PTH and hyperinsulinemic hypoglycemia, familial, 4: Through DBP, it is delivered to the kidneys, where the enzyme alpha-hydroxylase (CYP27B1) oxidizes 25(OH)D to form 1,25-dihydroxyvitamin D (1,25-(OH)2D), which is the biologically active form of vitamin D. Additionally, the levels of the biologically active form of vitamin D (1,25-(OH)2D) are controlled and balanced in terms of activating its conversion to the active form by parathyroid hormone (PTH) and 1,25-(OH)2D itself in cases of deficiency, and it is inhibited by fibroblast growth factor 23 (FGF23), as well as the minerals calcium and phosphate [17].