The most common cause of high blood calcium is primary hyperparathyroidism, which results in overproduction of parathyroid hormone by the parathyroid glands. The parathyroid glands play a role in the body’s regulation of calcium levels in the blood.he most common cause of high blood calcium is primary hyperparathyroidism, which results in overproduction of parathyroid hormone by the parathyroid glands. The parathyroid glands play a role in the body’s regulation of calcium levels in the blood.