Your neck has four pea-sized glands called the parathyroid glands. Hyperparathyroidism happens when they don't make enough parathyroid hormone (PTH), are surgically removed, are damaged or when your body is resistant to that hormone. PTH controls the blood level of important vitamins and minerals such as vitamin D and calcium.
You may also have hypoparathyroidism because you have another condition that affects how much PTH is in your body, such as a low magnesium level.