Overview
SIADH (Syndrome of Inappropriate Antidiuretic Hormone) and Diabetes Insipidus (DI) are opposing disorders of ADH (vasopressin) regulation that produce mirror image sodium and fl...
SIADH (Syndrome of Inappropriate Antidiuretic Hormone) and Diabetes Insipidus (DI) are opposing disorders of ADH (vasopressin) regulation that produce mirror-image sodium and fluid imbalances. SIADH involves excess ADH → free water retention → dilutional hyponatremia (Na⁺ <135 mEq/L). DI involves absent or ineffective ADH → massive free water loss → hypernatremia (Na⁺ >145 mEq/L) and profound dehydration. Both conditions are missed when sodium changes are attributed to IV fluids or dietary intake without checking urine osmolality — the key differentiating test. Severe SIADH (Na⁺ <120 mEq/L) causes cerebral oedema and seizures. Severe DI with inadequate replacement causes hypernatremic encephalopathy, vascular collapse, and death. Top 3 Nursing Priorities: 1. Monitor serum sodium and urine output trends — small changes over hours signal impending crisis 2. In SIADH: restrict free water (800–1000 mL/day); in DI: replace free water aggressively (IV hypotonic solution or DDAVP) 3. Never correct hyponatremia faster than 8–10 mEq/L per 24 hours — osmotic demyelination syndrome (ODS/CPM) is irreversible Common NCLEX Trap: In SIADH, the serum sodium is LOW but the urine is concentrated (high urine osmolality, high urine...
