A Normal FGF23 Does Not Preclude Tumor-Induced Osteomalacia

Area of Science:

• Endocrinology
• Bone Metabolism
• Oncology

Background:

• Tumor-induced osteomalacia (TIO) is a rare paraneoplastic condition characterized by impaired bone mineralization.
• It is primarily caused by excessive secretion of fibroblast growth factor 23 (FGF23) from mesenchymal tumors.
• Patients often present with profound hypophosphatemia and bone pain, leading to fragility fractures.

Purpose of the Study:

• To present a case of TIO in a middle-aged man with multiple fragility fractures.
• To highlight the diagnostic challenges and pitfalls in interpreting laboratory findings for TIO.
• To emphasize the importance of timely diagnosis and surgical intervention for TIO management.

Main Methods:

• A case report of a 45-year-old male with multiple fragility fractures and hypophosphatemia.
• Diagnostic workup included biochemical tests, genetic analysis for bone disorders, and imaging (PET/CT).
• Tumor resection followed by assessment of clinical and biochemical response.

Main Results:

• The patient experienced progressive functional deterioration due to multiple fragility fractures.
• Initial FGF23 levels were not significantly elevated, complicating the diagnosis.
• PET/CT identified a suprapatellar lesion, confirmed as a phosphaturic mesenchymal tumor upon biopsy.
• Post-resection, the patient showed marked improvement in physical function, pain, and resolution of hypophosphatemia.

Conclusions:

• TIO diagnosis requires careful exclusion of other causes of bone loss and meticulous interpretation of FGF23 levels.
• Prompt surgical resection of the causative tumor is the definitive treatment for TIO.
• This case underscores the importance of a high index of suspicion for TIO in patients with unexplained hypophosphatemia and fractures.