20th June 2023

A 64-year-old man presents to the emergency department with acute onset confusion

A 64-year-old man presents to the emergency department with acute onset confusion, with a recent history of weight loss, lethargy, and intermittent headaches. He has a past medical history of COPD, hypertension, and hiatus hernia. His regular medications include ramipril, omeprazole and a tiotropium inhaler. He has a 60-pack-year smoking history and drinks no alcohol. On admission, he is cachectic and unkempt, with clinical and biochemical signs of dehydration. His serum sodium is 119. Further biochemical testing is performed, and the results are as follows:

Serum osmolality 120mOsm/kg (275-295mOsm/kg)
Urinary sodium 112mEq/L
Urinary osmolality 200mOsm/kg (500-800mOsm/kg)

A CT head reveals multiple intracranial lesions consistent with metastases. A CT chest/abdomen/pelvis demonstrates a right hilar mass, widespread intrathoracic lymphadenopathy, and liver lesions consistent with metastatic malignancy of lung origin. A biopsy is obtained and confirms adenocarcinoma of lung origin.

What is the most likely underlying mechanism of his hyponatraemia?

Select the SINGLE most appropriate option from the list below. Select ONE option only.