Most common cause of Hyperparathyroidism is?

Primary hyperparathyroidism is caused by autonomous secretion of PTH, usually by a single parathyroid adenoma, which can vary in diameter from a few millimetres to several centimetres. It should be distinguished from secondary hyperparathyroidism, in which there is a physiological increase in PTH secretion to compensate for prolonged hypocalcaemia (such as in vitamin D deficiency, p. 1126), and from teiary hyperparathyroidism, in which continuous stimulation of the parathyroids over a prolonged period of time results in adenoma formation and autonomous PTH secretion (Box 20.39). This is most commonly seen in individuals with advanced chronic kidney disease . DAVIDSON'S PRINCIPLES AND PRACTICE OF MEDICINE 22ND EDITION PAGE NO-769