Conditions · Metabolism (biochemistry)
Hypernatraemia !
Blood sodium is high because water is lacking; it happens in people who cannot drink or lose a lot of water, and neurons shrink.
Updated 2026-11-01
Structures involved
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In biochemistry
In physiology
- Body fluid compartments
- Osmosis, osmolarity and tonicity
- Cell volume regulation
- The posterior pituitary: ADH and oxytocin
- Heat exhaustion and heat stroke
- The countercurrent mechanism, urea recycling and urine concentration
- ADH, thirst and water balance
- Reasoning through fluid and electrolyte imbalances
- Fluid shifts between compartments
- Urine dilution, free water and osmotic diuresis
What it is
A high serum sodium that nearly always reflects a deficit of free water relative to sodium, with hyperosmolality.
Who gets it
It mainly affects dependent older adults, infants with diarrhoea and intensive care patients; thirst protects anyone with free access to water.
Causes
- Water loss without replacement: fever, sweating, burns, osmotic diarrhoea, vomiting
- Central diabetes insipidus (no ADH: head trauma, pituitary surgery, tumours) or nephrogenic (lithium, hypercalcaemia, hypokalaemia)
- Osmotic diuresis: hyperglycaemia, mannitol, urea from high-protein feeds
- No access to water or impaired thirst: dementia, infants, intubated patients
- Sodium gain: hypertonic solutions or bicarbonate, errors in mixing infant formula
Risk factors
- Older age or dementia
- Infants with diarrhoea
- Critically ill or tube-fed patients
- Lithium treatment
- Surgery or trauma near the pituitary
Pathophysiology
When free water is lacking, plasma osmolality rises; water leaves cells by osmosis and neurons shrink, stretching and tearing bridging veins (haemorrhage) and disturbing their function: irritability, lethargy, seizures and coma. Normally hyperosmolality triggers thirst and ADH release from the posterior pituitary; if thirst, water access or ADH action fails, sodium rises. Over days neurons generate osmolytes to regain volume, so overly fast correction produces brain oedema.
Classification
- Hypovolaemic: loss of water and some sodium (diarrhoea, sweat)
- Euvolaemic: pure water loss (diabetes insipidus, fever)
- Hypervolaemic: sodium gain (hypertonic solutions)
- Acute (< 48 h) or chronic
Symptoms and signs
- Intense thirst (if preserved)
- Irritability and high-pitched cry in infants
- Lethargy, confusion
- Weakness and muscle twitching
- Seizures and coma when severe
- Polyuria with dilute urine in diabetes insipidus
Diagnosis
- Clinical
- Assess hydration, access to water, urine output and drugs.
- Tests
- Serum sodium and osmolality, urine osmolality and sodium, glucose, calcium, potassium, creatinine; water deprivation test and response to desmopressin to tell central from nephrogenic diabetes insipidus.
- Imaging
- MRI of the sella and hypothalamus if central diabetes insipidus is confirmed; head CT if neurological status worsens.
Treatment
- Medical
- Replace the free water deficit (oral, by tube or with intravenous hypotonic fluids) gradually
- Medical
- Central diabetes insipidus: desmopressin; nephrogenic: treat the cause (stop lithium, correct calcium and potassium) and thiazides or a low-solute diet
- Medical
- Hypovolaemia or shock: isotonic fluid first to restore perfusion
- Follow-up
- Frequent sodium checks so it is not lowered faster than is safe in chronic forms
Complications
- Brain haemorrhage from shrinkage
- Venous sinus thrombosis
- Brain oedema from fast correction
- Rhabdomyolysis
Prognosis
Hypernatraemia in older adults and critically ill patients is linked to high mortality, partly from the underlying illness.
Prevention
- Offering water to dependent people
- Mixing infant formula correctly
- Monitoring sodium and urine output on lithium or after pituitary surgery
Test yourself
Think of the answer, then tap the question to see it.
What does hypernatraemia nearly always reflect?
A deficit of free water relative to sodium, not an excess of body sodium.
Which mechanism protects a healthy person from hypernatraemia?
Thirst, triggered by hyperosmolality, together with ADH release.
How is central diabetes insipidus told from nephrogenic?
After water deprivation, urine concentrates with desmopressin in the central form and not in the nephrogenic form.
Why is chronic hypernatraemia corrected slowly?
Neurons have generated osmolytes; lowering sodium fast draws water in and causes brain oedema.
Related conditions
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Sodium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Sodium blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Dehydration: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Diabetes insipidus: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Nephrogenic diabetes insipidus: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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