Unit Converter
Sodium (Na)
(Major Extracellular Cation - Essential for Fluid Balance, Nerve Conduction, Acid–Base Status & Critical-Care Management)
Synonyms
- Sodium
- Na⁺
- Serum sodium
- Plasma sodium
- Extracellular sodium
Units of Measurement
- mmol/L (SI unit)
- mEq/L (equivalent unit)
Unit Conversion
For monovalent ions (Na⁺):
1 mmol/L=1 mEq/L1\ \text{mmol/L} = 1\ \text{mEq/L}1 mmol/L=1 mEq/L
Description
Sodium is the primary extracellular cation, responsible for:
- Maintaining osmotic balance
- Regulating extracellular fluid (ECF) volume
- Acid–base equilibrium
- Neuromuscular function
- Renal water handling
Serum sodium concentration reflects the balance between water and sodium rather than total body sodium.
A small change in Na⁺ may indicate dangerous fluid shifts, making it one of the most critical emergency laboratory parameters.
Physiological Role
1. Osmoregulation
Controls plasma osmolality:
Plasma Osmolality≈2×Na⁺+Glucose18+BUN2.8\text{Plasma Osmolality} ≈ 2 \times \text{Na⁺} + \frac{\text{Glucose}}{18} + \frac{\text{BUN}}{2.8}Plasma Osmolality≈2×Na⁺+18Glucose+2.8BUN
2. Fluid Homeostasis
Regulated by:
- ADH (Vasopressin)
- Aldosterone
- Renin–angiotensin system
- Natriuretic peptides
3. Nerve & Muscle Function
Necessary for:
- Action potentials
- Neuromuscular transmission
4. Acid–Base Balance
Sodium bicarbonate is the primary extracellular buffer.
Clinical Significance
HYPERNATREMIA
Usually due to water loss, not sodium gain.
Causes
- Dehydration
- Diabetes insipidus
- Osmotic diuresis
- Excessive sweating
- Severe diarrhea
- Tube feeding complications
- Hyperaldosteronism
Symptoms
- Thirst
- Confusion
- Irritability
- Seizures
- Coma
- Intracranial hemorrhage (rapid rise)
HYPONATREMIA
Most common electrolyte disorder.
Types
- Hypertonic hyponatremia (e.g., hyperglycemia)
- Hypotonic hyponatremia (true hyponatremia)
- Hypovolemic
- Euvolemic (SIADH, hypothyroidism, adrenal insufficiency)
- Hypervolemic (CHF, cirrhosis, CKD)
- Hypovolemic
Symptoms
- Headache
- Nausea
- Confusion
- Seizures
- Cerebral edema
Dangerous Correction Rule
Correct by <8−10 mmol/L per 24 hours\text{Correct by } < 8-10\ \text{mmol/L per 24 hours}Correct by <8−10 mmol/L per 24 hours
Rapid correction → osmotic demyelination syndrome (ODS).
Reference Intervals
(Tietz 8E + Mayo + ARUP)
Serum / Plasma Sodium
- 135 – 145 mmol/L
Critical Values
- <120 mmol/L → high seizure risk
- >160 mmol/L → severe dehydration, neurologic injury
Diagnostic Uses
1. Evaluation of Fluid Status
- Hypovolemia
- Hypervolemia
- Euvolemic hyponatremia (SIADH)
2. Renal Disorders
- AKI
- CKD
- Tubular dysfunction
3. Endocrine Disorders
- Adrenal insufficiency
- SIADH
- Hyperaldosteronism
4. Critical Care Monitoring
- Shock
- Sepsis
- Head injury
- Severe dehydration
- Diabetic emergencies
5. Monitoring Therapy
- IV fluids
- Diuretics
- Desmopressin
- Hypertonic saline
Analytical Notes
- Serum sodium measured by ion-selective electrode (ISE)
- Direct ISE (blood gas) unaffected by protein/lipid abnormalities
- Indirect ISE (chemistry analyzers) affected by pseudohyponatremia in:
- Hyperlipidemia
- Hyperproteinemia (e.g., multiple myeloma)
- Hyperlipidemia
Clinical Pearls
- Sodium reflects water balance, not actual sodium stores.
- SIADH = low sodium + low serum osmolality + high urine sodium.
- Hyperglycemia lowers sodium (corrected Na⁺ increases by 1.6 mmol/L per 100 mg/dL glucose rise).
- In head injury, keep sodium slightly high-normal to reduce cerebral edema.
- Normal saline (0.9%) contains 154 mmol/L Na⁺.
Interesting Fact
Despite making up only 1% of total body sodium, serum sodium determines critical neurologic function because it controls extracellular osmolality.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Electrolytes
- Mayo Clinic Laboratories - Sodium
- ARUP Consult - Electrolyte Disorders
- Endocrine Society - Hyponatremia Guidelines
- ACLS / Critical Care Electrolyte Management