Pathologic Femur Fracture Due to a Brown Tumor in a Patient With Secondary Hyperparathyroidism and Vitamin D-Resistant Rickets

Vitamin D-resistant rickets is the common clinical outcome of multiple genetic mutations that alter the regulation of phosphorus and vitamin D metabolism, mainly through their effects on fibroblast growth factor 23 (FGF-23). These diseases typically present in childhood with the classic physical examination finding of nutritional rickets, such as genu varum/valgum and rachitic rosary. Treatment, which is aimed at improving severe bone disease with vitamin D and phosphorus supplementation, can cause secondary hyperparathyroidism and/or kidney failure from nephrocalcinosis over the life of the patient. Although FGF-23 has been shown to downregulate parathyroid hormone in vitro, its effect on parathyroid secretion in disease states such as chronic kidney disease and X-linked hypophosphatemic rickets is unclear because elevations in FGF-23 and parathyroid hormone levels characterize both of these disease states. We describe a case of vitamin D-resistant rickets that presented with a femur fracture through a brown tumor. Radiographs show the combination of severe bony abnormalities associated with both long-standing hyperparathyroidism and vitamin D-resistant rickets.