Severe hyponatraemia in an amiloride/hydrochlorothiazide-treated patient

A 85-year-old woman treated with, among other drugs, a thiazide diuretic presented with a severe hyponatraemia. She met several of the criteria for SIADH and, besides drugs, no cause for SIADH was found. After stopping the thiazide diuretic and restricting fluid intake the patient recovered fully. It was later proved that the thiazide was the cause of the water intoxication by rechallenging the patient with a single dose of amiloride/hydrochlorothiazide 5/50 mg. This "thiazide provocation test" showed its usefulness in the differential diagnosis of suspected SIADH. Moreover, the test demonstrated the paradoxal effect of thiazide diuretics to cause water retention in susceptible patients.