The parathyroid glands are four small glands located in the neck, around the thyroid. They make and release a hormone that helps control the level of calcium in the blood, in the bones and throughout the body.
Calcium is very important for the functioning of the nervous and muscular systems, and it is the only element that has its own regulation system: the parathyroid glands. They continuously regulate the amount of calcium in the blood. If the calcium level goes down a bit, the parathyroid glands release parathyroid hormone (PTH) and remove calcium to release it into the blood. Conversely, when the calcium level in the blood is high, they stop producing PTH. PTH also has an effect on the kidneys and indirectly on the intestines (through Vitamin D).