A 45-year-old male presented to the emergency department with acute malaise and fatigue. He had a past medical history of depression, anxiety and childhood bladder surgery for atonic bladder and was not prescribed any regular medications. He was clinically euvolaemic and his serum sodium was 121 mmol/L. Full blood examination, liver function tests, other electrolytes, creatinine, urea and tests for secondary causes of hyponatraemia were normal. Paired serum and urine osmolality were 252 mOsm/kg and 261 mOsm/kg respectively, with urine sodium 143 mmol/L. CT brain was unremarkable and CT chest revealed no significant findings.
A diagnosis of idiopathic syndrome of inappropriate antidiuretic hormone (SIADH) was made and the patient was treated with a 750 mL fluid restriction which, over the subsequent year, maintained serum sodium between 128 and 134 mmol/L.
Two years after the initial diagnosis, the patient’s hyponatraemia became progressively more challenging to manage. Episodes of pain, or the administration of intravenous fluids, predictably precipitated severe hyponatraemia. Serum copeptin was dramatically elevated (>5000 pmol/L) on multiple occasions (reference range: non-water deprived, non-fasting adults <16.3 pmol/L). Tolvaptan therapy has been effective at maintaining safe levels of serum sodium.
This case study explores a novel cause of SIADH. We suggest a more stringent approach to the investigations of apparent idiopathic SIADH in younger subjects.