Musculoskeletal Imaging

Parathyroid hormone

the secretory product of the chief cells of the parathyroid glands, which plays an important role in calcium and phosphorus metabolism. Its main functions are:

  • to stimulate and regulate bone remodelling, and

  • to influence the control of plasma levels of calcium.

    Parathyroid hormone has a direct effect on both bone and kidney; it also exerts indirect effects on the intestines. It can lead to bone resorption, bone formation, or both. At the cellular level, parathyroid hormone influences osteoclasts, osteoblasts, osteocytes and bone lining cells. Bone-regulating hormones have a dramatic effect on the appearance and function of osteoclasts. Rapid development of ruffled borders is one of the immediate actions of parathyroid hormone, leading to osteoclastic and osteocytic resorption.

    In the maintenance of calcium homeostasis, a fall in the serum level of calcium leads to a release of parathyroid hormone and a relatively rapid activation of osteocytes and osteoclasts. The effect is to promote bone resorption and mobilization of calcium from the skeleton. The initial effect of parathyroid hormone on the osteoblast appears to be inhibitory, but later the osteoblasts are stimulated, resulting in the incorporation of calcium into the skeleton and an increase in bone synthesis.

    In the kidney, the excretion of calcium, phosphate, bicarbonate, and other ions is regulated directly by parathyroid hormone.

    Parathyroid hormone also has a significant regulatory action on the formation of vitamin D and thus plays a role in rickets and osteomalacia.

    DR