PTH and Hypocalcemia: In addition to this tissue specificity, FGF23 is regulated by other factors in addition to the Pi concentration, mainly calcitriol (positive regulation), calcium (hypocalcemia acts as a “brake” on FGF23 secretion) and PTH (unknown regulation; in animal models, it acts as a positive regulator, though in humans, the results of the studies are conflicting) [19]