A 58-year-old heavy smoker presents with weight loss, hyponatraemia (sodium 118 mmol/L) and euvolaemia. Bronchoscopic biopsy shows small cell carcinoma. The electrolyte abnormality results from which mechanism?
- A Ectopic secretion of ADH by the tumour ✓
- B Secretion of PTH-related peptide
- C Inappropriate production of calcitonin
- D Renal tubular damage from hypercalcaemia
Correct answer: A. Ectopic secretion of ADH by the tumour
Explanation
Small cell lung carcinoma is a classic cause of the paraneoplastic syndrome of inappropriate ADH secretion, producing euvolaemic hyponatraemia. PTHrP secretion causes humoral hypercalcaemia of malignancy, classically with squamous cell carcinoma of the lung, which is the key distractor because both are paraneoplastic endocrinopathies of lung cancer but produce opposite calcium pictures. Calcitonin excess is seen in medullary thyroid carcinoma.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
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