Sodium
Na· also: Sodium, Serum Sodium
Clinical Overview
Sodium is the primary extracellular cation and the main determinant of blood osmolality (tonicity). It regulates fluid balance, blood pressure, and nerve and muscle function. Sodium is tightly regulated by the kidneys under the influence of aldosterone, ADH (vasopressin), and ANP.
Why This Test Matters
Sodium disorders are the most common electrolyte abnormalities in hospitalized patients. Hyponatremia (low sodium) is the most frequent electrolyte disorder and a leading cause of hospital mortality if corrected too rapidly (causing osmotic demyelination syndrome/central pontine myelinolysis). Hypernatremia typically indicates severe dehydration or diabetes insipidus. Both extremes affect brain function.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 136 – 145 | mEq/L | — |
Also reported in: mmol/L.
Critical (Panic) Values
Critical Low: < 120 mEq/L. Critical High: > 160 mEq/L. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High Na Causes
- Dehydration (insufficient water intake, especially in elderly)
- Diabetes insipidus (central or nephrogenic — ADH deficiency or resistance)
- Excessive sweating without water replacement
- Hyperaldosteronism
- Hypertonic saline or sodium bicarbonate administration
Low Na Causes
- SIADH (syndrome of inappropriate ADH secretion) — most common in hospital
- Heart failure (dilutional hyponatremia)
- Liver cirrhosis (dilutional hyponatremia)
- Hypothyroidism and adrenal insufficiency
- Diuretics (especially thiazides)
- Excessive water intake (psychogenic polydipsia)
- Vomiting and diarrhea (sodium loss)
Signs & Symptoms to Watch For
How to Prepare for This Test
No special preparation. Drawn as part of a basic or comprehensive metabolic panel (BMP/CMP).
Factors That Can Affect Results
- Pseudohyponatremia: falsely low sodium in severe hyperlipidemia or hyperproteinemia with older analyzers (not a problem with modern ion-selective electrode methods)
- Hyperglycemia: each 100 mg/dL rise in glucose above normal lowers measured sodium by ~1.6 mEq/L (transcellular water shift)
- Sample hemolysis: high intracellular potassium released but minimal sodium effect
Related Topics
Frequently Asked Questions
Why must hyponatremia be corrected slowly?
When sodium is chronically low, brain cells adapt by expelling osmoles to prevent swelling. If sodium is corrected too rapidly (more than 8–10 mEq/L in 24 hours), brain cells rapidly lose water, causing osmotic demyelination syndrome (ODS), also called central pontine myelinolysis. ODS causes irreversible brain damage with symptoms ranging from confusion and dysarthria to locked-in syndrome. The correction rate must be carefully controlled with frequent sodium monitoring.
What is SIADH and what causes it?
SIADH (syndrome of inappropriate antidiuretic hormone secretion) is the most common cause of hyponatremia in hospitals. In SIADH, ADH (vasopressin) is secreted despite normal or low blood osmolality, causing water retention and sodium dilution. Common causes include certain medications (SSRIs, carbamazepine, cyclophosphamide), lung diseases (pneumonia, COPD, tuberculosis), brain diseases (stroke, meningitis, head injury), and malignancies (especially small cell lung cancer).