Diabetes insipidus, syndrome of inappropriate ADH secretion (SIADH)

Big picture

This topic is about water balance disorders caused by abnormal antidiuretic hormone (ADH/arginine vasopressin) action.

There are two opposite clinical pictures:

Disorder	ADH problem	Main water problem	Serum sodium tendency	Urine
Diabetes insipidus (DI)	Too little ADH effect	Water loss	Hypernatremia if patient cannot drink	Large volume, dilute urine
SIADH	Too much ADH effect	Water retention	Hyponatremia	Inappropriately concentrated urine

Exam shortcut:

DI = “dry patient, dilute urine.” SIADH = “wet physiology, low sodium, concentrated urine.”