4th April 2024

Background

Highly sensitive calcium-sensing receptors (CaSR) on the parathyroid glands are responsible for detecting any changes in calcium concentration and regulating secretion of PTH. Together with calcitriol (1,25-dihydroxyvitamin D) and fibroblast growth factor 23 (FGF23), PTH regulates calcium and phosphate homeostasis. PTH acts to counter detected hypocalcaemia via three mechanisms:

• Increased bone resorption: targets osteoblasts and osteocytes, particularly in cortical bone, to enhance bone remodelling
• Decreased urinary calcium excretion: PTH increases calcium reabsorption in the distal tubule, as well as inhibiting proximal tubular reabsorption of phosphate
• Increased intestinal calcium and phosphate absorption: mediated by increased activated vitamin D