An overactive parathyroid gland results in a condition called primary hyperparathyroidism, which can impact your health. Four small glands located behind the thyroid gland in your neck are called parathyroid glands. They make a hormone called parathyroid hormone (PTH). When your parathyroid glands are working normally, PTH keeps your calcium levels normal by moving the right amounts of calcium in and out of your bones. Primary hyperparathyoidism is caused by an enlargement of one or more of the parathyroid glands occasionally due to cancer of the parathyroid gland. In primary hyperparathyroidism, your parathyroid glands send out too much PTH to your body and your blood level of calcium becomes high. Sensipar lowers PTH by telling your parathyroid glands to stop releasing too much PTH into your blood. It also lowers your blood calcium levels.