Overactivity of the parathyroid glands, causing parathyroid hormone to be produced in amounts greater than normal. Parathyroid hormone controls the level of calcium in the blood, and overproduction of the hormone leads to a rise in blood calcium. If this is great, the person complains of exhaustion, depression, loss of appetite, weakness nausea, vomiting, constipation, and occasionally excessive thirst. Calcium is removed from the bones; pain and tenderness may follow, and the bones (being fragile) are more prone to fracture. This may lead to a diagnosis of the condition, since the X-ray appearances of the bones alter.

Increased excretion of calcium in the urine often leads to the formation of kidney stones; about 5 percent of persons with kidney stones are likely to show features of hyperthyroidism.

There are four parathyroid glands, one pair situated on each side of the thyroid gland, and they are normally about the size of a pea. Overactivity may be due to the development of a tumor of one of the glands (primary hyperparathyroidism) or may be the result of a chronically low level of calcium in the blood---such as occurs in kidney disease or deficient absorption of calcium from the intestine. Treatment of primary hyperparathyroidism is surgical removal of the tumor; treatment of secondary hyperparathyroidism requires correction of the underlying condition. It is possible to raise the blood calcium level by reducing the phosphate level, so that in chronic kidney failure a diet low in protein and phosphorus, with added aluminum hydroxide and vitamin D, may be recommended.