PTH and X-linked hypophosphatemia: However, there were still 12.9% (8/62) of patients having serum iFGF23 levels within the reference range, which suggested that serum iFGF23 in patients with XLH is very likely associated with other metabolic factors, such as serum PTH, circulating α-Klotho, hypoxia, and inflammatory cytokines, in addition to serum phosphate (Courbebaisse and Lanske, 2018).