The syndrome of inappropriate secretion of antidiuretic hormone

The syndrome of inappropriate secretion of antidiuretic hormone (SIADH) has diverse causes. These most commonly relate to intrathoracic or intracranial pathology, malignancy or drugs that can impair water excretion.

In the absence of diuretic drugs or another obvious cause, a diagnosis of SIADH is likely when the following results apply. These are a:

• serum sodium concentration lower than 130 mmol/L
• serum osmolality lower than 275 mmol/kg
• urine osmolality higher than 100 mmol/kg
• urine sodium concentration higher than 30 mmol/L.

The diagnosis can generally be established from the clinical context and the relationship between urine and serum osmolality, which should be assessed in concurrent samples. Assaying plasma arginine vasopressin (AVP) (also known as antidiuretic hormone) is usually not required. Serum urea, uric acid and potassium concentrations are often low.