Tumor-Induced Osteomalacia in the Setting of Normal FGF-23

Program: Abstracts - Orals, Poster Previews, and Posters
Session: FRI 368-390-Metabolic Bone Disease Case Reports (posters)
Clinical
Friday, April 1, 2016: 1:15 PM-3:15 PM
Exhibit/Poster Hall (BCEC)

Poster Board FRI 381
Leah Elizabeth Kang*, Pedram Javanmard and Anoop Kapoor
Stony Brook University Hospital, Stony Brook, NY
Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by severe hypophosphatemia and osteomalacia that occurs in association with a tumor. These tumors are typically benign, however, are small in nature and difficult to locate.

A previously healthy 19-year-old male was referred to our clinic after suffering bilateral femoral neck fractures. As per orthopedic surgery, bones appeared soft during repair and X-rays showed osseous abnormalities. He endorsed aches and pains. Physical examination revealed a well-nourished male with normal muscle strength. Initial tests revealed a severely low phosphorous of 1.4 (2.5-4.5 mg/dL), Mg 2.2 (1.5-2.5 mg/dL), Ca 9.5 (8.9-10.4 mg/dL), BUN 10 (7-20 mg/dL), and Cr 0.66 (0.6-1.26 mg/dL). To evaluate the etiology of hypophosphatemia, further workup showed an inappropriately low vitamin D 1,25 (OH)2 of 16 (18-72 pg/mL), low vitamin D 25(OH) 18 (30-100 ng/mL), normal iPTH, inappropriately normal 24 hour urine phosphorous 385 (360-1600 mg/24h), low 24 hour urine calcium 12 (55-300 mg/24h), elevated alkaline phosphatase 319 (39-117 units/L), and elevated bone isoenzyme 75% (28-66%). Hypophosphatemic osteomalacia was diagnosed, and Neutra Phos (2g/day), calcitriol (0.5 μg BID), and ergocalciferol (50,000 IU biweekly) were initiated. PHEX gene was negative, making X-linked hypophosphatemic rickets unlikely. DXA showed very low bone density, with Z scores of -3.9 (L-spine) and -4.7 (forearm). Bone scan revealed multiple chronic rib fractures. Despite aggressive phosphorous repletion, the maximum phosphorous level achieved was 2.3 (2.5-4.5 mg/dL). Testing was then initiated to evaluate for TIO. Fibroblast growth factor 23 (FGF-23) was normal at 164 (<180 RU/mL), and MRI/PET showed a FDG avid, benign appearing 0.6 x 0.9 cm cortical lesion in the distal right anterior tibial shaft. He was referred orthopedic surgery to have excision of the lesion. Surgical pathology from excision of right distal tibia revealed a phosphaturic mesenchymal tumor (1.5 x 0.9 cm) with focal cortical breakthrough and extension into the adjacent soft tissue with no involvement of the medullary cavity, confirming the diagnosis of TIO. Postoperative laboratory results showed a normal phosphorous of 4.6 and alkaline phosphatase 251 off of phosphorous supplements.

The diagnosis of TIO is a challenging one in that the causative tumor is typically small and difficult to locate. Serum FGF-23 levels were normal in this case, suggesting that TIO may be mediated by other tumor-secreted phosphatonins. It is prudent to consider TIO early in the differential diagnosis of hypophosphatemic osteomalacia, and to initiate the search for an underlying tumor even with normal FGF-23 levels.

Nothing to Disclose: LEK, PJ, AK

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