PTH and secondary hyperparathyroidism: Hyperparathyroidism could also be classified into three categories: (1) primary hyperparathyroidism, which is primarily caused by adenomas of parathyroid glands; (2) secondary hyperparathyroidism, which is caused by resultant hyperplasia of parathyroid glands in response to hypocalcemia; (3) tertiary hyperparathyroidism, a term used to describe progressive stage of secondary hyperparathyroidism along with unresponsive flow of PTH.