Tumor-induced Osteomalacia: A Sherlock Holmes Approach to Diagnosis and Management

Tumor-induced osteomalacia (TIO) is a subtype of paraneoplastic syndrome associated with hypophosphatemia due to renal phosphate wasting in adults. The humoral factor responsible for clinical picture known as fibroblast growth factor 23 (FGF23) is most often secreted by benign yet elusive mesenchymal tumors, difficult to localize, access, and excise completely; rarely, they are multiple and malignant. Paradoxical inappropriately normal or low levels of 1,25-dihydroxyvitamin D in the setting of hypophosphatemia is due to suppressive effect of FGF23. The following case report describes a 31-year-old male with symptoms of multiple fractures and severe muscle weakness, hypophosphatemia with elevated tubular maximum reabsorption of phosphate/glomerular filtration rate with low active Vitamin D, prompted assay for C-terminal FGF23, which was elevated multifold. The tumor was localized with whole body 68-Gadolinium DOTANOC positron emission tomography-computed tomography fusion scan in the left nasal cavity with ipsilateral maxillary antrum. It was excised through transnasal approach and found to be mesenchymal tumor on histopathology. At 1 week of follow-up, serum phosphate became normalized without supplementation. The patient is in follow-up for further measurement of FGF23 level and signs of recurrence. Because the occurrence of such a condition is rare and most often misdiagnosed or mismanaged for years, it is important to recognize this condition in differential diagnosis as potential curative surgical option is a reality.