Inclusion Criteria:  Male or female, aged ≥ 18 years, inclusive, at the time of enrollment  Diagnosis of X-linked Hypophosphatemia confirmed by  documented PHEX mutation in either the patient, or in a directly related family member positive family history of XLH and symptoms of the disease or Phosphaturia + elevated serum levels of c-term FGF23 or iFGF23 and symptoms of the disease Written informed consent  Exclusion Criteria:  - Suspected of confirmed diagnosis of another phosphate wasting disorder