PTH
Parathyroid hormone (PTH) is an 84-amino-acid peptide secreted by parathyroid chief cells and the master regulator of systemic calcium and phosphate homeostasis. Through the Gs-coupled PTH1 receptor it acts on bone to stimulate osteoblastic RANKL expression and osteoclastic resorption, and on kidney to enhance distal calcium reabsorption, suppress phosphate reabsorption and activate vitamin D hydroxylation. Secretion is exquisitely tuned by the calcium-sensing receptor, falling when calcium rises. While chronic PTH excess drives bone loss in hyperparathyroidism, intermittent administration of the 1-34 fragment (teriparatide) exerts net anabolic effects and is used to treat severe osteoporosis. PTH assays are central to diagnosing calcium disorders and secondary hyperparathyroidism in renal disease.
