This case highlights the critical role of intact FGF-23 assays over C-terminal assays for improved diagnostic accuracy in TIO, the indicative value of hypophosphatemia with low or inappropriately normal 1,25 Vit D, the necessity of understanding phosphate homeostasis feedback loops, and the efficacy of burosumab therapy in cases of strong clinical suspicion despite elusive tumors. This evidence concerns the gene FGF23 and hypophosphatemia.