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Tumour-Induced Osteomalacia FGF23 Diagnosis — ESENeph MCQ

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HardTubular DisordersTumour-Induced Osteomalacia FGF23 DiagnosisESENeph

A 48-year-old man presents with progressive proximal muscle weakness, bone pain, and multiple pathological fractures. Bloods: phosphate 0.3 mmol/L (severely low), calcium 2.20 mmol/L, ALP 350 U/L, PTH 8 pmol/L, 25-OH vitamin D 65 nmol/L, 1,25(OH)2 vitamin D very low. eGFR is 88 mL/min/1.73m2. FGF23 level is markedly elevated. CT shows a small mass in the left femur. What is the diagnosis?

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Correct answer: B — Tumour-induced osteomalacia

The combination of severe hypophosphataemia, low 1,25(OH)2D, markedly elevated FGF23, normal eGFR, and a localised mesenchymal tumour is diagnostic of tumour-induced osteomalacia (TIO). TIO is a rare paraneoplastic syndrome caused by phosphaturic mesenchymal tumours that secrete excess FGF23, which inhibits sodium-phosphate cotransporters in the proximal tubule (causing phosphaturia) and suppresses renal 1-alpha-hydroxylase (causing low calcitriol). Treatment is surgical excision of the tumour, which is curative. Octreotide scanning or DOTATATE PET/CT can help localise occult tumours. If unresectable, treatment with phosphate supplements and calcitriol is required. Burosumab (anti-FGF23 antibody) is available for TIO.

Reference: Jan de Beur 2005 – Tumour-Induced Osteomalacia; Imanishi et al 2022 – TIO Diagnosis and Treatment