REASONING GP · Clinical algorithm · Quick reference

Hyponatraemia in adults — triage and diagnostic approach

The commonest electrolyte abnormality in primary care. Symptoms and speed of onset decide urgency, not the number alone; volume status decides the cause; and the correction rate is itself a safety issue.

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AdultsLab resultNa⁺ below 135v1.0 · Sep 2026

Core rule. Symptoms and the speed of the fall drive urgency, not the absolute number. Seizure, drowsiness, confusion or vomiting at any level is an emergency; Na⁺ below 125, or a rapid fall, is an emergency even in a patient who feels well. Then three questions in order: is it acute or chronic (a slow fall over months is far better tolerated), what is the volume status (hypovolaemic, euvolaemic or hypervolaemic — this is what names the cause), and what drugs (thiazides and SSRIs are the two commonest causes in primary care). Never correct faster than 8–10 mmol/L in 24 hours — over-rapid correction causes osmotic demyelination, which is irreversible.

On seeing the result: is this an emergency now? Arrange emergency admission — 999 or immediate discussion with the acute team

Any neurological feature at any sodium level — seizure, drowsiness, reduced GCS, confusion, agitation, severe headache, vomiting, unsteadiness · Na⁺ below 125 mmol/L, however well the patient seems · a rapid fall — for example more than 10 mmol/L within 48 hours, or a drop from normal on a recent result · hypovolaemic shock — systolic BP below 90, tachycardia, postural collapse, ongoing vomiting or diarrhoea · suspected adrenal crisis (low sodium with a high potassium, hypotension, pigmentation) · new-onset seizure with hyponatraemia after a marathon, MDMA or excessive water intake. Hand over the sodium with its previous values and dates, the volume assessment and the full drug list.

Three questions, in this order Urgency, then tempo, then volume status — the volume assessment is what names the cause

Urgency

Symptomatic at any level, below 125, or falling fast: emergency admission. 125–129 asymptomatic: same-day assessment. 130–134 asymptomatic and stable: primary-care investigation. A well patient at 128 that has been 128 for two years is a different problem from 128 last week and 138 last month.

Acute or chronic?

Find the previous sodium results and dates — this single step changes management more than any test. A fall over under 48 hours is acute and dangerous (cerebral oedema). A fall over weeks to months is chronic, better tolerated, and must be corrected slowly.

Volume status — examine, do not guess

Hypovolaemic: postural drop, tachycardia, dry mucous membranes, reduced skin turgor, low urine output. Hypervolaemic: oedema, raised JVP, ascites, crackles. Euvolaemic: neither — which points at SIADH, drugs, hypothyroidism or adrenal insufficiency.

Drugs — the commonest cause

Thiazides and thiazide-like diuretics (indapamide especially) and SSRIs and SNRIs lead by far. Also carbamazepine and oxcarbazepine, PPIs, NSAIDs, tricyclics, antipsychotics, desmopressin, chemotherapy, MDMA, and low-dose amitriptyline. Note start and dose-change dates.

Red-flag screen: trigger → concern → destination → what to do before handover One row is enough; do not add up reassuring features

