PROFOUND HYPONATRAEMIA FOLLOWING AN IDIOSYNCRATIC REACTION TO DIURETICS

SUMMARY A case report of a 79‐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.