Sodium (Hypo-Hypernatraemia)

What is Hyponatraemia?

Hyponatraemia is defined as serum sodium < 133 mmol/L. Persistent levels < 130 mmol/L should always prompt investigation. Rapid correction should be avoided to reduce risk of osmotic demyelination.

Classification by Severity

  • Mild: 125–133 mmol/L
  • Moderate: 115–125 mmol/L
  • Severe: < 115 mmol/L or symptomatic

Symptoms to Watch

  • Lethargy, dizziness, nausea
  • Headache, confusion, disorientation
  • Seizures, altered consciousness, coma (in severe cases)

General Stepwise Assessment

Step 1: Confirm and Repeat

  • Primary care: repeat UE after 1 week (see primary care management below)
  • Secondary care: repeat UE + check plasma/urine osmolarity (see secondary care management below)

Step 2: Identify Fluid Status

  • Hypovolaemia: vomiting, diarrhoea, diuretics, Addison’s
  • Euvolaemia: SIADH, hypothyroidism, drug-induced
  • Hypervolaemia: heart failure, cirrhosis, nephrotic syndrome

Step 3: Additional Tests

  • Urine sodium and osmolality: only request in secondary care to evaluate renal handling and differentiate SIADH
  • Plasma osmolality: useful in true hyponatraemia (< 275 mOsm/kg)
  • 9 am cortisol or ACTH test: to exclude Addison’s (secondary care)
  • TSH and FT4: exclude hypothyroidism (can be done in primary care)
  • SIADH diagnosis = plasma osmolality < 275 + urine osmolality > 100
  • Normal urine osmolality range = 300–900 mOsm/kg

Primary Care Management

  • Mild hyponatraemia (125–133 mmol/L) and asymptomatic → repeat sodium, urea, creatinine, glucose, calcium, and TSH after 1 week
  • Review and stop contributing medications: e.g. thiazides, SSRIs, carbamazepine
  • If sodium improves → monitor in primary care every 3–6 months
  • If stable, no underlying cause, and no red flags → annual monitoring

When to Treat in Primary Care

  • If hypovolaemic and mild symptoms → oral rehydration with isotonic fluids
  • If related to reversible cause (e.g. recent diuretic or SSRI) → stop drug, recheck in 1 week
  • Do not start sodium supplements in primary care without specialist input

When to Refer to Secondary Care

  • Sodium < 115 mmol/L
  • Any red flag symptoms: confusion, seizures, drowsiness, collapse
  • Persistent sodium < 125 despite correcting reversible causes
  • Suspected SIADH, Addison’s, or unclear cause needing urine/plasma osmolality testing
  • Rapid drop in sodium over days or increasing symptoms

Secondary Care Management

Step 1: Confirm True Hyponatraemia

  • Check serum osmolality: < 275 mOsm/kg confirms hypotonic hyponatraemia
  • Exclude pseudohyponatraemia (e.g. hyperglycaemia, hyperlipidaemia)

Step 2: Determine Cause Using Urine Tests

  • Urine osmolality:
    • < 100 mOsm/kg → primary polydipsia or low solute intake
    • > 100 mOsm/kg → kidneys are concentrating, investigate further
  • Urine sodium:
    • > 30 mmol/L → renal loss (e.g. diuretics, SIADH, Addison’s)
    • < 30 mmol/L → extrarenal loss (e.g. diarrhoea, vomiting)

Urine osmolality reference range: 300–900 mOsm/kg

Step 3: Assess for Endocrine and Systemic Causes

  • Request TSH and free T4 to rule out hypothyroidism
  • Request 9am cortisol or Synacthen test if Addison’s suspected
  • Review drug history (SSRIs, diuretics, carbamazepine)

Step 4: Diagnose SIADH

  • Low plasma osmolality (< 275)
  • High urine osmolality (> 100)
  • Urine sodium > 30
  • No hypovolaemia or fluid overload
  • Normal adrenal and thyroid function

Common causes: SSRIs, malignancy (e.g. lung), CNS disease, pain, infection

Step 5: Treatment Strategy

  • SIADH: fluid restrict to 800–1000 mL/day
  • Hypovolaemia: give IV 0.9% sodium chloride
  • Severe symptomatic or Na < 115: consider cautious hypertonic saline under specialist supervision
  • Monitor closely: aim to raise sodium ≤ 10 mmol/L in 24 hours to avoid osmotic demyelination

Step 6: When to Refer or Escalate

  • Severe or rapidly worsening hyponatraemia (< 115 mmol/L)
  • Neurological symptoms or altered consciousness
  • Unclear diagnosis despite basic tests
  • Suspected malignancy or adrenal failure
  • Persistent hyponatraemia despite management

Discuss with endocrinology or nephrology if complex, recurrent, or unclear.

