A confounding factor in our study is that both cases of XLH‐HOPT had CKD unlike the cases in the FDH group (Table 1).(13, 14) Given that both FGF23 and serum phosphate increase with the decline in eGFR and given that there is dysregulated production of FGF23 in XLH, it is possible that CKD might explain both the TmP/GFR results and the FGF23 results in the two cases of XLH‐HOPT. This evidence concerns the gene FGF23 and chronic kidney disease.