2nd May 2024

A 28-year-old woman presents to the clinic with a 3-month history of excessive thirst and frequent urination.

A 28-year-old woman presents to the clinic with a 3-month history of excessive thirst and frequent urination. She reports having to wake up multiple times at night to drink water and urinate. She denies any recent head trauma or surgery. Her medical history is unremarkable. Physical examination is normal. Urinalysis shows low urine osmolality and specific gravity.

Investigations:

Serum sodium: 145 mmol/L (135-145)

Serum osmolality: 300 mOsm/kg (285-295)

Water deprivation test: No increase in urine osmolality

Desmopressin challenge test: Increase in urine osmolality

Which is the most appropriate initial treatment?