FGF23 and hypophosphatemia: The biochemical hallmark of TIO includes renal phosphate wasting-induced hypophosphatemia, elevated levels of parathyroid hormone (PTH) and alkaline phosphatase (ALP), inappropriately normal or frankly low 1, 25-dihydroxyvitamin D, and inappropriately normal or elevated FGF23 levels (1, 2).