Parathyroid hormone (PTH), also called parathormone or parathyrin, is a hormone secreted by the parathyroid glands that is important in bone remodeling, which is an ongoing process in which bone tissue is alternately resorbed and rebuilt over time. PTH is secreted in response to low blood serum calcium (Ca2+) levels. PTH indirectly stimulates osteoclast activity within bone marrow, in an effort to release more ionic calcium (Ca2+) into the blood to elevate serum calcium (Ca2+) levels. The bones act as a (metaphorical) "bank of calcium" from which the body can make "withdrawals" as needed to keep the amount of calcium in the blood at appropriate levels despite the ever-present challenges of metabolism, stress, and nutritional variations. PTH is "a key that unlocks the bank vault" to remove the calcium. In consequence, PTH is vital to health, and health problems that yield too little or too much PTH (such as hypoparathyroidism, hyperparathyroidism, or paraneoplastic syndromes) can wreak havoc in the form of bone disease, hypocalcaemia, and hypercalcaemia.