Hypernatraemia (Guidelines)

TAM (Treatments and Medicines) NHS Highland
NHS Highland

Audience

  • All NHS Highland

  • Primary and Secondary Care

  • Adults only

Definition

Hypernatraemia is defined as a serum sodium concentration of >145mmol/L.

Severity can be classified as:

  • Mild (146 to 149 mmol/L)

  • Moderate (150 to 159 mmol/L)

  • Severe (≥160 mmol/L)

Hypernatraemia is most often caused by water loss with inadequate replacement.
Less commonly, it can result from ‘salt gain’.

Causes of hypernatraemia:

  • Extra-renal water loss

    • GI losses

    • Insensible water loss: Fever, diaphoresis, hyperventilation

    • Impaired thirst mechanism

  • Renal water loss: Inability of the kidneys to conserve water appropriately.
    Characterised by polyuria >3 litres of urine/ 24 hours.
    Measure serum osmolality.

    • Osmotic diuresis (urine osmolality >300mOsm/L) – Think HHS in patients with T2DM (For patients with HHS, please follow the HHS guidelines)

    • Diabetes Insipidus (DI) (urine osmolality <150mOsm/L) (If Diabetes Insipidus is suspected/ already diagnosed – please contact the renal or endocrine physicians for further advice.)

      • Central DI

      • Nephrogenic DI

  • Iatrogenic: Administration of intravenous hypertonic saline or intravenous hypertonic sodium bicarbonate.

Symptoms associated with hypernatraemia

Symptoms can range from mild to severe:

  • Extreme thirst
  • Fatigue
  • Confusion
  • Muscle cramps or twitching
  • Nausea
  • Decreased level of consciousness
  • Seizures

Management in primary care before and after referral

The key priority is to identify the underlying cause, as this will guide appropriate management and determine the need for referral to secondary care.

Initial assessment should include a clinical history, physical examination (including assessment of volume status), and measurement of urine osmolality.

Patients with mild hypernatraemia who are clinically well, normotensive, and able to access and drink water independently should be encouraged to increase their oral water intake to replace the estimated water deficit. Repeat blood tests should be arranged to monitor the trend in serum sodium concentration and assess response to treatment.

Referral to specialist service

For patients requiring hospital admission, contact the acute medical team via bleep 4000.

If urgent clinical advice is required, contact the medical registrar via bleep 2050.

Management in secondary care

Management of hypernatraemia:

  1. Obtain clinical history to help determine potential causes. Aim to treat the underlying cause as well as correcting the hypernatraemia.

  2. Determine if hypernatraemia is acute or chronic.

  3. Calculate the approximate water deficit using the formula:

  • Female: TBW = 0.5 x body weight(kg)

  • Male: TBW = 0.6 x body weight (kg)

OR using the online Free Water Deficit Calculator: https://ukidney.com/calculators/free-water-deficit-calculator

  • Aim to decrease the serum sodium concentration by about 0.5 to 1mmol/L per hour, and no more than 12mmol/L in the first 24 hours.

  • Complete correction should be achieved over 36 to 72 hours.

  • The desired serum sodium concentration may be either the target to be achieved over the subsequent 24 hours or a value within the normal reference range (135 to 145 mmol/L). If the latter is selected, care must be taken to avoid overly rapid correction of hypernatraemia, as this increases the risk of cerebral oedema. The rate of correction should remain within recommended limits.

     4. Monitor serum sodium levels every 4 hours until target rate of correction is attained.

Fluid therapy:

Mild / Moderate hypernatraemia:

  • Push water via oral / enteral routes. If oral route unavailable, treat as per severe below.

Severe:

  • In the absence of hypotension, patients should be given 1000mL of Dextrose 5% IV over 6 hours.

  • If the patient is hypotensive, initial fluid therapy should be with Plasmalyte ® to help restore ECF volume. Subsequent replacement fluids should be hypotonic.

Escalation criteria within secondary care

  • All patients with severe hypernatraemia should be discussed with caring consultant.
  • Neurological signs or symptoms at presentation, or the development of neurological signs once treatment is started (may signify the development of cerebral oedema).
  • Persistent or worsening hypernatraemia despite appropriate fluid replacement.
  • A rapid fall in serum sodium concentration during treatment (suggesting overly rapid correction of hypernatraemia).

Cases where there is difficulty obtaining route for enteral/ IV fluid replacement

Abbreviations

  • DI: diabetes insipidus

  • ECF: extracellular fluid

  • HHS: hyperosmolar hyperglycaemic state

  • T2DM: Type 2 diabetes mellitus

  • TBW: Total body water

Editorial Information

Last reviewed: 27/08/2026

Next review date: 27/08/2029

Author(s): Renal.

Version: 1

Approved By: TAM Subgroup of the ADTC

Reviewer name(s): S Chetcuti, Renal/GIM ST4.

Document Id: TAM809