A 48-year-old man presents with two years of worsening proximal muscle weakness, bone pain, and difficulty rising from a squat. Labs: calcium 9.2 mg/dL, phosphate 1.4 mg/dL, alkaline phosphatase mildly raised, PTH normal, 25-hydroxyvitamin D adequate, 1,25-dihydroxyvitamin D inappropriately low, marked urinary phosphate wasting. Which hormone mediates this disorder?

• A Parathyroid hormone related peptide
• B Insulin like growth factor 1
• C Calcitonin
• D Fibroblast growth factor 23 ✓

Explanation

Tumour-induced osteomalacia is caused by secretion of FGF23, usually from a small benign phosphaturic mesenchymal tumour. FGF23 suppresses renal sodium phosphate cotransporters and inhibits 1-alpha hydroxylase, producing hypophosphataemia, phosphaturia and low or inappropriately normal 1,25-dihydroxyvitamin B with normal calcium and PTH. Cure follows surgical excision of the tumour after localisation, often with functional imaging. PTHrP causes hypercalcaemia of malignancy, not renal phosphate wasting.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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