Tumor-Induced Osteomalacia With Normal Fibroblast Growth Factor-23 (FGF23) and Idiopathic Hypercalciuria

Tumor-induced osteomalacia (TIO) is a rare acquired paraneoplastic syndrome characterized by low serum phosphate, phosphaturia, inappropriately low/normal levels of serum calcitriol, and normal or elevated levels of fibroblast growth factor-23 (FGF23). Finding this mesenchymal tumor is challenging since it is usually benign, small, slow-growing, and is localized in the appendicular skeleton. We report a 58-year-old male patient who arrived at the endocrinology outpatient clinic due to slowly progressive low back pain and generalized weakness since the age of 48. On physical examination, only a reduced range of motion was noted. Laboratory tests revealed hypophosphatemia with normal parathyroid hormone (PTH) levels, normal serum calcium, high 24-hour urine calcium, normal 1,25-dihydroxyvitamin D levels, low renal threshold phosphate concentration (TmPO4/GFR), and high FGF23. The 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) reported a hypermetabolic extramedullary lesion in the C1-C2 vertebral bodies measuring 1.5 x 1.1 cm. Two months after the 18F-FDG PET/CT, complete excision of the cervical tumor was performed. The pathology ward reported a histiocytic mesenchymal neoplasm with accumulations of multinucleated giant cells compatible with phosphaturic mesenchymal tumor. After surgery, the patient's hypophosphatemia was completely resolved. With this, the diagnosis of TIO was confirmed. The patient remains asymptomatic, with normal phosphate levels at one year of followup. Hypophosphatemia due to renal losses in an adult patient is a challenging diagnosis and one must consider TIO, autosomal dominant hypophosphatemic rickets, fibrous dysplasia, and even Fanconi syndrome. FGF23 can be extremely useful during the diagnostic approach since acquired dependent hypophosphatemia (FGF23) 30 RU/mL) highly suggests TIO. In this case report, we want to highlight the paramount importance of adequate tumor screening in adult patients with acquired FGF23-dependent hypophosphatemia. TIO is a reversible cause of hypophosphatemia with potentially disabling consequences if left untreated. These manifestations are non-specific (bone pain and muscle weakness), while others are progressive and severely disabling (hone deformities and multiple fractures). In this case report, we want to highlight the paramount importance of adequate tumor screening in adult patients with acquired hypophosphatemia, and the crucial lead that phosphate and vitamin D regulating hormones (FGF23) have for suspecting TIO.