Chronic hyponatraemia from SIADH — the correction-ceiling tightrope

Presentation

A 58-year-old woman is admitted from the clinic with a three-week history of progressive malaise, anorexia and confusion, and a serum sodium of 114 mmol/L found on routine bloods. She is euvolaemic on examination, with no oedema, no signs of dehydration, and a normal blood pressure. She is a long-term smoker, and a recent chest CT for a persistent cough showed a central right hilar mass with mediastinal nodes, biopsied as small-cell lung cancer.^[3]

Her serum osmolality is 252 mOsm/kg (low), her urine osmolality is 360 mOsm/kg (inappropriately concentrated) and her urine sodium is 64 mmol/L (inappropriately high). Her thyroid and cortisol are normal, and she takes no diuretics.^[3]