A condition of abnormally elevated output of parathyroid hormone (or PTH) triggering responses that increase blood calcium. It is characterized by hypercalcemia and bone resorption, eventually leading to bone diseases. PRIMARY hyperparathyroidism is caused by parathyroid hyperplasia or parathyroid neoplasms. SECONDARY hyperparathyroidism is increased PTH secretion in response to hypocalcemia, usually caused by chronic kidney diseases.