PTH and secondary hyperparathyroidism: An intact 1,25(OH)2D3-VDR system is important for both basal and PTH-induced osteoclastogenesis; 1,25(OH)2D3 administration inhibits PTH synthesis and parathyroid cell growth, thus rendering 1,25(OH)2D3 therapy effective in treating the secondary hyperparathyroidism of chronic kidney disease (CKD) [98].