Hypernatraemia in Primary Care

Defined as serum sodium > 145 mmol/L. It reflects a relative water deficit. Most cases in primary care are due to inadequate fluid intake, medications, or subtle fluid losses.

Stepwise Primary Care Approach

Step 1: Confirm Diagnosis and Repeat

  • Repeat sodium to exclude error (especially if > 150)
  • Also request:
    • U&Es (to assess renal function)
    • Glucose (exclude diabetes)
    • Calcium (to rule out hypercalcaemia)
    • TSH and free T4 (exclude hypothyroidism)
  • Assess hydration and clinical history

Step 2: Determine Likely Cause Based on Fluid Status

  • Hypovolaemic: elderly, vomiting, diarrhoea, diuretics, poor fluid intake
  • Euvolaemic: diabetes insipidus (rare - usually polyuria & polydipsia)
  • Hypervolaemic: iatrogenic (e.g. IV saline), often in secondary care

Check medications: thiazide diuretics, lithium, corticosteroids

Step 3: Treat Mild Hypernatraemia in Primary Care (Na 145–149 mmol/L)

  • Encourage oral fluids (e.g. water, not sugary drinks)
  • Address underlying causes (e.g. constipation, infection, meds)
  • If due to recent reversible issue (e.g. acute illness), recheck in 1 week
  • If sodium improves, monitor every 3-6 months

Step 4: When to Refer or Escalate

  • Sodium > 150 mmol/L or rising
  • Unable to maintain hydration orally
  • Evidence of confusion, delirium, falls, or reduced consciousness
  • Suspected diabetes insipidus or neurological symptoms
  • No clear cause found or persistent abnormality

→ Refer to acute medical team or discuss with endocrinology

Secondary care approach

Step 1: Assess Hydration and Volume Status

  • Hydration status: Check mucous membranes, capillary refill, skin turgor, BP lying/standing, HR, urine output, daily weight.
  • Urine output: Oliguria < 0.5 mL/kg/hour over 6 hours → suspect dehydration or renal dysfunction.
  • Look for: polyuria (e.g. diabetes insipidus), vomiting, diarrhoea, diuretics, fever, or poor fluid intake.

Step 2: Check Key Blood and Urine Tests

  • U&Es, glucose, calcium, serum osmolality
  • Urine osmolality and urine sodium to assess cause

Interpretation Examples:

  • Hypovolaemic: low BP, dry mucosa, high serum osmolality, high urine osmolality, urine Na < 20 mmol/L
  • Euvolaemic: normal BP and exam; urine osmolality > 300 = diabetes insipidus; low if primary polydipsia
  • Hypervolaemic: oedema, fluid overload, raised JVP; may occur in iatrogenic sodium administration or Cushing’s

Mild Hypernatraemia (Na 145–150 mmol/L)

  • Check hydration status and urine output
  • Review contributing causes (e.g. inadequate fluid intake, medications)
  • Consider oral rehydration if able to drink
  • If IV required: use 0.9% saline initially if hypovolaemic
  • Once euvolaemic, switch to 5% dextrose or 0.45% saline to slowly correct sodium
  • Recheck sodium and osmolality in 6-8 hours

Moderate Hypernatraemia (Na 150–159 mmol/L)

  • Discuss with on-call medical registrar
  • Calculate water deficit (see below)
  • If hypovolaemic → give 500-1000 mL 0.9% saline over 1-2 hours, then reassess
  • Once euvolaemic, correct water deficit slowly over 48 hours using 5% dextrose
  • Monitor fluid input/output and sodium every 4-6 hours

Severe Hypernatraemia (Na ≥160 mmol/L)

  • Urgent discussion with SpR and consider HDU/ICU
  • If confused, drowsy or unstable → high risk of complications like seizures or intracranial bleeding
  • If known central diabetes insipidus or desmopressin use, consult endocrine
  • Correct slowly to avoid cerebral oedema (≤10 mmol/L drop in Na per 24 hrs)

Step 3: Calculate Water Deficit

Water deficit (L) = (measured Na / 140 - 1) × 0.6 × weight (kg)

Example: 75 kg patient with Na 160 mmol/L

(160 / 140 - 1) × 0.6 × 75 = 0.14 × 75 = 10.5 L deficit

  • Replace over 48 hours
  • Use 5% dextrose or 0.45% saline if euvolaemic
  • If hypovolaemic, prioritise correction with 0.9% saline before switching
mypanotes