A Case of Tumor-Induced Osteomalacia Masked by Parathyroid Carcinoma

Background: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by fibroblast growth factor 23 (FGF-23)-secreting tumors, typically of mesenchymal origin, leading to renal phosphate wasting and severe bone demineralization and fragility fractures. Diagnosing TIO remains a significant clinical challenge, particularly when coexisting mineral metabolism disorders, such as hypercalcemic hyperparathyroidism, are masking its clinical presentation. Case Presentation: A 74-year-old woman with fragility fractures, generalized bone pain, and nephrolithiasis was initially diagnosed with primary hyperparathyroidism due to concomitant hypercalcemia, hypophosphatemia, and elevated parathyroid hormone (PTH). Despite a successful parathyroidectomy, which normalized calcium levels, severe hypophosphatemia persisted due to renal phosphate wasting. High FGF-23 levels and subsequent functional imaging indicating a somatostatin receptor-positive lesion in the left popliteal fossa led to the diagnosis of TIO. Surgical resection immediately normalized FGF-23 levels, leading to a slower rise in phosphorus during follow-up. Histopathology revealed a tophaceous-like giant cell granulomatous reaction, recalling the earlier report by Prader. Conclusions: This case highlights that parathyroid disorders can coexist with TIO, and they may delay its diagnosis. In this circumstance, a high index of clinical suspicion is represented by the persistence of hypophosphatemia post-parathyroidectomy.