Trigger / red flag	Leading concern · destination	Do now · do not delay
Any neurological feature at any sodium level: seizure, drowsiness, reduced GCS, confusion, agitation, severe headache, vomiting or unsteadiness.	Cerebral oedema — needs hypertonic saline and monitoring Emergency admission now	Symptoms, not the number, define severity: a symptomatic patient at 128 needs admission while an asymptomatic patient at 128 may not. Do not fluid-restrict a symptomatic patient at home or wait for a repeat. Send the previous sodium values and dates so the receiving team can judge the tempo and set a safe correction rate.
Na⁺ below 125 mmol/L, however well the patient seems; or a rapid fall — more than 10 mmol/L in 48 hours, or a fall from a normal recent result.	Severe or acute hyponatraemia · risk of seizure and herniation Emergency admission now	Wellness is not reassurance below 125, and a rapid fall is dangerous at a level that would be tolerated if chronic. Stop the likely culprit drug now — usually a thiazide or an SSRI — and say what was stopped. Do not start fluid restriction and review in a week.
Hypovolaemic with hypotension: systolic BP below 90, tachycardia, postural collapse, ongoing vomiting, diarrhoea or high stoma output, or a patient unable to drink.	Hypovolaemic hyponatraemia with circulatory compromise Emergency admission now	These patients need IV saline with monitoring, not oral fluids at home. Assess postural BP and mucous membranes. Do not fluid-restrict a hypovolaemic patient — the treatment is the opposite, and this is the commonest management error in hyponatraemia.
Low sodium with a high potassium, postural hypotension, fatigue, weight loss, pigmentation, hypoglycaemia, or after stopping long-term steroids.	Adrenal insufficiency · adrenal crisis 999 if hypotensive, vomiting or unwell Same-day endocrine advice and a 9 am cortisol	Each number looks mild alone, which is why this is missed — hyponatraemia plus hyperkalaemia in an unwell patient is an adrenal crisis until excluded. Take a 9 am cortisol but do not wait for it before treating a crisis. Never stop long-term steroids abruptly.
Na⁺ 125–129 mmol/L asymptomatic, or any fall that is clearly progressive on serial results.	Moderate hyponatraemia — needs assessment today Same-day clinical assessment and bloods	Assess volume status face to face; a remote review cannot do it. Send paired serum and urine osmolality with a urine sodium, plus TFTs, 9 am cortisol, glucose, lipids and potassium, before starting any treatment — once fluid restriction or a drug change begins, the diagnostic window closes. Stop or hold the culprit drug. Repeat within 24–48 hours.
Hypervolaemic hyponatraemia: oedema, raised JVP, ascites or crackles with heart failure, cirrhosis or advanced CKD; or worsening sodium after a diuretic increase.	Decompensated heart failure · cirrhosis · nephrotic syndrome · advanced CKD Same-day assessment; specialist advice before changing diuretics	The sodium is a marker of the underlying disease severity and a poor prognostic sign — treat the disease, not the number. Do not simply increase the diuretic, and do not give saline. Seek cardiology, hepatology or renal advice on the diuretic balance; fluid restriction is appropriate here but needs the specialist's dose.
Acute water loading: marathon or endurance event, MDMA or ecstasy use, psychogenic polydipsia, excessive water intake, or a recent colonoscopy bowel preparation — particularly with headache, vomiting or confusion.	Acute dilutional hyponatraemia · exercise-associated hyponatraemia Emergency admission if any symptom Same-day assessment if asymptomatic	This is acute hyponatraemia — the brain has had no time to adapt, so seizures occur at levels a chronic patient tolerates. Do not give further hypotonic fluid. Ask directly about water intake, energy drinks and recreational drugs; the history will not appear in the notes otherwise.
Euvolaemic hyponatraemia with a cancer feature: a smoker or ex-smoker with unexplained cough, weight loss, haemoptysis, chest pain or breathlessness; clubbing; supraclavicular nodes; or unexplained SIADH at any age.	SIADH from small-cell lung cancer, or another malignancy Urgent chest X-ray within 2 weeks · NICE NG12	SIADH is a recognised presentation of small-cell lung cancer and is often the first abnormality. Hyponatraemia is not itself an NG12 criterion, but where there is unexplained cough, weight loss, haemoptysis, chest pain or breathlessness, or the patient is 40 or over and has ever smoked, arrange an urgent chest X-ray within 2 weeks and refer on the urgent suspected-cancer route if it suggests cancer (NG12). A normal film with persistent unexplained SIADH still needs investigation — do not stop at "SIADH, cause unknown".

Trigger / red flag

Leading concern · destination

Do now · do not delay

Any neurological feature at any sodium level: seizure, drowsiness, reduced GCS, confusion, agitation, severe headache, vomiting or unsteadiness.

Cerebral oedema — needs hypertonic saline and monitoring
Emergency admission now

Symptoms, not the number, define severity: a symptomatic patient at 128 needs admission while an asymptomatic patient at 128 may not. Do not fluid-restrict a symptomatic patient at home or wait for a repeat. Send the previous sodium values and dates so the receiving team can judge the tempo and set a safe correction rate.

Na⁺ below 125 mmol/L, however well the patient seems; or a rapid fall — more than 10 mmol/L in 48 hours, or a fall from a normal recent result.

Severe or acute hyponatraemia · risk of seizure and herniation
Emergency admission now

Wellness is not reassurance below 125, and a rapid fall is dangerous at a level that would be tolerated if chronic. Stop the likely culprit drug now — usually a thiazide or an SSRI — and say what was stopped. Do not start fluid restriction and review in a week.

Hypovolaemic with hypotension: systolic BP below 90, tachycardia, postural collapse, ongoing vomiting, diarrhoea or high stoma output, or a patient unable to drink.

