When blood-calcium levels are too high, the parathyroid glands produce less PTH. But sometimes one or more of these glands produce too much hormone, leading to abnormally high levels of calcium (hypercalcemia) and low levels of phosphorus in your blood.hen blood-calcium levels are too high, the parathyroid glands produce less PTH. But sometimes one or more of these glands produce too much hormone, leading to abnormally high levels of calcium (hypercalcemia) and low levels of phosphorus in your blood.