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Syndrome of inappropriate antidiuresis — ABIM Board MCQ

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ModerateEndocrinology/MetabolismSyndrome of inappropriate antidiuresisABIM Board

A 70-year-old with small-cell lung cancer has sodium 118 mEq/L, serum osmolality 248 mOsm/kg, urine osmolality 510 mOsm/kg and urine sodium 72 mEq/L. He is clinically euvolemic, takes no diuretic and has normal cortisol, thyroid and kidney function. What is the diagnosis?

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Correct answer: A — Paraneoplastic syndrome of inappropriate antidiuresis from small-cell cancer

The best answer is “Paraneoplastic syndrome of inappropriate antidiuresis from small-cell cancer”. The low serum osmolality confirms hypotonic hyponatremia, while concentrated urine and high urine sodium despite euvolemia show persistent antidiuretic action. Normal adrenal, thyroid and kidney function and absence of diuretics complete the exclusion-based diagnosis of SIAD. Primary polydipsia should produce dilute urine, hypovolemia would alter the examination, and pseudohyponatremia is not hypotonic.

Reference: JCEM Approach to Hyponatremia and SIAD: https://academic.oup.com/jcem/article/107/8/2362/6578514