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.