FGF23 and autosomal dominant hypophosphatemic rickets: TIO is usually caused by phosphaturic mesenchymal tumors (PMTs) of mixed connective tissue variant type (MCT) that produce and secrete fibroblast growth factor 23 (FGF-23).(3,4) FGF-23 is a 32-kDa protein identified in 2000 as the protein mutated in autosomal dominant hypophosphatemic rickets (ADHR)(5–7) and recognized as an important regulator of phosphate and vitamin D metabolism.