Parathyroid function

The parathyroid glands are the glands in charge of the calcium in your blood, and they do it with a hormone that has a long name: parathyroid hormone, almost always shortened to PTH. When blood calcium drops, their chief cells release PTH.

PTH raises calcium along several routes at once. It pulls calcium out of bone by switching on the osteoclasts, the cells that break bone down; at the same time it slows the osteoblasts, which are the ones that lay it down; and it tells the kidney to reclaim more calcium instead of letting it go. Once calcium has risen, PTH shuts itself off. And if it rises too far another hormone steps in, calcitonin, which comes from the thyroid and pushes it back down. The two work like the thermostat in a house, which turns the heat on when it is cold and off once the room is warm enough; here what is kept at its set point is calcium.