Usually a water deficit — often in patients who can't drink or ask for water.
In chronic hypernatraemia (over 48 hours) lower sodium by no more than about 10 mmol/L in 24 hours.
Act now
- 1
Oral water if safe to swallow; stop unnecessary sodium-containing fluids.
- 2
Hypovolaemic and shocked: resuscitate with 0.9% sodium chloride first.
Assess
- Fluid status, access to water, confusion, seizures.
- Causes: insensible loss (fever), GI loss, osmotic diuresis (hyperglycaemia), diabetes insipidus (e.g. lithium), excess sodium (IV saline, bicarbonate).
Investigations
- U&E, glucose, calcium, serum and urine osmolality, urine sodium, fluid balance.
- Dilute urine (osmolality under 300 mOsm/kg) despite high Na⁺: think diabetes insipidus.
Manage
- Estimate the free-water deficit: total body water (about 0.6 × weight in men, 0.5 in women; lower in older adults) × (Na⁺ ÷ 140 − 1).
- Replace with oral or NG water, or IV 5% glucose, plus ongoing losses.
- Check Na⁺ every 4–6 hours at first to avoid correcting too fast.
Escalate when
- Na⁺ over 160 mmol/L, neurological symptoms, diabetes insipidus or HHS: senior and endocrinology.
Doses and prescribing
Glucose
5% IV, rate guided by the free-water deficit and repeat Na⁺ checks.
Check the BNF and your local formulary before prescribing.
Written to current guidance (NICE CG174) · sources checked 14 Nov 2026. We watch NICE for updates weekly.
Sources (2)
- NICE CG174 — Intravenous fluid therapy in adults in hospitalVersion 5 May 2017, checked 14 Nov 2026
- BNF — check doses, cautions and renal adjustmentchecked 14 Nov 2026
Reviewed by Aboodee Alhasso, MPharm MRCPharm IP on 14 Nov 2026
Calculators for this
For healthcare professionals, as an aid to clinical judgement — not a substitute for it. Follow your local policy and ask a senior when in doubt.