Parathyroid Hormone (PTH)

Parathyroid hormone (PTH) is an 84-amino-acid peptide hormone secreted by the parathyroid glands, small endocrine glands usually located behind the thyroid gland. Its principal physiological role is the regulation of extracellular calcium and phosphate concentrations, making it a central hormone in calcium homeostasis and skeletal mineral metabolism .^1,2

PTH secretion is controlled primarily by the concentration of ionized calcium in extracellular fluid. Calcium-sensing receptors on parathyroid cells detect changes in calcium concentration. When ionized calcium falls, PTH secretion increases; when calcium rises, increased activation of these receptors suppresses PTH release. This forms a negative-feedback system that helps keep blood calcium within a narrow physiological range .¹

PTH acts mainly on the kidneys and skeleton. In the kidney, it increases calcium reabsorption while decreasing phosphate reabsorption, thereby promoting phosphate excretion. It also stimulates production of calcitriol, the active hormonal form of vitamin D, which increases intestinal calcium absorption. In bone, sustained elevations of PTH can promote bone resorption indirectly through signaling from osteoblast-lineage cells that affects osteoclast activity .^1,2

PTH should not be confused with calcitriol or parathyroid hormone-related protein (PTHrP). Calcitriol is a vitamin D-derived hormone whose production is partly stimulated by PTH, whereas PTHrP is a related signaling protein produced in many tissues. PTH is the principal hormone secreted by the parathyroid glands for the regulation of calcium and phosphate homeostasis .^1,2