Parathyroid glands have calcium-sensing receptors that respond to extracellular calcium concentrations by secreting parathyroid hormone (PTH), which effects its action at the following 3 sites:
1. PTH acts on the bone to increase osteoclastic resorption, thereby liberating calcium and increasing systemic calcium concentrations.
2. PTH acts on the kidney to increase calcium reabsorption and phosphorus excretion and also to increase calcidiol conversion to calcitriol.
3. Calcitriol acts on the intestine to increase calcium and phosphorus absorption.
Courtesy of Dr. Kathleen Ham.