Conditions · Metabolism (biochemistry)
Hyponatraemia !
Blood sodium is low because water is in excess of sodium; if it falls fast the brain swells and seizures can occur.
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
- Water and electrolyte absorption and how diarrhea happens
- Heat exhaustion and heat stroke
- Functions of the hypothalamus
- The countercurrent mechanism, urea recycling and urine concentration
- ADH, thirst and water balance
- Sodium and extracellular volume regulation (with natriuretic peptides)
- Reasoning through fluid and electrolyte imbalances
- Where and how diuretics act
- Fluid shifts between compartments
- Urine dilution, free water and osmotic diuresis
- IV fluids: tonicity and where they go
What it is
A low serum sodium, nearly always from a relative excess of water retained under antidiuretic hormone (ADH); it is the most common electrolyte disorder in hospital.
Who gets it
It is the most common electrolyte disorder in hospital patients and older adults, especially with thiazide diuretics, heart failure, cirrhosis or after surgery.
Causes
- Hypovolaemic: diarrhoea, vomiting, thiazide diuretics, adrenal insufficiency, renal salt wasting
- Euvolaemic: SIADH, severe hypothyroidism, cortisol deficiency, primary polydipsia, very low solute intake ('beer potomania', tea-and-toast diet)
- Hypervolaemic: heart failure, cirrhosis, nephrotic syndrome, advanced kidney disease
- Drugs: thiazides, SSRI antidepressants, carbamazepine, oxcarbazepine, cyclophosphamide, MDMA
- Pseudohyponatraemia (hypertriglyceridaemia or paraproteins) and hypertonic hyponatraemia (hyperglycaemia, mannitol)
Risk factors
- Older age
- Thiazide diuretics
- After surgery and pain
- Endurance exercise with excess water intake
- Heart failure or cirrhosis
Pathophysiology
ADH opens aquaporin 2 channels in the collecting duct and retains free water. If ADH is high from a non-osmotic stimulus (hypovolaemia, low effective circulating volume, pain, nausea, drugs, tumours) or water intake exceeds the ability to excrete it, sodium is diluted and osmolality falls. Water enters neurons by osmosis: brain oedema, headache, vomiting, confusion, seizures and herniation if the fall is fast. Over days brain cells expel osmolytes (potassium, glutamate, myo-inositol) and adapt; so overly rapid correction suddenly dehydrates neurons and causes osmotic demyelination (central pontine myelinolysis).
Classification
- By time: acute (< 48 h) or chronic (≥ 48 h or unknown)
- By symptoms: mild, moderate or severe
- By osmolality: hypotonic (true), isotonic (pseudohyponatraemia), hypertonic (glucose, mannitol)
- By volume: hypovolaemic, euvolaemic, hypervolaemic
Symptoms and signs
- Nausea and malaise
- Headache, confusion, gait problems and falls in older adults
- Cramps
- Drowsiness
- Seizures and coma when severe
Diagnosis
- Clinical
- Assess extracellular volume (signs of dehydration or oedema), drugs, fluid intake and duration.
- Tests
- Serum sodium, measured serum osmolality, urine osmolality and sodium, glucose, triglycerides and protein (to exclude false values), TSH and cortisol, creatinine, uric acid.
- Imaging
- Head CT if neurological status worsens; chest X-ray or CT if an ADH-producing tumour is suspected.
Treatment
- Acute
- Severe symptomatic hyponatraemia: hypertonic saline boluses by protocol, raising sodium just enough to halt brain oedema
- Medical
- Hypovolaemic: restore volume with isotonic fluid; stop diuretics
- Medical
- Euvolaemic or hypervolaemic: water restriction, treat the cause (heart failure, cirrhosis), increase solute; vaptans in selected cases
- Follow-up
- Close monitoring of sodium and urine output so the safe daily correction limit is not exceeded and osmotic demyelination is avoided
Complications
- Brain oedema and herniation
- Seizures
- Falls and fractures in older adults
- Osmotic demyelination syndrome from rapid correction
Prognosis
Hyponatraemia, even mild, is linked to higher mortality and longer hospital stay; most cases correct when the cause is treated.
Prevention
- Checking sodium when starting thiazides or antidepressants in older adults
- Avoiding hypotonic fluids after surgery
- Drinking to thirst during prolonged exercise
Test yourself
Think of the answer, then tap the question to see it.
Why does severe acute hyponatraemia cause seizures?
Water enters neurons by osmosis and produces brain oedema before cells can adapt.
Why is chronic hyponatraemia not corrected quickly?
Neurons have already lost osmolytes; a sudden rise in sodium dehydrates them and causes osmotic demyelination.
Which hormone explains most hyponatraemia?
Antidiuretic hormone (ADH), which retains free water in the collecting duct through aquaporin 2.
What is pseudohyponatraemia?
A falsely low sodium with normal osmolality, from excess lipids or proteins in the sample with some methods.
Related conditions
- Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
- Heart failure
- Cirrhosis
- Adrenal insufficiency
- Hypothyroidism
- Acute (viral) gastroenteritis
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Low blood sodium: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Sodium Blood Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Sodium urine test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Osmolality Tests: MedlinePlus Medical Test (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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