It is also important to check 25OHD levels, as normal values (as seen here) help differentiate TIO from nutritional vitamin D deficiency, where low 25OHD would lead to secondary hypophosphatemia with appropriately low 1,25 Vit D but without the FGF-23-driven renal wasting profile [13,14]. Here, FGF23 is linked to hypophosphatemia.