<prompt> Most common cause of Hyperparathyroidism is?Parathyroid adenomaParathyroid hyperplasiaThyroid carcinomaMedullary carcinoma thyroid  <response>: 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 . DAVIDSONS PRINCIPLES AND PRACTICE OF MEDICINE 22ND EDITION PAGE NO-769 