• Hyponatraemia and reduced plasma osmolality
• Urine sodium concentration >20 mmol/L and urine osmolality greater than plasma osmolality or not maximally dilute
• No oedema or signs of hypovolaemia
• Normal renal, thyroid and adrenal function
• The patient is not taking diuretics or purgatives

Causes of SIADH

Malignant disease

• Small cell carcinoma of bronchus, thymoma, lymphoma, sarcoma, mesothelioma, carcinoma of pancreas and duodenum

Chest disorders

• Pneumonia, tuberculosis, empyema, asthma, pneumothorax, positive-pressure ventilation

Neurological disorders

• Meningitis, encephalitis, head injury, brain tumour, cerebral abscess, subarachnoid haemorrhage, Guillain–Barré syndrome, acute intermittent porphyria

Drugs

• Antidepressants, carbamazepine, cytotoxics, MDMA (‘ecstasy’), NSAIDs, opioids, oxytocin, phenothiazines, thiazides, vincristine

Others

• Post-operative state
• HIV infection
• Idiopathic
• Reset osmostat