Practice question

A 68-year-old man is found to have hyponatraemia (Serum Sodium 125 mmol/L) during a routine blood test. He is asymptomatic. His serum osmolality is 260 mOsm/kg (low), urine osmolality is 400 mOsm/kg (high), and urine sodium is 50 mmol/L (high). He is clinically euvolaemic (no oedema or dehydration). Thyroid and adrenal function are normal. He recently started taking Citalopram for depression. What is the most likely cause of his hyponatraemia?