Sodium (Hypo-Hypernatraemia)

What is Hyponatraemia?

Hyponatraemia means the sodium level is low. It is usually defined as serum sodium < 133 mmol/L.

If sodium is low, the key questions are: how low is it, is the person unwell and why is it happening.

Correcting sodium too quickly can cause brain injury (osmotic demyelination), so correction must be slow and careful.

Classification by Severity

  • Mild: 130-132 mmol/L
  • Moderate: 125-129 mmol/L
  • Severe: < 125 mmol/L or any red flag symptoms

Red Flag Symptoms (same-day assessment)

  • Confusion, severe drowsiness
  • Seizure
  • Collapse
  • Severe headache or vomiting

Common Causes

  • Medicines: thiazides, SSRIs, carbamazepine, antipsychotics
  • Dehydration: vomiting, diarrhoea, poor intake
  • Fluid overload: heart failure, cirrhosis
  • SIADH: often due to chest infection, lung disease, cancer, or medicines
  • Hormone problems: adrenal insufficiency (Addison’s), hypothyroidism
  • High glucose: can make sodium look lower than it is

Primary Care Approach (what to do first)

Step 1: Confirm the result and check how urgent it is

  • Check symptoms and observations if available
  • Look at previous sodium results (trend matters)
  • Check if there is a clear trigger (new medicine, diarrhoea, poor intake)

Step 2: Repeat and add simple blood tests

  • If mild and well: repeat U&Es within 1 week
  • Add: glucose, calcium, TSH (and creatinine/egfr if not already included)

Step 3: Medication review

  • Stop or switch likely causes where safe: thiazides, SSRIs, carbamazepine
  • Document the plan and recheck sodium after changes

Step 4: Simple fluid advice (only if safe)

  • If dehydration likely: encourage oral rehydration (small frequent fluids)
  • If heart failure or fluid overload suspected: avoid pushing fluids and arrange urgent assessment

Refer same-day to hospital from primary care if:

  • Any red flag symptoms
  • Sodium < 125 mmol/L
  • Rapid fall in sodium (new or worsening over days)
  • Clinical concern: sepsis, severe dehydration, fluid overload, or confusion

Secondary Care Assessment (what hospital will do)

  • Repeat U&Es and check trends frequently
  • Assess fluid status: dehydrated, normal volume, or overloaded
  • Check serum osmolality and urine osmolality
  • Check urine sodium
  • Consider cortisol testing if adrenal insufficiency possible
  • Consider chest imaging or further tests if SIADH suspected

Secondary Care Management (simple overview)

  • If severe symptoms (e.g. seizure): urgent treatment with hypertonic saline under protocol and close monitoring
  • If dehydrated (hypovolaemic): IV fluids (usually sodium chloride 0.9%) and treat the cause
  • If SIADH (euvolaemic): fluid restriction, stop causative drugs, treat underlying trigger, consider specialist treatments if persistent
  • If fluid overloaded (hypervolaemic): fluid restriction, treat heart failure/cirrhosis, consider diuretics as appropriate
  • Correction rule: avoid rapid correction (aim slow rise, with close monitoring)

Simple Worked Examples

Example 1: Primary care

  • 72 year old on bendroflumethiazide, well, sodium 129 mmol/L found on routine bloods
  • Likely cause: thiazide-related hyponatraemia
  • Action: stop thiazide (if safe), repeat U&Es in 1 week, check glucose and TSH, advise to avoid excess free water intake
  • If sodium improves: continue monitoring
  • If sodium falls or symptoms develop: same day hospital assessment

Example 2: Hospital

  • 60 year old with chest infection, confused, sodium 118 mmol/L
  • Likely cause: SIADH triggered by infection
  • Action in hospital: urgent assessment, serum/urine osmolality and urine sodium, fluid restriction, treat infection, close sodium monitoring, specialist input, avoid rapid correction

What is Hypernatraemia?

Hypernatraemia means sodium is high: > 145 mmol/L.

It usually reflects a water deficit rather than too much salt. It is common in older adults who are dehydrated.

Severity Guide

  • Mild: 145-149 mmol/L
  • Moderate: 150-159 mmol/L
  • Severe: ≥ 160 mmol/L

Primary Care Approach

Step 1: Confirm the Result and Decide Urgency > following guide on when to repeat bloods

  • If sodium 145-149 mmol/L and patient well:
    • Repeat U&Es within 1 week
    • Encourage adequate oral fluids while awaiting repeat
  • If sodium 150-159 mmol/L and patient well:
    • Repeat U&Es within 24-48 hours
    • Assess hydration and medications urgently
  • If sodium ≥ 160 mmol/L:
    • Same-day hospital referral
    • Do not delay waiting for repeat bloods
  • If any red flag symptoms (confusion, drowsiness, seizures):
    • Same day hospital assessment regardless of level
  • Always check previous sodium results to assess trend (rising levels are more concerning)

Step 2: Look for the cause

  • Dehydration (poor intake, vomiting, diarrhoea)
  • Recent infection or fever
  • Medication review: diuretics, lithium, steroids
  • Symptoms of diabetes (polyuria, polydipsia)

Step 3: Basic blood tests

  • Repeat U&Es
  • Glucose (exclude hyperglycaemia)
  • Calcium
  • Renal function (eGFR)

Step 4: What To Do After Repeat Results

  • If sodium has returned to normal (≤ 145 mmol/L):
    • No further urgent action needed
    • Monitor at next routine blood test if risk factors remain
  • If sodium remains 145-149 mmol/L but stable and patient well:
    • Reinforce fluid advice
    • Review medications again
    • Repeat U&Es in 2-4 weeks
  • If sodium has increased (now ≥ 150 mmol/L):
    • Arrange urgent samem day assessment
    • Do not wait for another repeat in primary care
  • If persistent mild elevation without clear cause:
    • Consider further investigation or discussion with medical team
  • Escalate immediately if symptoms develop at any stage

Secondary Care Assessment

  • Frequent sodium monitoring
  • Assess fluid status carefully
  • Check serum and urine osmolality if diabetes insipidus suspected
  • Strict fluid balance chart

Secondary Care Management

  • If dehydrated: IV fluids (usually sodium chloride 0.9% initially if unstable, then hypotonic fluids as guided)
  • Correction rule: reduce sodium slowly (generally no more than 10 mmol/L in 24 hours)
  • If diabetes insipidus: treat underlying cause, consider desmopressin under specialist care
  • Monitor U&Es every 4-6 hours in severe cases

Worked Examples

Example 1: Primary Care

  • 82 year old with poor oral intake during hot weather
  • Sodium 147 mmol/L, mildly dry, otherwise well
  • Likely dehydration
  • Encourage fluids, review medications, repeat U&Es in 1 week

Example 2: Hospital

  • 65 year old confused, sodium 162 mmol/L
  • Severe dehydration
  • Managed with IV fluids and close sodium monitoring
  • Sodium reduced gradually to avoid cerebral oedema
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