Differential diagnosis of polyuria and polydipsia

1. Big picture

Polyuria and polydipsia are common symptoms, but the examiner wants one central idea:

First prove true polyuria, then decide whether the urine is full of solute or mostly free water.

The practical diagnostic split is:

Polyuria → measure urine volume → check glucose and electrolytes → urine osmolality/specific gravity → classify as osmotic diuresis vs water diuresis.

The most important causes for the state exam are:

1. Diabetes mellitus → osmotic diuresis from glucosuria.
2. Central diabetes insipidus → lack of antidiuretic hormone/arginine vasopressin.
3. Nephrogenic diabetes insipidus → kidney resistance to antidiuretic hormone.
4. Primary/psychogenic polydipsia → excessive water intake suppresses antidiuretic hormone.
5. Chronic kidney disease/polyuric renal failure → impaired concentrating ability.

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