🧭 When to suspect

Hyponatraemia (serum sodium < 135 mmol/L) is the commonest electrolyte abnormality in clinical practice and is usually a disorder of water excess rather than true sodium deficiency. Most cases are found incidentally on a routine U&E, and 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, and consider it actively in older adults with new confusion, drowsiness, gait instability or recurrent falls. The two clinical priorities in primary care are to assess volume status (which directs the diagnosis) and to 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). Crucially, 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 – and 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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