The parathyroid glands are four small glands located in the neck which produce parathyroid hormone, a hormone involved in the regulation of calcium and phosphate levels. Overactivity of one or more of the glands results in excessive parathyroid hormone production.
Hyperparathyroidism may be:

• primary - caused by problems in the glands themselves and leads to high levels of calcium in the blood (hypercalcaemia) or
• secondary - caused by low levels of calcium in the body.