Hypovolaemic hyponatraemia with circulatory compromise
Emergency admission now

These patients need IV saline with monitoring, not oral fluids at home. Assess postural BP and mucous membranes. Do not fluid-restrict a hypovolaemic patient — the treatment is the opposite, and this is the commonest management error in hyponatraemia.

Low sodium with a high potassium, postural hypotension, fatigue, weight loss, pigmentation, hypoglycaemia, or after stopping long-term steroids.

Adrenal insufficiency · adrenal crisis
999 if hypotensive, vomiting or unwell Same-day endocrine advice and a 9 am cortisol

Each number looks mild alone, which is why this is missed — hyponatraemia plus hyperkalaemia in an unwell patient is an adrenal crisis until excluded. Take a 9 am cortisol but do not wait for it before treating a crisis. Never stop long-term steroids abruptly.

Na⁺ 125–129 mmol/L asymptomatic, or any fall that is clearly progressive on serial results.

Moderate hyponatraemia — needs assessment today
Same-day clinical assessment and bloods

Assess volume status face to face; a remote review cannot do it. Send paired serum and urine osmolality with a urine sodium, plus TFTs, 9 am cortisol, glucose, lipids and potassium, before starting any treatment — once fluid restriction or a drug change begins, the diagnostic window closes. Stop or hold the culprit drug. Repeat within 24–48 hours.

Hypervolaemic hyponatraemia: oedema, raised JVP, ascites or crackles with heart failure, cirrhosis or advanced CKD; or worsening sodium after a diuretic increase.

Decompensated heart failure · cirrhosis · nephrotic syndrome · advanced CKD
Same-day assessment; specialist advice before changing diuretics

The sodium is a marker of the underlying disease severity and a poor prognostic sign — treat the disease, not the number. Do not simply increase the diuretic, and do not give saline. Seek cardiology, hepatology or renal advice on the diuretic balance; fluid restriction is appropriate here but needs the specialist's dose.

Acute water loading: marathon or endurance event, MDMA or ecstasy use, psychogenic polydipsia, excessive water intake, or a recent colonoscopy bowel preparation — particularly with headache, vomiting or confusion.

Acute dilutional hyponatraemia · exercise-associated hyponatraemia
Emergency admission if any symptom Same-day assessment if asymptomatic

This is acute hyponatraemia — the brain has had no time to adapt, so seizures occur at levels a chronic patient tolerates. Do not give further hypotonic fluid. Ask directly about water intake, energy drinks and recreational drugs; the history will not appear in the notes otherwise.

Euvolaemic hyponatraemia with a cancer feature: a smoker or ex-smoker with unexplained cough, weight loss, haemoptysis, chest pain or breathlessness; clubbing; supraclavicular nodes; or unexplained SIADH at any age.

SIADH from small-cell lung cancer, or another malignancy
Urgent chest X-ray within 2 weeks · NICE NG12

SIADH is a recognised presentation of small-cell lung cancer and is often the first abnormality. Hyponatraemia is not itself an NG12 criterion, but where there is unexplained cough, weight loss, haemoptysis, chest pain or breathlessness, or the patient is 40 or over and has ever smoked, arrange an urgent chest X-ray within 2 weeks and refer on the urgent suspected-cancer route if it suggests cancer (NG12). A normal film with persistent unexplained SIADH still needs investigation — do not stop at "SIADH, cause unknown".

Safety rule. Never fluid-restrict a hypovolaemic patient. Never correct faster than 8–10 mmol/L in 24 hours — osmotic demyelination is irreversible and is caused by the treatment, not the disease. Send the diagnostic bloods before treating, or the cause becomes unknowable. And a sodium that has been stable for years at 130 is not the same finding as one that fell there last month.

Colour is semantic: red = emergency now · amber = same-day assessment · blue = define/assess · green = primary-care management · purple = uncertain / specialist route (including 2WW). Continued on page 2: examination and tests, classification, cause classifier, routine referral, primary-care management, endpoint.

REASONING GP · Hyponatraemia in adults — triage and diagnostic approach

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Assessment and investigation Volume status cannot be assessed remotely — below 130 needs a face-to-face examination

Assess

GCS and orientation; lying and standing BP, pulse, mucous membranes, skin turgor; JVP, oedema, ascites, crackles; weight against previous. Decide and record hypovolaemic, euvolaemic or hypervolaemic.

