A 35-year-old man reports drinking approximately 6 L of water daily because of anxiety about dehydration. He is alert, euvolaemic and not taking diuretics or antidepressants. Serum sodium is 116 mmol/L, serum osmolality is 258 mOsm/kg, urine osmolality is 74 mOsm/kg and urine sodium is 12 mmol/L. Which diagnosis best accounts for the hyponatraemia?
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