Hyperfunction of the parathyroid glands resulting in the overproduction of parathyroid hormone. It is caused by parathyroid adenoma, parathyroid hyperplasia, parathyroid carcinoma, and multiple endocrine neoplasia. It is associated with hypercalcemia and hypophosphatemia. Signs and symptoms include weakness, fatigue, nausea, vomiting, constipation, depression, bone pain, osteoporosis, cystic bone lesions, and kidney stones.