FGF23 and hypophosphatemia: TIO is caused by overproduction of FGF23 by small, typically benign mesenchymal tumors, leading to fragility fractures and diffuse bone and muscle pain.(18) Laboratory findings are similar to those of the FGF23 excess syndromes described above, with hypophosphatemia, renal phosphate wasting, and low to inappropriately normal 1,25(OH)2D3 for the degree of hypophosphatemia, however, without a previous history of these lab abnormalities.(19) Serum calcium, 25OHD, and PTH levels are normal, though persistently low 1,25(OH)2D3 may lead to secondary hyperparathyroidism, with elevated ALP.(19)