A 62-year-old male smoker develops a small cell carcinoma of the lung. He presents with hyponatremia, concentrated urine with elevated urine sodium, and euvolemia. The most likely underlying mechanism is:

• A Ectopic PTHrP production causing renal tubular sodium wasting
• B Ectopic ACTH production causing secondary hyperaldosteronism
• C Ectopic calcitonin production inhibiting renal calcium reabsorption
• D Ectopic ADH (AVP) production by tumor cells causing SIADH ✓

Explanation

Small cell carcinoma (neuroendocrine origin) is the tumor most classically associated with ectopic ADH secretion causing SIADH. The neuroendocrine tumor cells synthesize and release AVP autonomously, leading to euvolemic hyponatremia with inappropriately concentrated urine and elevated urine sodium. PTHrP-mediated hypercalcemia is the most common humoral paraneoplastic syndrome but is more characteristic of squamous cell carcinoma. Ectopic ACTH causing Cushing syndrome is also seen with small cell carcinoma but does not explain hyponatremia.

Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.