Hyperparathyroidism is overactivity of the tiny glands in the neck that are next to the thyroid gland (parathyroid glands). The parathyroid glands release a substance called parathyroid hormone that helps control the amount of calcium in the bloodstream.

When the parathyroid glands produce too much hormone, the amount of calcium in the blood increases. The body removes calcium from the bones, absorbs more calcium from the intestines, and releases less calcium into the urine. This can result in constipation, nausea, vomiting, fatigue, and other symptoms. It can also lead to kidney stones and weakening of the bones (osteoporosis).

Hyperparathyroidism may develop because of a tumour on the parathyroid gland, as a side effect of treatment for other conditions (such as Paget's disease), or for unknown reasons. Surgery may be needed to remove part or all of the parathyroid glands.