Hyperparathyroidism occurs when one or more of your parathyroid glands are overactive. You have four of these tiny glands. Each one is about the size of a pea. They are found in your neck, behind your thyroid gland. These glands make parathyroid hormone. It regulates calcium, phosphorus, and vitamin D levels in your blood. Magnesium levels are important for correct functioning of the parathyroid glands. If these glands are overactive, they make too much parathyroid hormone. That raises the level of calcium in your blood.
Parathyroid hormone causes calcium to be released from your bones. This loss of calcium from the bones can lead to weak, brittle bones (osteopenia and osteoporosis) and bone fractures. When blood that is high in calcium goes through the kidneys, the calcium may be filtered into the urine. That can lead to kidney stones.