A 61-year-old man was referred to the Endocrinology clinic with a 5-month history of hyponatraemia. It was first detected as part of a routine test, which revealed a serum sodium of 124 mmol/l. His serum osmolality was 253 mOsmol/kg, urine sodium was 32 mmol/l and urine osmolality 317 mOsmol/kg. His results suggested SIADH. He already had a chest X-ray and brain MRI scan done which were reported as normal. His thyroid function test and short synacthen test were normal. He had been commenced on fluid restriction and demeclocycline, however his sodium remained low with a mean of 122±3.4 mmol/l and when seen in clinic it was 118 mmol/l. Accordingly a CT scan of his chest was arranged. The patient was admitted to hospital and commenced on Tolvaptan (a selective vasopressin V2- receptor antagonist). After only two doses of 15 mg/day his sodium came up to 132 mmol/l within 48 h and the Tolvaptan was stopped. Over the next 6 days his sodium remained stable between 128 and 133 mmol/l and he was discharged on fluid restriction. His CT scan revealed a 2.2 cm lesion in his left lung and a biopsy confirmed small cell carcinoma. Nine days after discharge a repeat test showed his sodium was down again to 116 mmol/l.
This case illustrates the importance of excluding an underlying malignancy in a patient with significant hyponatraemia even if the chest X-ray is reported as normal. It also demonstrates the efficacy of Tolvaptan in rectifying hyponatraemia in SIADH.