FGF23 and autosomal recessive hypophosphatemic rickets: HR can be classified as FGF23-associated and non-associated, and HR is also subdivided into several forms, including X-linked hypophosphataemia (XLH), autosomal dominant hypophosphataemic rickets (ADHR), autosomal recessive hypophosphataemia (ARHR), and hereditary hypophosphataemic rickets with hypercalciuria (HHRH).