The grouping of four small glands, about 5 mm in diameter, which lie to the side of and behind the THYROID GLAND. These glands regulate the metabolism of calcium and of phosphorus. If for any reason there is a deficiency of the secretion of the parathyroid glands, the amount of calcium in the blood falls too low and the amount of phosphorus increases. The result is the condition known as TETANY characterised by restlessness and muscle spasms – sometimes severe. The condition is checked by the injection of calcium gluconate, which causes an increase in the amount of calcium in the blood.

The most common cause of this condition (hypoparathyroidism) is accidental injury to or removal of the glands during the operation of thyroidectomy for the treatment of hyperthyroidism (see THYROID GLAND, DISEASES OF – Thyrotoxicosis). If there is over-production of the parathyroids, there will be an increase of calcium in the blood: this extra calcium is drawn from the bones, causing cysts to form with resulting bone fragility. This cystic disease of bone is known as OSTEITIS FIBROSA CYSTICA. Tumours of the parathyroid glands result in this overactivity of the parathyroid hormone, and the resulting increase in the amount of calciumin the blood leads to the formation of stones in the kidneys. The only available treatment is surgical removal of the tumour. Increased activity of the parathyroid glands, or hyperparathyroidism, may cause stones in the kidneys. (See KIDNEYS, DISEASES OF.)