Familial hypophosphatemic vitamin D-resistant rickets or X-linked dominant hypophospha- temia presents with growth retardation, rachitic and osteomalacic bone disease, hypophosphatemia, renal defects in phosphate reabsorption and vitamin D metabolism, and dental findings. The patients often have multiple periodontal abscesses, but no evidence of dental caries, trauma, or periodontal disease on the corresponding teeth. Radiographic examination of teeth shows root dysplasia and enlarged pulp chamber. The histological examination shows marked globular dentin and increased predentin width.

The disease is caused by various mutations in the PHEX gene (Xp22.1) and is transmitted as an X-linked dominant trait with complete penetrance, but variable expressivity. PHEX encodes an endopeptidase expressed predominantly in bone and teeth that regulates fibroblast growth factor 23 (FGF-23) synthesis through unknown mechanisms. PHEX mutations lead to increased circulating levels of FGF-23, a phosphate-regulating hormone (phosphatonin), that leads to reduced renal phosphate reabsorption and consequently abnormal bone mineralization.