SIADH
Formal Definition
Syndrome of Inappropriate Antidiuretic Hormone secretion — a disorder of water balance in which excess ADH (vasopressin) is released inappropriately relative to serum osmolality, causing pathological water retention, dilutional hyponatremia (serum sodium <135 mEq/L), and decreased serum osmolality (<280 mOsm/kg) in the absence of hypovolemia, adrenal insufficiency, hypothyroidism, renal failure, or diuretic use.
How It's Used on the Ward
"Syndrome of inappropriate antidiuretic hormone" or just "SIADH" — the patient is holding onto too much water and diluting their sodium; causes low sodium that can cause confusion, seizures; most commonly caused by small cell lung cancer (making ectopic ADH).
Example
""59-year-old male smoker with new-onset hyponatremia (serum Na 121 mEq/L), serum osmolality 252 mOsm/kg, urine osmolality 420 mOsm/kg (concentrated despite low serum osmolality), urine sodium 68 mEq/L, euvolemic on exam. CXR shows right hilar mass. Diagnosis: SIADH secondary to suspected small cell lung cancer. Correcting sodium slowly with hypertonic saline (3% NaCl) — goal 6-8 mEq/L in first 24h to avoid osmotic demyelination syndrome.""
Clinical Context
Common causes: small cell lung cancer (most common malignant cause), CNS pathology (stroke, meningitis, brain tumor, neurosurgery), pulmonary disease (pneumonia, COPD, asthma), medications (SSRIs, carbamazepine, cyclophosphamide, vincristine), pulmonary infection, pain, nausea. Diagnostic criteria: hyponatremia, serum osmolality <280, euvolemia, urine osmolality >100 (inappropriately concentrated), urine sodium >40, normal thyroid/adrenal function. Treatment: treat underlying cause, fluid restriction (first-line for mild), hypertonic saline (3% NaCl for severe <120 or symptomatic seizures), vaptans (tolvaptan — V2 receptor antagonist, for refractory cases), urea. CRITICAL: correct sodium slowly — max 6-8 mEq/L in first 24h, <10-12 mEq/L in first 48h — too fast causes osmotic demyelination syndrome (central pontine myelinolysis).