PTH (parathyroid hormone) is an 84-amino acid peptide hormone secreted by the parathyroid gland that serves as the primary regulator of calcium homeostasis 1. The hormone elevates serum calcium by stimulating bone resorption and reducing renal calcium excretion through binding to PTH1R, a G protein-coupled receptor that activates dual signaling cascades 12. PTH engages both adenylyl cyclase/PKA and phospholipase C/PKC pathways, enabling context-dependent anabolic or catabolic effects on bone 2. In bone tissue, PTH acts directly on osteoblasts and osteocytes while indirectly regulating osteoclasts through RANKL/RANK/OPG and Wnt signaling pathways; intermittent administration promotes bone formation while continuous exposure drives resorption 1. Beyond skeletal regulation, PTH influences adrenocortical steroid secretion and modulates immediate-early gene transcription in renal cells 34. PTH secretion is regulated by extracellular calcium sensing through CASR and modulated by magnesium levels 56. Loss-of-function mutations cause familial isolated hypoparathyroidism 7. Recombinant human PTH(1-84) replacement therapy, FDA-approved in 2015, effectively treats hypoparathyroidism with sustained efficacy over 6+ years 7.