Parathyroid Hormone (PTH)
An 84 amino acid peptide hormone produced by the parathyroid glands that raises blood calcium levels by stimulating bone resorption and kidney calcium reabsorption. When given in intermittent pulses, PTH paradoxically stimulates bone formation. Teriparatide (PTH 1-34) and abaloparatide exploit this anabolic effect for osteoporosis treatment.
Technical Context
PTH (84 aa) is secreted from parathyroid glands in response to low serum calcium (detected by calcium-sensing receptor, CaSR). PTH acts on PTH1R (a class B GPCR) in bone and kidney: it mobilises calcium from bone (stimulating both osteoblasts and osteoclasts), increases renal calcium reabsorption, and stimulates renal 1,25-dihydroxyvitamin D production (which increases intestinal calcium absorption). The paradoxical bone-forming effect of intermittent PTH — the basis for teriparatide therapy — occurs because brief exposure preferentially activates osteoblastic (bone-building) pathways (via Wnt/β-catenin signalling) over osteoclastic (bone-resorbing) pathways. Continuous exposure, conversely, favours net bone resorption. Teriparatide (PTH 1-34) contains the biologically active N-terminal fragment.