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

Initial investigations for a patient presenting with hyponatraemia should include a repeat serum sodium measurement to confirm the diagnosis and assess for rapid changes in sodium levels, which may indicate a need for urgent intervention.

Assess the patient's volume status clinically to help guide further investigations and management.

Measure serum osmolality to determine if the hyponatraemia is hypotonic, isotonic, or hypertonic.

Measure urine sodium concentration and urine osmolality to help differentiate causes of hyponatraemia, such as syndrome of inappropriate antidiuretic hormone secretion (SIADH), hypovolaemia, or hypervolaemia.

Review the patient's medication history to identify any drugs that may contribute to hyponatraemia and consider stopping them if appropriate.

Check renal function (urea and creatinine) and glucose levels to exclude other metabolic causes and assess kidney status.

If the cause remains unclear after initial tests, or if SIADH or an endocrine cause is suspected, further imaging such as CT chest/abdomen/pelvis and MRI head may be required to exclude malignancy or other underlying pathology.

In cases of suspected Addison's disease or other endocrine disorders, urgent referral or admission may be necessary for further endocrine evaluation.

Hospital admission is indicated for patients with acute onset or severe hyponatraemia (serum sodium <125 mmol/L), symptomatic patients, or those with signs of hypovolaemia.

For asymptomatic patients with mild hyponatraemia (130–135 mmol/L), initial investigation and management may be performed in primary care with close monitoring.

Overall, the initial investigations aim to confirm hyponatraemia, assess severity, identify the underlying cause, and guide appropriate management or referral.

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