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Drug-Induced SIADH Sequential Withdrawal — ESENeph MCQ

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ModerateElectrolyte DisordersDrug-Induced SIADH Sequential WithdrawalESENeph

A 55-year-old woman is found to have serum sodium 118 mmol/L. She is euvolaemic. Serum osmolality 255 mOsm/kg, urine osmolality 580 mOsm/kg, urine sodium 55 mmol/L. She takes sertraline and carbamazepine. TSH and cortisol are normal. Which investigation is most helpful to determine the relative contribution of each drug?

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Correct answer: A — Trial withdrawal of carbamazepine first

Both sertraline (SSRI) and carbamazepine cause SIADH, but their mechanisms differ. Carbamazepine both stimulates ADH release AND potentiates its renal action. In practice, carbamazepine is considered the more potent cause and should be trialled off first (switching to an alternative anticonvulsant such as levetiracetam). Serum ADH levels are unreliable in clinical practice due to assay variability and pulsatile secretion. A water deprivation test is used for diabetes insipidus, not SIADH. Sequential drug withdrawal, starting with the more likely culprit, is the pragmatic clinical approach.

Reference: Verbalis et al 2013 – Hyponatraemia Treatment Guidelines; BNF 2024 – Carbamazepine ADH Effects