In a primary care setting, differentiating the causes of hyponatraemia involves a systematic clinical and laboratory assessment focusing on the patient's volume status, symptoms, medication history, and underlying conditions. First, assess the patient's volume status to classify hyponatraemia as hypovolaemic, euvolaemic, or hypervolaemic, which guides the differential diagnosis. Hypovolaemic hyponatraemia is often due to fluid losses (e.g., vomiting, diarrhoea, diuretics), euvolaemic hyponatraemia may suggest syndrome of inappropriate antidiuretic hormone secretion (SIADH) or endocrine causes, and hypervolaemic hyponatraemia is commonly associated with heart failure, liver, or kidney disease NICE CKS.
Next, review medications that can cause hyponatraemia, such as thiazide diuretics or antipsychotics, and consider stopping them if appropriate, with follow-up sodium measurements after 2 weeks NICE CKS.
Laboratory investigations include repeat serum sodium to confirm persistence and exclude rapid decline, and if possible, urinary sodium and osmolality to help differentiate SIADH from other causes, although interpretation can be difficult in primary care NICE CKS.
Consider acute illness as a contributing factor; treating the underlying illness may resolve hyponatraemia NICE CKS.
Referral to specialists is recommended if the cause remains unclear after initial assessment, if SIADH or endocrine causes are suspected, or if malignancy is suspected as an underlying cause NICE CKS.
In summary, differentiation in primary care relies on clinical assessment of volume status, medication review, repeat sodium measurements, and targeted investigations, with specialist referral when diagnosis is uncertain or complex NICE CKS.