Hyperparathyroidism Which Developed after Resection of a Fibroblast Growth Factor 23-producing Tumor

A 53-year-old woman presented with bone pain and was diagnosed with osteomalacia because of hypophosphatemia, hyperphosphatasemia, bone pain, and radiographic findings. Because her intact-fibroblast growth factor 23 (FGF23) levels were high and contrast-enhanced computed tomography revealed a mass in the anterior ethmoid sinus, FGF23-related osteomalacia was diagnosed. The tumor was resected, but she developed hypercalcemia and elevated blood parathyroid hormone (PTH) levels. Primary hyperparathyroidism (PHPT) was diagnosed, and surgical resection was performed. To our knowledge, this is the first case of a FGF23-producing tumor complicated by PHPT. Because PHPT manifested after resecting the FGF23-producing tumor, FGF23 is thus considered to suppress PTH secretion in humans.