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.