Diagnostic Journey in the Recognition of Oncogenic Osteomalacia

A 73-year-old woman with a remote history of successfully treated primary hyperparathyroidism was referred to our office because of multiple skeletal fractures and proximal muscle weakness. Prior laboratory evaluation demonstrated hypocalcemia, vitamin D deficiency, elevated PTH, and elevated alkaline phosphatase. Updated evaluation in our clinic additionally showed hypophosphatemia prompting measurement of serum fibroblast growth factor 23 (FGF23). FGF23 proved to be markedly elevated prompting consideration of tumor-induced osteomalacia. Positron emission tomography with Cu-64 oxodotreotide revealed an enhancing focus in the left thoracic spine that was confirmed on magnetic resonance imaging to be a 1.5-cm extradural mass. The patient underwent surgical resection of the mass by the neurosurgery spine service. Pathologic analysis demonstrated a 2.4-cm mesenchymal tumor; mRNA in situ hybridization was positive for FGF23. Laboratory studies 12 weeks postoperatively showed near normalization of the serum FGF23 as well as improvement in her other metabolic abnormalities and clinical symptomatology. This case exemplifies the necessity of measurement of serum inorganic phosphate and a high level of suspicion for hypophosphatemic osteomalacia in patients with numerous fractures beyond that expected for their degree of osteoporosis.