Hyperfunction of the parathyroid glands resulting in the overproduction of parathyroid hormone. It may be primary or secondary; primary hyperparathyroidism 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. Secondary hyperparathyroidism is caused by the chronic stimulation of the parathyroid glands in patients with chronic renal failure, rickets, and malabsorption syndromes.