What initial investigations should I perform for a patient presenting with hyponatraemia?

Initial investigations for a patient presenting with hyponatraemia should include:

• Repeat serum sodium measurement to confirm hyponatraemia and assess for rapid changes, which may indicate a need for urgent admission ¹.
• Assessment of volume status clinically to classify the hyponatraemia as hypovolaemic, euvolaemic, or hypervolaemic ¹.
• Measurement of urinary sodium concentration and urinary osmolality to help determine the underlying cause, although interpretation can be difficult especially if the patient is on diuretics or has renal impairment ¹.
• Blood tests including plasma potassium, urea, creatinine, and glucose to assess electrolyte balance and renal function ¹.
• Consider measurement of venous blood acid–base status and chloride concentration if shock or hypovolaemia is suspected ⁴.
• Review of current medications that may contribute to hyponatraemia and stopping any non-essential ones if appropriate ¹.

If the cause remains unclear after these initial investigations, referral to a specialist (endocrinologist or nephrologist) is recommended for further evaluation ¹.