What is syndrome of inappropriate antidiuretic hormone secretion (SIADH)?

It is a syndrome of renal sodium loss and hyponatremia resulting from inappropriate secretion of antidiuretic hormone. Diagnostic criteria for SIADH:

Essential criteria:

• Effective serum osmolality <275 mOsm/kg

• Urine osmolality >100 mOsm/kg at some level of decreased effective osmolality

• Urine sodium concentration >30 mmol/L with normal dietary salt and water intake

• Clinical euvolemia

• Absence of adrenal, thyroid, pituitary or renal insufficiency

• No recent use of diuretic agents

Supplemental criteria:

• Serum uric acid <4 mg/dL

• Serum urea <21.6 mg/dL

• Failure to correct hyponatremia after 0.9% saline infusion

• Fractional sodium excretion >0.5%

• Fractional urea excretion >55%

• Fractional uric acid excretion >12%

• Correction of hyponatremia through fluid restriction