Phase 1 · Understand the Body — Why the disease behaves the way it does.
The One-Line Truth
The whole disease in one chain
Everything on this page exists to explain this line.
DI happens when your body makes too little ADH or your kidneys ignore it, so water floods out in your pee.
Water Tap, Sensor, and Valve Gone Wrong
Your kidneys are like a bathtub with a smart drain valve. Normally, a sensor (your brain) watches how salty the bathwater is. When water gets too salty, the sensor tells the valve to close so water stays in. When water gets too dilute, the sensor tells the valve to open so water drains out. In DI, the sensor breaks (central) or the valve stops listening (nephrogenic), so the valve jams open and bathwater floods out no matter what—your blood gets dangerously salty because you lose pure water but keep the salt. In SIADH, the opposite happens: the sensor gets stuck on 'close,' so the valve clamps shut even when the bathwater is already too dilute, and water pours in from outside, drowning your blood in freshness and washing away sodium.
Endocrine Society Clinical Practice Guideline on Diabetes Insipidus · American Association of Clinical Endocrinology (AACE) SIADH/Hyponatremia Guidelines · UpToDate Evidence-Based Clinical Decision Support · The New England Journal of Medicine (NEJM) Review on Disorders of Water Balance · reviewed 2026-07-07