FGF23 and X-linked hypophosphatemia: FGF23 is an osteocyte-derived phosphaturic hormone that suppresses the synthesis of 1,25-dihydroxyvitamin D (calcitriol), ultimately leading to hypophosphatemia and impaired bone mineralization in patients with disorders characterized by elevated FGF23 secretion, such as tumor-induced osteomalacia (TIO) or X-linked hypophosphatemia (XLH).