Elderly patient with cerebral disease who has hypo-osmolar hyponatremic with increased urine sodium (>40) and urine osmolarity >100, consistent with SIADH
Key idea: Causes of SIADH include ectopic ADH (small cell lung cancer), pulmonary disease, CNS disorders or head trauma (such as infarction), and drugs(cyclophosphamide)
Dehydration โ> Low urine sodium (<40) because the body is trying to retain as much sodium and water as possible
Water intoxication โ> Low urine osmolality (often <100) because so much water is being consumed and excreted that the urine is very dilute
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