Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

Key Features

• A diagnosis of exclusion

• Euvolemic hyponatremia

• Other causes of hyponatremia should be ruled out, including

  • Low effective circulating volume

  • Decreased solute intake

  • Cortisol deficiency

  • Severe hypothyroidism

• See Hyponatremia

Clinical Findings

• Characterized by

  • Hyponatremia

  • Decreased plasma osmolality (< 280 mOsm/kg)

  • Euvolemia

  • Absence of heart, kidney, or liver disease

  • Normal thyroid and adrenal function

  • Urine sodium usually > 20 mEq/L

Diagnosis

• Clinical

• Patients with SIADH may have low blood urea nitrogen (BUN) (< 10 mg/dL [3.6 mmol/L]) and hypouricemia (< 4 mg/dL [238 mcmol/L]), which are not only dilutional but result from increased urea and uric acid clearances in response to the volume-expanded state

Treatment

• See Hyponatremia