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SIADH — RACP Adult Medicine MCQ

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EasyNephrologySIADHRACP Adult Medicine

A 55-year-old man presents with a serum sodium of 118 mmol/L. He is euvolaemic. Serum osmolality is 255 mOsm/kg, urine osmolality 580 mOsm/kg, and urine sodium 55 mmol/L. TSH and cortisol are normal. He has small-cell lung carcinoma. What is the most likely diagnosis?

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Correct answer: D — SIADH

Euvolaemic hyponatraemia with concentrated urine (>100 mOsm/kg), elevated urine sodium (>30 mmol/L), normal thyroid and adrenal function, in a patient with small-cell lung carcinoma is classic SIADH. Small-cell lung cancer is the most common malignant cause. Treatment includes fluid restriction, and if severe/symptomatic, hypertonic saline.

Reference: eTG Endocrine – 2024 – Hyponatraemia; European Hyponatraemia Guidelines 2023