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Psychogenic Polydipsia — RACP Adult Medicine MCQ

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ModerateNephrologyPsychogenic PolydipsiaRACP Adult Medicine

A 22-year-old woman with schizophrenia presents with acute confusion. She has been drinking >15 L water daily. Serum sodium is 112 mmol/L, serum osmolality 238 mOsm/kg, urine osmolality 45 mOsm/kg, and urine sodium 8 mmol/L. What is the most likely diagnosis?

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Correct answer: E — Psychogenic polydipsia

Maximally dilute urine (osmolality <100 mOsm/kg) with very low urine sodium in the setting of excessive water intake indicates psychogenic polydipsia. The kidneys are functioning normally (appropriately diluting urine) but are overwhelmed by the volume of water consumed. This is common in psychiatric illness. Treatment is water restriction.

Reference: European Hyponatraemia Guidelines 2023