Bloods — send before treating

Paired serum and urine osmolality with a urine sodium, before fluid restriction or stopping a drug. Plus U&E with potassium, TFTs, 9 am cortisol, glucose, lipids and total protein, LFTs, calcium.

Interpreting them

Osmolality normal or high: pseudohyponatraemia or hyperglycaemia — not true hyponatraemia. Low osmolality, urine Na⁺ below 20: hypovolaemia or hypervolaemia. Urine Na⁺ above 30, concentrated, euvolaemic: SIADH.

Do not do

Do not diagnose SIADH without excluding hypothyroidism, hypocortisolism and diuretics. Do not restrict fluid before the osmolalities. Do not give saline to a hypervolaemic patient. Do not act on one result without the previous ones.

Classify: three exits, not two "Mild hyponatraemia, repeat in a month" is only safe when it is chronic, stable and asymptomatic

Emergency or same-day

Any symptom, below 125, a rapid fall, hypovolaemic shock, adrenal features, water loading, or 125–129 today. Output: destination, sodium with previous values and dates, volume status, drugs stopped with the time.

Chronic, mild, managed in primary care

Na⁺ 130–134, asymptomatic, stable across previous results, volume status assessed, cause identified. Output: cause, drug changed, osmolality results, repeat date, escalation thresholds.

Unclassified or persistent

Persisting after the culprit drug is stopped, SIADH with no cause found, or discordant osmolalities. Output: state the uncertainty, take the §6 route, name the reviewer.

Cause classifier: volume status first, then the osmolalities Only once the urgency is settled

Drug-induced

The commonest cause in primary care and usually euvolaemic SIADH-like: thiazides and thiazide-like diuretics (indapamide especially), SSRIs and SNRIs, carbamazepine and oxcarbazepine, PPIs, NSAIDs, tricyclics, antipsychotics, desmopressin. Check start and dose-change dates against the sodium trend — the answer is usually there.

Hypovolaemic, and hypervolaemic

Hypovolaemic — urine Na⁺ below 20 with postural drop and dry membranes: vomiting, diarrhoea, bleeding, poor intake, high stoma output. Urine Na⁺ above 20 despite hypovolaemia points to renal loss (diuretics, adrenal insufficiency). Treated with salt and water, never fluid restriction. Hypervolaemic — oedema with a low urine sodium: heart failure, cirrhosis, nephrotic syndrome, advanced CKD. The sodium reflects disease severity and carries prognostic weight; treat that disease, with specialist-directed restriction, not saline.

SIADH, and the endocrine causes it must not hide

SIADH: euvolaemic, low serum osmolality, inappropriately concentrated urine with urine Na⁺ above 30, normal thyroid and adrenal function, no diuretic. Causes: malignancy (especially small-cell lung), chest disease, CNS disease and stroke, drugs, pain, nausea, surgery. A diagnosis of exclusion that still needs its own cause found. Exclude first: hypothyroidism and adrenal insufficiency — both treatable, and the latter classically pairs a low sodium with a high potassium and postural hypotension.

Excess water, and pseudohyponatraemia

Water excess: primary polydipsia, beer potomania, post-marathon or MDMA, excessive hypotonic fluid — dilute urine with a low osmolality. Pseudohyponatraemia: a normal or raised serum osmolality from marked hyperlipidaemia or paraproteinaemia, or translocational hyponatraemia from hyperglycaemia — the sodium is not truly low and needs no correction.

Do not conclude by default

"SIADH" without TFTs, a 9 am cortisol and a drug review · "chronic and stable" without finding the previous results · "the thiazide" in a clearly hypovolaemic patient — the mechanism changes the treatment · "unexplained SIADH" in a smoker without a chest X-ray.

Refer or seek advice routinely from this consultation Not same-day — the cases that leave by letter; §7 is what primary care does today

Hyponatraemia persisting after the culprit drug is stopped and the cause corrected → endocrinology or nephrology · confirmed SIADH with no cause identified → endocrinology, with the paired osmolalities, TFTs and cortisol in the letter, and imaging in parallel · recurrent hyponatraemia, or a patient needing a thiazide or SSRI that cannot be replaced → specialist advice on alternatives and monitoring · chronic hyponatraemia in heart failure, cirrhosis or CKD → the relevant team for the fluid and diuretic balance, since the sodium tracks disease severity · suspected primary polydipsia → psychiatry alongside medical monitoring · confirmed hypothyroidism or adrenal insufficiency → endocrinology.

