The Impact of Phosphate Metabolism on Healthy Aging

Inclusion Criteria:

* Children above the age of 13 years
* Younger and older adults with XLH and HHRH with confirmed NPT2c mutations affecting both copies of the NPT2c gene (HHRH) or one copy of the PHEX gene (XLH)
* Be willing to provide access to prior medical records to determine eligibility including imaging, biochemical, medical, and surgical history data
* Be willing and able to complete all aspects of the study
* Be willing to adhere to the study visit schedule and comply with the assessments (in the opinion of the investigator).

Exclusion Criteria:

* Subjects will be excluded, if they are children younger than age 13 years
* Subjects that have other diseases likely to impact bone and mineral metabolism (e.g. renal, hepatic, gastrointestinal disorders, and malignancy),
* Subjects that are currently pregnant,
* Subjects that received medical therapy or developed any condition, which in the opinion of the investigator, could present a concern for either subject safety or difficulty with data interpretation.
* Subjects will be excluded from Aim 2, if they are unable to tolerate supplemental phosphate.