What is a Critical Number for Sodium?

October 14, 2025 •

3 min read

According to Mayo Clinic, a normal blood sodium level is between 135 and 145 millimoles per liter (mmol/L). A critical number for sodium exists at both the low and high extremes of this range, representing an immediate threat to life that requires prompt medical attention.

Quick Summary

This article explains the critical number for sodium, detailing the dangerous thresholds of hyponatremia (low sodium) and hypernatremia (high sodium). It explores the causes, symptoms, and potential complications associated with these life-threatening electrolyte imbalances.

Key Points

Normal Range: A healthy serum sodium level is typically between 135 and 145 mmol/L.
Low Critical Number: Severe hyponatremia is generally defined as a serum sodium concentration below 125 mEq/L.
High Critical Number: Severe hypernatremia is typically a serum sodium level greater than 155–160 mEq/L, though some labs use 155 mEq/L as a critical alert.
Associated Risks: Dangerously low or high sodium levels can lead to severe neurological complications, including cerebral edema (swelling) with hyponatremia and brain cell shrinkage with hypernatremia.
Causes: Imbalances can result from dehydration, excessive fluid intake, use of certain medications like diuretics, or underlying chronic illnesses affecting the heart, kidneys, or liver.
Urgent Care: Critical sodium levels are a medical emergency requiring prompt diagnosis and supervised, careful correction to prevent irreversible damage.
Target Correction: Safe correction of sodium levels is gradual; over-correction can be just as dangerous as the imbalance itself.

Understanding the Normal Range for Sodium

Sodium is a vital electrolyte that helps regulate fluid balance and supports nerve and muscle function. A healthy serum sodium level is typically between 135 and 145 mmol/L. The body uses mechanisms like thirst and hormones to maintain this balance. However, imbalances can lead to dangerously low (hyponatremia) or high (hypernatremia) sodium levels.

The Critical Low Number: Severe Hyponatremia

Severe hyponatremia is generally defined as a serum sodium level below 125 mEq/L, with some thresholds even lower for emergency action. At these levels, cells, particularly brain cells, swell due to water shifting into them, leading to cerebral edema.

Symptoms often include:

Nausea and vomiting
Headaches
Confusion
Seizures
Decreased consciousness

Causes and Risks of Low Sodium

Causes of low sodium can be varied:

Overhydration
Certain diuretics
Medical conditions like heart failure, kidney disease, liver cirrhosis, and SIADH
Severe fluid loss from vomiting or diarrhea

The Critical High Number: Severe Hypernatremia

A critically high sodium level, or severe hypernatremia, is typically above 155 or 160 mEq/L, with levels over 160 mEq/L having high mortality rates. In hypernatremia, water is drawn out of cells, causing them to shrink. Brain cell shrinkage can cause serious complications.

Symptoms can include:

Excessive thirst
Confusion
Muscle weakness
Seizures or coma

Causes and Risks of High Sodium

Hypernatremia usually results from a water deficit. It's uncommon if the thirst mechanism is working and water is available. Common causes are:

Dehydration, often in elderly or incapacitated individuals
Increased water loss from fever, severe diarrhea, or conditions like diabetes insipidus
Excessive sodium intake (rare)

Comparison of Sodium Imbalances


Feature	Hyponatremia (Low Sodium)	Hypernatremia (High Sodium)
Critical Threshold	Typically < 125 mEq/L	Typically > 155 mEq/L
Underlying Mechanism	Excess free water or sodium loss; cell swelling	Water deficit or excess sodium; cell shrinkage
Key Symptoms	Confusion, headaches, seizures, vomiting	Extreme thirst, confusion, seizures, muscle weakness
Primary Cause	Overhydration, medical conditions, diuretics	Dehydration, diabetes insipidus
Brain Impact	Cerebral edema	Brain cell shrinkage

Medical Management of Critical Sodium Levels

Treatment is tailored to the specific situation. For severe hyponatremia, hypertonic saline may be used cautiously. For hypernatremia, addressing the cause and slowly correcting water deficit is key. Rapid correction of either can be dangerous.

Conclusion: Recognizing and Responding to Critical Sodium Levels

Recognizing a critical number for sodium is vital. A critically low level is typically below 125 mEq/L, risking cerebral edema, while a critically high level above 155–160 mEq/L can cause brain cell shrinkage. A blood test is necessary for diagnosis. Seek immediate medical attention for symptoms of imbalance. Careful medical intervention is needed to correct imbalances and prevent complications. Monitoring is crucial for at-risk individuals, including the elderly and those with chronic heart, kidney, or liver conditions.

The Role of Sodium in the Body

Sodium helps regulate your body's fluid balance, nerve function, and muscle contractions.

Symptoms of High vs. Low Sodium

Low sodium can cause confusion and seizures due to cell swelling, while high sodium can lead to confusion, weakness, and seizures from cell shrinkage.

Causes of Critical Sodium Imbalance

Causes include overhydration and diuretics for low sodium, and dehydration and impaired thirst for high sodium.

Medical Intervention for Sodium Imbalance

Treatment requires medical supervision and may involve IV fluids and addressing underlying conditions.

Risks of Rapid Correction

Correcting sodium levels too quickly can cause severe neurological complications.

Who is at Risk?

Elderly individuals, athletes, and people with heart, kidney, or liver disease are particularly vulnerable.

Prevention Through Diet and Hydration

Proper hydration and managing underlying health conditions are critical for preventing sodium imbalances.

Frequently Asked Questions

The normal range for serum sodium is 135 to 145 millimoles per liter (mmol/L) or mEq/L.

A critically low sodium number, indicating severe hyponatremia, is typically a serum sodium concentration below 125 mEq/L, though some lab guidelines may use a different cutoff.

A critically high sodium number, indicating severe hypernatremia, is typically a serum sodium concentration above 155 mEq/L, with levels over 160 mEq/L considered highly dangerous.

With critically low sodium (hyponatremia), the brain's cells swell (cerebral edema), while with critically high sodium (hypernatremia), brain cells shrink. Both can lead to severe neurological symptoms, including seizures and coma.

Symptoms can include confusion, headaches, nausea, muscle cramps, and seizures. Excessive thirst is a key symptom of high sodium.

Causes can include overhydration, fluid loss from severe vomiting or diarrhea, use of certain diuretics, or chronic conditions affecting the heart, kidneys, or liver.

Rapid correction of chronic hyponatremia can lead to osmotic demyelination syndrome, while rapid correction of hypernatremia can cause cerebral edema, both of which are severe neurological conditions.