causes of hyponatraemia

Clinical answer with reasoning, red flags and references. Clinically reviewed by Dr Kola Tytler MBBS CertHE MBA MRCGP.

Dr Kola Tytler MBBS CertHE MBA MRCGPClinical Lead • iatroX

Hyponatraemia can arise from various causes, often multifactorial, making interpretation of diagnostic tests like urinary sodium concentration and osmolality sometimes difficult . Common contributing factors include acute illnesses and certain medications . If a person is taking a medication that may be causing hyponatraemia, it should be stopped if appropriate, with a repeat serum sodium measurement taken after two weeks . If a medication cannot be stopped, specialist advice should be sought to discuss monitoring or referral . If the hyponatraemia persists after stopping a medication, another underlying cause should be assessed, or the person referred to an endocrinologist .

In cases of acute illness contributing to hyponatraemia, treating the underlying problem and rechecking the serum sodium after two weeks, or sooner based on clinical judgement, is recommended . For asymptomatic, mild hyponatraemia (serum sodium 130–135 mmol/L), primary care management may be appropriate, ensuring a repeat serum sodium measurement is taken to rule out a rapidly decreasing concentration, which would necessitate hospital admission .

Severe or acute onset hyponatraemia (less than 48 hours duration or serum sodium <125 mmol/L), or symptomatic hyponatraemia, requires immediate hospital admission . For asymptomatic, moderate hyponatraemia (serum sodium 125–129 mmol/L), discussion with an endocrinologist regarding admission or referral is advised . An urgent 2-week wait referral is indicated if malignant disease is suspected as a cause of SIADH . Referral to an endocrinologist is also recommended if SIADH or another endocrine cause is suspected, or if reset osmostat syndrome or cerebral salt-wasting is suspected . Hyponatraemia related to endocrine disorders is uncommon and requires specialist input . Referral to an appropriate specialist is also recommended if hyponatraemia is thought to be caused by heart failure, kidney disease, or liver disease .

Secondary care management focuses on identifying and treating the underlying cause, with strategies dependent on the onset, symptoms, and volume status . For acute hyponatraemia with moderate to severe symptoms, hypertonic saline is used to restore serum sodium levels and reduce cerebral oedema . In cases of acute hyponatraemia with mild or no symptoms, non-essential parenteral fluids and medications provoking hyponatraemia are stopped, and treatment targets the underlying cause . For chronic hyponatraemia without significant symptoms, similar management of fluids and medications is advised, with treatment directed at the underlying cause . Fluid restriction is recommended for individuals with hypervolaemia or SIADH . Extracellular volume is restored with 0.9% saline for those with hypovolaemia . Tolvaptan is indicated for hyponatraemia secondary to SIADH and should be initiated in hospital or under specialist supervision . Recent literature highlights that hyponatraemia can be a complex condition with various underlying aetiologies [ ].

Key References

Related Knowledge Topics