Diabetes Insipidus

Your patient is putting out 8 liters of urine a day and still can't quench their thirst — but their blood sugar is perfectly normal. The problem isn't glucose; it's a missing hormone.

Diabetes insipidus (DI) results from either insufficient antidiuretic hormone (ADH) production (central DI) or the kidneys' inability to respond to ADH (nephrogenic DI). Without effective ADH action, the collecting ducts remain impermeable to water, producing massive volumes of very dilute urine — often 4–20 liters per day with a specific gravity below 1.005 and urine osmolality under 200 mOsm/kg. Serum sodium rises (often >145 mEq/L) and serum osmolality climbs above 295 mOsm/kg as the body loses free water. The water deprivation test differentiates DI from psychogenic polydipsia: in DI, urine stays dilute despite fluid restriction. Desmopressin (DDAVP) administration then distinguishes central from nephrogenic — central DI responds with concentrated urine; nephrogenic does not. Central DI is treated with intranasal or oral desmopressin. Nephrogenic DI management focuses on thiazide diuretics (paradoxically reduce urine output), low-sodium diet, and treating the underlying cause. Nursing priorities include strict I&O with hourly monitoring in acute cases, daily weights, assessing for dehydration (poor skin turgor, tachycardia, hypotension), replacing fluids to match output, and monitoring serum sodium and osmolality closely.

Key Distinctions

Don't confuse DI with SIADH — they are physiologic opposites. DI means too little ADH effect (dilute urine, high serum sodium); SIADH means too much ADH (concentrated urine, low serum sodium). Students mix up central and nephrogenic DI on treatment: desmopressin works for central DI only because the kidneys can still respond. Diabetes insipidus has nothing to do with blood glucose — the name misleads students into connecting it with diabetes mellitus.

Clinical Pearl

DI is "drying out" — Dilute urine, Insatiable thirst, high sodium. Think of ADH as the body's water-saving valve: DI means the valve is broken open.