The only decisions that belong in this consultation What primary care manages today — chronic mild hyponatraemia in a well patient

Send the diagnostic bloods before you treat, then change the drug

Bloods first: paired serum and urine osmolality with a urine sodium, TFTs, 9 am cortisol, glucose, lipids and total protein — all before fluid restriction or stopping the drug. Once treatment starts the results become uninterpretable and the cause is lost, which turns a solvable problem into "SIADH, cause unknown". Then the drug: stop the thiazide and choose another antihypertensive class, and do not restart it; for an SSRI, weigh the mental-health risk — stop, switch class or reduce with a plan, and recheck the sodium at 1–2 weeks after any change. Review carbamazepine, PPIs, NSAIDs and tricyclics.

Match the treatment to the volume status

Hypovolaemic: oral salt and water, treat the loss — never fluid restriction. Euvolaemic (SIADH): fluid restriction, typically 1–1.5 L/24 h, with written advice on what counts as fluid, and only after the osmolalities are sent. Hypervolaemic: treat the underlying disease, with restriction at the specialist's dose. Tolvaptan and demeclocycline are specialist-only.

Correct slowly, then recheck — both are safety limits

Correction: no more than 8–10 mmol/L in 24 hours, slower still where the hyponatraemia is chronic, or in alcohol excess, malnutrition, liver disease or hypokalaemia — over-rapid correction causes osmotic demyelination, irreversible and iatrogenic. A sodium rising faster than this needs urgent advice, not congratulation. Recheck: U&E at 3–7 days after any change (sooner if below 130), then widening intervals once stable, with the thresholds written down: any confusion, drowsiness, seizure or vomiting, or Na⁺ below 125 → emergency admission; 125–129 → same-day assessment.

What not to do

Do not fluid-restrict a hypovolaemic patient, or give saline to a hypervolaemic one. Do not treat before the osmolalities. Do not accept one low sodium without the previous values. Do not restart the thiazide that caused it. Do not file a sodium of 128 as "borderline" with no plan and no reviewer.

Endpoint and documentation Finish with one of four conclusions

1 · Emergency

Admission arranged; sodium with previous values and dates, symptoms, GCS, volume status, potassium, drugs stopped with the time.

2 · Same-day

Assessment today; volume status recorded; osmolalities, urine sodium, TFTs and cortisol sent before treatment; drug held; repeat in 24–48 h.

3 · Chronic mild, managed

Level, tempo, volume status and cause recorded; drug changed; treatment matched to volume status; repeat date and escalation thresholds written down.

4 · Unclassified or persistent

Uncertainty stated; §6 referral sent with the osmolalities, TFTs and cortisol; NG12 chest X-ray where indicated; review owner.

Why primary-care management is safe today, in one line: Na⁺ is 130–134 · asymptomatic — no confusion, drowsiness, headache or vomiting · chronic and stable on previous results, which have been found and compared · volume status assessed face to face and recorded · potassium normal, so adrenal insufficiency is not in play · osmolalities, urine sodium, TFTs and cortisol sent before any treatment · a modifiable cause identified and acted on · repeat booked at 3–7 days with a named reviewer and written escalation thresholds. Safety-net wording: contact us or 111 the same day if you become confused, unusually drowsy or unsteady, develop a bad headache, start vomiting or cannot keep fluids down; call 999 for a fit or collapse. Stick to the agreed fluid limit, and attend the repeat test even if you feel well.

Remote and result-handling limitation. Volume status cannot be assessed on the telephone, so a sodium below 130 needs a face-to-face examination. Convert to face-to-face or emergency assessment where the patient cannot attend, there is vomiting, diarrhoea or poor intake, a relative reports cognitive change, or the result arrives out of hours with no one able to act.

Clinical decision support only; follow local acute-transfer, renal and prescribing policies and check doses against the current BNF. Source hierarchy: NICE guidelines and CKS first, then BNF/MHRA for doses and safety, then national specialty guidance. Sources: NICE NG136 Hypertension · NG106 Chronic heart failure · NG203 Chronic kidney disease · NG148 Acute kidney injury · NG12 Suspected cancer (May 2025) · NICE CKS Hyponatraemia, Heart failure · European Society of Endocrinology / ERA-EDTA guideline on the diagnosis and treatment of hyponatraemia · Society for Endocrinology emergency guidance · UK Kidney Association · BNF · MHRA. Review after any guidance or medicines-safety update, otherwise every 6 months. © Reasoning GP · gpreasoning.uk