26th April 2024

A 30-year-old woman has been experiencing excessive thirst and a markedly increased urine output for the past 4 months.

A 30-year-old woman has been experiencing excessive thirst and a markedly increased urine output for the past 4 months. She reports drinking up to 4 litres of water daily and frequently waking up at night to urinate. She denies any significant past medical history or medication use. Clinical examination shows normal vital signs and no significant abnormalities. She is not displaying any signs of dehydration.

Investigations:

Serum sodium: 147 mmol/L (135–145)

Serum osmolality: 305 mOsm/kg (275–295)

Urine osmolality: 100 mOsm/kg (300–900)

Random blood glucose: 5.2 mmol/L (3.9–5.6)

Serum creatinine: Normal

Which is the most likely diagnosis?