MON-817 - Unmasking FGF23 Excess: A Case of Bilateral Femoral Neck Fractures

Introduction:
Fibroblast growth factor 23 (FGF23) is a key mediator in phosphate homeostasis that in excess results in hypophosphatemia and decreased bone mineralization. Tumor-induced osteomalacia (TIO) presents with excess FGF23 secretion by phosphaturic mesenchymal tumors (PMT), fractures, and nonspecific symptoms. We present a case of a young female who, after complaining of worsening hip pain, was found to have bilateral femoral neck fractures due to TIO.
Case summary:
A 35-year-old female noticed anterior right hip pain after jumping on a trampoline that worsened one year later after a child jumped into her lap. MR arthrogram of the right lower extremity was obtained to investigate for labral injury but instead revealed bilateral femoral neck fractures. These were rapidly repaired with open reduction and internal fixation. A DEXA performed showed Z-scores in the normal range. One year after fixation, the patient was evaluated for metabolic bone disease. Physical examination showed a height of 160 cm, weight of 111.4 kg, BMI 43.52 kg/m2, sclerae without blue discoloration, and legs without bowing. She denied history of other fractures, long-term glucocorticoid use, iron infusions, or nephrolithiasis. Labs revealed normal serum calcium 9.6 mg/dL (8.7-10.4), high alkaline phosphatase 148 U/L (46-116), low 1,25 (OH)2D < 8 pg/mL (18-78), 25-OH-vitamin D 41.9 ng/mL (20.0-80.0), low phosphate 1.2 mg/dL, high FGF23 101 pg/mL ( < 59), and inappropriately normal 24-hour urine phosphorus 846 mg/day (400-1300). Genetic testing was negative for phosphate disorders. Ga-68 DOTATE PET CT was initially reviewed, revealing multiple healing bilateral rib fractures without a source of FGF23. Technetium-99m oxidronate bone scan revealed a small focus of increased uptake in the right femur. After revisiting the DOTATE PET CT and comparing to the initial MR arthrogram, radiology identified a right femoral neck lesion concerning for PMT. During this time, musculoskeletal pain improved with calcitriol and sodium phosphate, without change in serum phosphate or FGF23. Burosumab was eventually initiated with phosphate correcting to low normal with high dose (90 mg monthly). Orthopedics has agreed to perform a right total hip arthroplasty but to improve outcome has requested weight loss before this revision. Treatment with tirzepatide has been initiated, with slow weight loss response.

Conclusion:
Both diagnosis and treatment were delayed by several years: first, as is common, by failure to recognize low serum phosphate, and thereafter, by post-surgical hardware that interfered with recognition of the femoral tumor location. Although rare, TIO should be considered in cases of hypophosphatemia and high or inappropriately normal FGF23. This case highlights the importance of comprehensive evaluation for metabolic bone disease, especially in young patients with atraumatic fractures.

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