion and non-osmotic ADH release
Psychogenic polydipsia — excessive water intake (>15–20 L/day) overwhelms the kidney's diluting capacity
Hypothyroidism — severe hypothyroidism (myxedema) can cause mild hyponatremia, though this is debated as a primary cause
Beer potomania / tea-and-toast diet — extremely low solute intake limits the kidneys' ability to excrete free water

Symptoms of Hyponatremia

Symptoms depend on severity and rate of onset:

Mild (130–135 mEq/L): Often subtle — fatigue, difficulty concentrating, unsteadiness, increased fall risk in elderly
Moderate (125–129 mEq/L): Nausea, headache, confusion, muscle cramps
Severe (<125 mEq/L): Vomiting, seizures, obtundation, respiratory arrest, cerebral herniation
Chronic hyponatremia may be relatively asymptomatic even at levels of 115–120 mEq/L, because the brain adapts by extruding intracellular osmoles

Related Tests & Calculators

Sodium disorders are best evaluated alongside related tests and calculations:

Sodium Correction for Hyperglycemia — corrects measured sodium for the dilutional effect of elevated glucose (add 1.6 mEq/L for each 100 mg/dL glucose above 100)
Free Water Deficit Calculator — estimates the free water deficit in hypernatremia to guide replacement
Serum Osmolality Calculator — calculates expected osmolality to identify osmolal gaps and guide hyponatremia workup

About This Test

Methodology

Serum sodium is measured by ion-selective electrode (ISE) methods — either direct ISE (on blood gas analyzers, undiluted sample) or indirect ISE (on chemistry analyzers, after sample dilution). The indirect method can produce falsely low sodium readings (pseudohyponatremia) in patients with markedly elevated lipids or proteins, because the lipid/protein phase displaces water in the diluted sample. Direct ISE and blood gas analyzers are not affected.

🔑 Clinical Pearls

Pseudohyponatremia: Always consider this artifact when sodium is low but the patient is asymptomatic with markedly elevated triglycerides (>1,500 mg/dL) or paraproteins (myeloma). Check serum osmolality — it will be normal in pseudohyponatremia.
Overcorrection danger: Correcting chronic hyponatremia too quickly (>8–10 mEq/L in 24 hours) risks osmotic demyelination syndrome (ODS, formerly central pontine myelinolysis) — devastating and often irreversible brainstem injury. Monitor sodium every 2–4 hours during correction.
Sodium correction rate: European guidelines (2014) recommend a maximum correction of 10 mEq/L in the first 24 hours and 8 mEq/L in any subsequent 24-hour period for chronic hyponatremia. High-risk groups (alcoholism, malnutrition, hypokalemia, Na <105) should be corrected even more slowly.
Hypertonic saline: For acute symptomatic hyponatremia with seizures, give 100 mL of 3% NaCl IV bolus over 10 minutes, repeated up to 3 times if needed. The goal is a 4–6 mEq/L rise to alleviate brain edema, not full correction.
DDAVP clamp: If sodium is correcting too quickly, DDAVP (desmopressin) can be given to halt aquaresis and prevent overcorrection — an increasingly used "safety clamp" strategy.
Hyponatremia in heart failure: Hyponatremia in heart failure is a poor prognostic marker. It is typically managed with fluid restriction and neurohormonal blockade, not salt administration.

References

Tietz NW, ed. Tietz Clinical Guide to Laboratory Tests. 4th ed. WB Saunders; 2006.
Sterns RH. Disorders of plasma sodium — causes, consequences, and correction. N Engl J Med. 2015;372(1):55–65.
Spasovski G, Vanholder R, Allolio B, et al. Clinical practice guideline on diagnosis and treatment of hyponatraemia. Eur J Endocrinol. 2014;170(3):G1–G47.
Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000;342(20):1493–1499.
Verbalis JG, Goldsmith SR, Greenberg A, et al. Diagnosis, evaluation, and treatment of hyponatremia: expert panel recommendations. Am J Med. 2013;126(10 Suppl 1):S1–S42.

References last verified: February 2026