RESUMO
Syndrome of inappropriate antidiuretic hormone (SIADH) is the most common cause of hyponatraemia. There are many causes of SIADH, but investigation tends to focus around the most common causes-particularly diseases of the brain and lung, malignancy and medication-induced SIADH [Ellison and Berl (2007, The Syndrome of Inappropriate Antidiuresis. N Engl J Med., 356, 2064-72]. We describe a case of SIADH secondary to atonic bladder in an 83-year old woman, which was discovered on MRI of the abdomen, performed for further characterisation of a known pancreatic lesion. Insertion of a urinary catheter alleviated retention and resulted in prompt resolution of hyponatraemia. This is an under-recognised cause of this common condition, with important implications for investigation and management.