🧭 When to suspect

Hyponatraemia (serum sodium < 135 mmol/L) is the commonest electrolyte abnormality in clinical practice.

• It is usually a disorder of water excess rather than true sodium deficiency.

• Most cases are found incidentally on a routine U&E.

• Symptoms are often vague and easily attributed to age or comorbidity.

Suspect it in anyone with non-specific headache, nausea, vomiting, muscle cramps, lethargy or unsteadiness.

• Consider it actively in older adults with new confusion, drowsiness, gait instability or recurrent falls.

The two clinical priorities in primary care:

• Assess volume status (which directs the diagnosis).

• Review the drug chart, since medication is one of the most common and most reversible causes.

Biochemical severity

Serum sodium

Significance & action

Mild

130–135 mmol/L

• Often asymptomatic

• Investigate the cause and recheck – usually managed in primary care

Moderate

125–129 mmol/L

• Investigate

• Discuss with endocrinology if asymptomatic

• Admit if symptomatic, acute or hypovolaemic

Severe (profound)

< 125 mmol/L

• High risk

• Arrange urgent admission, particularly if acute or symptomatic

Classify each case three ways: by biochemical severity (above), by onset (acute < 48 hours versus chronic ≥ 48 hours or unknown), and by volume status (hypovolaemic, euvolaemic or hypervolaemic). Symptom severity drives urgency more than the absolute number.

⚠️ Common pitfall

• Acting on the number before confirming it is real.

• A low sodium may be spurious (a drip-arm sample), or reflect pseudohyponatraemia (very high lipids or protein, with a normal serum osmolality) or hyperglycaemia drawing water into the blood.

• Exclude these first.

• Remember that a profoundly low sodium can be asymptomatic, while a modest fall can cause significant symptoms if it developed quickly.

Source: Society for Endocrinology · European Hyponatraemia Guideline

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