Increased expression of the CYP27B1 gene and thus calcitriol production is stimulated by hypocalcemia, hypophosphatemia, parathyroid hormone (PTH), growth hormone, insulin growth factor 1 (IGF-1), and prolactin, while it is dissimulated by hypercalcemia, hyperphosphatemia, high levels of calcitriol, and a bone-derived hormone fibroblast growth factor 23 (FGF23). Here, PTH is linked to hyperphosphatemia.