The parathyroids are four glands, each about the size of a lentil, that sit behind the thyroid gland in the lower part of the neck.
Parathyroid glands control the level of calcium in the blood. They do this by controlling calcium loss from the bones and kidneys, and calcium absorption from the intestine.
In around 3 people per 1000 one or more of the parathyroid glands can become enlarged, causing a condition called hyperparathyroidism, in which the blood calcium level becomes raised.
The following section explains hyperparathyroidism in more detail.