The only purpose of the parathyroid glands is to control calcium within the blood in a very tight range between 8.8 and 10.1 mg/dl (2.17 to 2.52 mmol/L). This is necessary because we use calcium molecules to perform many body functions. Parathyroid glands control blood calcium by taking calcium out of the bones when more is needed, or by allowing calcium go to into the bones when blood calcium is in the normal range. Thus, the parathyroid glands control the amount of calcium in the blood and have a dramatic role determining how strong and dense the bones are by controlling how much calcium is in the bones.
Although the four parathyroid glands are quite small, they have a very good blood supply so they can monitor the calcium level in the blood 24 hours a day. The parathyroid glands measure the amount of calcium present in the blood and react by making more or less of a hormone called “parathyroid hormone”, which we abbreviate as PTH. When the calcium level in the blood is too low, the cells of the parathyroids sense it and make more PTH. The PTH is secreted into the blood and it circulates to act in several places: the intestines, the kidneys, and the bones. Within the intestines, the extra PTH helps us absorb more calcium from our diet. Within the kidneys PTH prevents us from losing calcium into our urine. But the most important and most powerful action of PTH is to cause the bones to release calcium into the blood. Thus, if the blood calcium ever gets a little too low, the parathyroid glands sense it and make some PTH which goes to the bones and removes some calcium from the bones, thereby increasing the amount of calcium in the blood. Please watch our video animation to see this process at work!
When the calcium level in the blood is adequate (normal), the cells of the parathyroids make less PTH (or stop making it completely), thereby allowing calcium to stay within the bones. In a normal person with normal parathyroid glands, the parathyroid glands will turn on and off hundreds of times per day to keep the calcium level in the normal range so our brain and muscles function properly and our bones stay hard. Think of parathyroid glands like the thermostat in your house: you set a temperature and the thermostat turns the heater on and off many times per day to keep the temperature very near the set point. Those of us with normal parathyroid function will have blood calcium levels that are in the normal range (with little variability) because the parathyroid glands are turning on and off many times per day so the calcium in our blood and bones stays in a constant, healthy level.
For more detailed information on this topic: http://www.parathyroid.com/parathyroid-function.htm