Human parathyroid hormone(hPTH) is an 84 amino acid residue peptide and one of the main regulators in maintenance of calcium homeostasis. It acts primarily on kidney and bone cells stimulating calcium back resorption or calcium mobilization, respectively. The whole mechanism is still under discussion, but low-dose hPTH triggers cyclic AMP-dependent protein kinase in some populations of bone cells bearing PTH receptors, which stimulates the proliferation of osteoblasts.