How do I differentiate between the causes of hyponatraemia in a primary care setting?

In primary care, differentiating the causes of hyponatraemia involves a systematic clinical and laboratory assessment focusing on the patient's volume status, symptoms, medication history, and relevant investigations.

• Assess volume status: Determine if the patient is hypovolaemic, euvolaemic, or hypervolaemic through clinical signs such as blood pressure, heart rate, mucous membranes, skin turgor, and oedema. Hypovolaemia suggests losses (e.g., vomiting, diarrhoea, diuretics), hypervolaemia suggests conditions like heart failure, liver or kidney disease, and euvolaemia may indicate syndrome of inappropriate antidiuretic hormone secretion (SIADH) or endocrine causes ¹.
• Review medication history: Identify drugs that can cause hyponatraemia, such as thiazide diuretics or antipsychotics. If possible, stop the offending medication and recheck sodium after 2 weeks ¹.
• Evaluate symptoms and severity: Acute onset (<48 hours), severe hyponatraemia (<125 mmol/L), or symptomatic patients require urgent hospital admission. Asymptomatic or mild cases (130–135 mmol/L) can be initially managed in primary care with close monitoring ¹.
• Laboratory investigations: Repeat serum sodium to confirm hyponatraemia and monitor trends. Urinary sodium and osmolality can help differentiate causes but may be unreliable in primary care. Persistent unexplained hyponatraemia warrants specialist referral ¹.
• Consider underlying conditions: Treat any acute illness that may contribute to hyponatraemia. If SIADH is suspected without clear cause, consider imaging to exclude malignancy. Endocrine causes like Addison’s disease require specialist input ¹.

In summary, primary care differentiation relies on clinical volume assessment, medication review, symptom evaluation, repeat sodium measurements, and targeted investigations, with referral when the cause is unclear or severe ¹.