The purpose of the study is to gain a better understanding of the effect of iron on fibroblast growth factor 23 (FGF23) in the inherited disorder, autosomal dominant hypophosphatemic rickets (ADHR). ADHR is an inherited disorder in which the body makes too much FGF 23 and causes low blood phosphorus levels and bone problems such as rickets (bowed legs in children) or bone pain and weakness in adults. This study is to test whether or not giving iron helps correct the high FGF23 and there by correcting the phosphate problem.

Iron will be provided in an open label treatment to all enrolled subjects. Iron levels will be monitored in blood and doses adjusted with the target of getting the iron levels to or a little above 100 mcg/dl.

The study will look to see if there is a decrease of FGF23 level. It will also look at how long does it take to decrease the level of FGF 23 and how long it takes for the serum and urine phosphate to normalize.

Inclusion Criteria:

• FGF Mutation in either Arginine 176 or arginine 179
• able and willing to provide consent or have a parent that is able/willing to consent, if a minor
• either serum iron <50mcg/dl (regardless of phosphate or intact FGF23 concentration); or iron between 500 and 100mcg/dl with serum phosphorus value below 3.0mg/dl for adults or less than or equal to 0.5 mg/dl the lower limit of normal for age in children and intact FGF23 about 30pg/ml
• age >2 years
• May be receiving treatment with phosphate and calcitriol, but must be willing to undergo dose adjustments by the investigators if iron resolves the phosphate wasting defect.

Exclusion Criteria:

• malignancy within the last 5 years, except treated squamous or basal cell skin carcinoma
• terminal illness/hospice.
• severe end-organ disease, e.g. cardiovascular, pulmonary, etc, which may limit ability to complete study.

estimated GFR <45ml/min/1.73m2, calculated using MDRD formula for adults or modified Schwartz equation for children

• pregnancy or plan on becoming pregnant