Hormone. The chief cells of the parathyroids secrete a hormone called parathyroid hormone (PTH) or parathormone or also called Collip's hormone after the name of its discoverer.

1. This hormone regulates the calcium and phosphate balance between the blood and other tissues.
2. PTH inhibits collagen synthesis by osteoblasts and bone resorption by osteoclasts. It mobilises the release of calcium into the blood from the bones.
3. PTH increases calcium absorption from the intestines. It increases calcium resorption from the nephrons (and inhibits phosphate resorption) of the kidneys.
4. Parathormone regulates the metabolism of calcium and phosphate.
5. Parathyroids are under the feeding back control of blood calcium level. A fall in blood calcium stimulates them to secrete parathormone, a rise in blood calcium inbihits parathormone secretion from them.