The word parathyroid means "next to the thyroid".
There are four parathyroid glands, each about the size of a small grain of rice. There are two on each side, usually behind or below the thyroid gland
Although they are located near the thyroid and share the same supplying blood vessels, their function is quite different. Parathyroid glands produce parathyroid hormone (PTH). PTH regulates calcium in the body, particularly by effects on bones.
Enlarged parathyroid glands produce too much parathyroid hormone, a condition known as hyperparathyroidism. This results in too much calcium being turned over in bones and leads eventually to osteoporosis.
It also results in a high level of calcium in the blood and this finding on a routine blood test is a frequent way of the disease first being detected. The high blood calcium can be associated with a range of symptoms and complications such as kidney stones and heart disease.
This problem may be cured by surgical removal of the enlarged gland or glands.
Further information on the parathyroid glands can also be found here.