The parathyroid glands are four tiny glands in the neck. They make parathyroid hormone (PTH). PTH controls the amount of calcium, vitamin D, and phosphorus in your blood. Primary hyperparathyroidism is when there's too much PTH produced in your blood. It occurs when one or more of the glands are too active.
The job of PTH is to tell the body how to control calcium. Too much PTH means the body raises the amount of calcium in the blood. This leads to a problem where there's too much calcium in the blood (hypercalcemia) and not enough in the bones. This can cause serious health problems.