SAT-362 A Bone to Pick with FGF23

A 48 yo M with a past medical history of bilateral femoral neck stress fractures at age 44 status post closed reduction percutaneous pinning (CRPP) presented to the emergency room with bilateral lower extremity pain after a ground level fall. Plain films were notable for a right intertrochanteric fracture and left hip nonunion. The patient underwent removal of right CRPP hardware and revision fixation with a short TFN-Advanced Femoral Nailing System (TFNA) and fixation of the left femoral neck nonunion. Given his injuries following a ground level fall, endocrinology was consulted for recommendations. Labs during his hospital course were notable for hypophosphatemia as well as normal levels of alkaline phosphatase, serum calcium, and PTH. Serum protein electrophoresis and urine protein electrophoresis were within normal limits. A 24-hour urine calcium and phosphorous were done and the patient was found to have a low calculated renal phosphate threshold (TmP04/GFR), concerning for hypophosphatemic osteomalacia. An FGF23 level was drawn and was elevated, suggestive of an FGF23-induced hypophosphatemic osteomalacia. A low vitamin D 1,25-OH was also found consistent with this process, given interference of alpha-hydroxylation of vitamin D 25-OH from FGF23. The patient therefore underwent a PET scan to rule out a mesenchymal tumor-induced osteomalacia but his PET scan was largely unremarkable. Given that there were no issues with bone development or fractures during childhood, the suspicion for X-linked hypophosphatemia or autosomal recessive hypophosphatemia was low. The patient was believed to have either a small mesenchymal tumor not seen on PET scan or an autosomal dominant hypophosphatemia, which can manifest clinically during adulthood, and so was treated with daily Calcitriol and phosphate with subsequent correction of his serum phosphorous. Such a delayed presentation of hereditary hypophosphatemic osteomalacia, however, has been reported only in women either soon after puberty, during pregnancy, or after delivery (1). This case thus represents a possible autosomal dominant hypophosphatemia in a male patient or a rare case of tumor-induced hypophosphatemic osteomalacia. Reference: (1) Drezner MK, Whyte MP. Heritable renal phosphate wasting disorders. Genetics of bone biology and skeletal disease, 2nd ed. 2018; 761–782.