FGFR1 and Hypocalcemia: Persistent hypocalcemia leading to hyperplasia of parathyroid cells and downregulation of VDRs and CaSR in the parathyroid glands increase PTH secretion [27]. It has been postulated that FGF-23 may decrease PTH secretion via the αKlotho-FGFR1 complex [20]. In advanced SHP, the Klotho-FGFR1 complex expression in parathyroid cells is impaired [28], leading to an increase in the inhibitory effect of FGF-23 resistance to PTH secretion.