Profound hyponatraemia following an idiosyncratic reaction to diuretics.

A case report of a79-year-old woman with a breast adenocarcinoma who presented with clouding of consciousness and anorexia with a hyponatraemia of 97 mmol/l is described. An initial diagnosis of secretion of inappropriate antidiuretic hormone (SIADH) was made on the clinical history and a urine: plasma osmolality ratio of 2.25. Further investigation revealed the correct diagnosis of a sodium-losing state with dehydration, and that her ADH level was normal. The actual cause of her hyponatraemia was the recent prescription of a thiazide diuretic. This case illustrates the unreliability of urine: plasma osmolality ratios for the diagnosis of SIADH.