Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

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

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

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

See Hyponatremia