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Hyponatraemia
Treat the symptoms, not the number — and never correct too fast. Decide first whether the brain is in trouble (severe symptoms mean hypertonic saline now). Then slow down, assess volume status, send paired serum and urine tests, and correct at a safe pace. Built for revision, not live patient decisions.
Settle them, and check the basics
is the brain in danger?Red flags — what each should make you think
How fast it fell matters as much as how low it is. Acute hyponatraemia (under 48 h) swells the brain and needs urgent treatment. Chronic hyponatraemia has adapted, and correcting it too quickly causes osmotic demyelination — a disaster you cause yourself.
Severe symptoms (seizures, reduced GCS, vomiting, cardiorespiratory distress) or moderately severe symptoms (nausea, confusion, headache) — Society for Endocrinology 2022
- 3%Hypertonic saline bolus
- NaRecheck sodium
- ↑5Aim for a 5 mmol/L rise
- ≤10Cap the 24-hour rise
- UOWatch the urine output
- ☎Senior + critical care
Understand the patient & find the cause
volume status + paired testsStep-by-step diagnosis
- 1 · Is it true hyponatraemia? Check serum osmolality. Normal or high → hyperglycaemia, mannitol, or lab artefact (lipids, proteins).
- 2 · Urine osmolality — below 100 mOsm/kg → too much water (primary polydipsia, beer potomania, low solute intake).
- 3 · Urine sodium — below 30 mmol/L → low effective circulating volume (losses, heart failure, cirrhosis). Above 30 → renal sodium loss, diuretics, adrenal insufficiency, or SIADH.
- 4 · Volume status — dry (losses, diuretics, Addison's), euvolaemic (SIADH, hypothyroidism, adrenal insufficiency), overloaded (heart failure, cirrhosis, nephrotic syndrome, renal failure).
Send paired serum and urine osmolality and urine sodium before giving fluids if you can. Once saline is in, the urine results are hard to interpret.
Common culprit drugs
- Thiazide diuretics (the classic)
- SSRIs, carbamazepine, antipsychotics
- PPIs, opioids, MDMA
- Chemotherapy (cyclophosphamide, vincristine)
Work A–E — assess and act as you go
The diagnoses you must not miss
tap to open eachWhat points toward it, what would rule it in, and how to manage it.
Seizures, reduced GCS, vomiting, respiratory arrest — especially when sodium fell quickly (post-op, MDMA, endurance exercise, primary polydipsia).
Clinical + sodium; don't wait for the cause.
150 mL 3% saline over 20 min, repeat as needed, aiming for a 5 mmol/L rise. Senior and ITU.
Correction of chronic hyponatraemia too fast. Days later: dysarthria, dysphagia, quadriparesis, "locked-in". High risk: alcohol use, malnutrition, low K⁺, liver disease, Na under 120.
MRI, but prevention is everything.
Cap the rise at 10 mmol/L in 24 h (8 if high risk). If overshooting, involve senior/renal: stop saline, consider 5% glucose and/or desmopressin to re-lower.
Hypotension, vomiting, abdominal pain, hypoglycaemia, raised potassium, pigmentation, on long-term steroids or stopped them suddenly.
Random cortisol (don't wait for it).
IV/IM hydrocortisone 100 mg immediately, then IV fluids. Endocrine review.
Bradycardia, hypothermia, reduced GCS, dry skin, known hypothyroidism off treatment.
TFTs.
ITU, endocrine; IV levothyroxine/liothyronine under specialist guidance, plus hydrocortisone until adrenal insufficiency is excluded.
Investigate — what to order, when, and what it tells you
test with a question in mindPaired tests
Rule out the treatable
Other bloods
Monitoring
SIADH — a diagnosis of exclusion
Euvolaemic, low serum osmolality, urine osmolality above 100, urine sodium above 30, normal thyroid and adrenal function, not on diuretics. Then look for the cause: chest (pneumonia, cancer), brain (stroke, bleed, infection), drugs, pain, nausea, post-op. Treat with fluid restriction (often about 1 L/day) first.
How the plan comes together
disposition · handoverward / outpatient
Mild chronic hyponatraemia, no symptoms, cause identified (e.g. a thiazide stopped). Recheck in days.
escalate
Severe symptoms, Na below 120, rapid fall, high ODS risk, or not responding → senior, endocrinology and HDU.
Hand over: starting Na, rate of change, fluids given, targets, next sodium due.
Ask three questions: how fast did it fall, is the brain affected, and what is the volume status? Then send the paired urine and serum tests, set a safe 24-hour target, and write it in the notes. Nicely done getting here.
Clerking template
copy or downloadHyponatraemia — documentation structure
A structure and prompt list to help you document and think systematically. It is not a completeness check and cannot guarantee nothing is missed — use your trust's own proforma, your seniors, and your clinical judgement.
Test yourself
active recallCover the answer, say it out loud, then reveal. Retrieval beats re-reading.
0 / 10 knownFrequently asked questions
quick answersWhat are the red flags for hyponatraemia?
- Seizures, reduced GCS, vomiting — think severe symptoms — 3% saline
- Na rising more than 10 in 24 h — think overcorrection — ODS risk
- Alcohol, malnourished, low K⁺ — think high ODS risk — limit 8/24 h
- Hypotension + low Na + high K⁺ — think adrenal crisis
- Marathon runner or MDMA — think acute hyponatraemia
- Very high glucose — think pseudo / translocational hyponatraemia
What is the initial management of hyponatraemia?
- Hypertonic saline bolus — 150 mL 3% saline (or equivalent) IV over 20 min, with senior/ITU involvement
- Recheck sodium — between boluses; repeat up to twice more until symptoms improve or Na has risen by 5
- Aim for a 5 mmol/L rise — in the first hour, then stop the hypertonic saline
- Cap the 24-hour rise — no more than 10 mmol/L in 24 h (8 if high risk), then 8 per 24 h after
- Watch the urine output — a sudden diuresis means sodium may overshoot
- Senior + critical care — hypertonic saline is an HDU-level treatment
Always alongside senior support and your local guideline.
What diagnoses must you not miss in hyponatraemia?
- Hyponatraemic encephalopathy — seizures · coma
- Osmotic demyelination — the iatrogenic killer
- Adrenal crisis — low Na · high K · low BP
- Severe hypothyroidism — myxoedema
Open each one in the must-not-miss section for what points toward it, how to rule it in, and how to manage it.
What investigations should a junior doctor order for hyponatraemia?
- Paired tests — Confirms true hypotonic hyponatraemia.
- Rule out the treatable — Corrects the sodium.
- Other bloods — Context and pseudohyponatraemia.
- Monitoring — Every 2–4 h while correcting actively; at least 6-hourly otherwise.
Admit or discharge: how is the plan decided for hyponatraemia?
- Ward / outpatient — Mild chronic hyponatraemia, no symptoms, cause identified (e.g. a thiazide stopped). Recheck in days.
- Escalate — Severe symptoms, Na below 120, rapid fall, high ODS risk, or not responding → senior, endocrinology and HDU.
Is there a clerking template for hyponatraemia?
Yes — there is a free clerking template for hyponatraemia on this page, structured as presenting complaint, red flags asked, history, examination (A–E), investigations, differentials, plan and escalation. You can copy it or download it as a Word document. Jump to the template.
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