The parathyroid glands are small glands located behind the larger thyroid gland in the neck. In the majority of people there are four parathyroid glands, however, some people may have additional or missing parathyroid glands. The parathyroid glands are important for maintaining adequate levels of blood calcium and phosphate for normal nervous system, muscle and bone function.
The parathyroid glands produce a hormone called parathyroid hormone (PTH). PTH is released from the parathyroid glands when the level of calcium in the bloodstream falls – the role of PTH is to increase the level of calcium in the bloodstream so that the nervous system (and muscles) can function adequately. PTH can act on the bone, the intestines and the kidney to increase the level of calcium in the blood.
In bone, PTH increases release of both calcium and phosphate. The bones are the body’s principal storage site of calcium, where it is stored in the form of calcium hydroxyapatite, which is mainly comprised of calcium and phosphorous. Hence, the action of PTH to increase the levels of calcium in the bloodstream will also cause the level of phosphate in the blood to rise. In bone, PTH acts indirectly on a type of cell known as an osteoclast, which is involved in breaking down bone. Osteoclasts do not have receptors for PTH, and thus cannot respond to its release directly. Instead, PTH acts directly on osteoblasts, the cells involved in building and depositing new bone, to change their signalling patterns. This indirectly stimulates the actions of osteoclasts; ultimately leading to increased breakdown of bone, and thus, increased release of both calcium and phosphate into the bloodstream.
The intestines are involved in absorbing the calcium we ingest with food and drink. Calcium is normally absorbed with an active form of vitamin D; a vitamin which is most commonly obtained from the sun. In the intestine, PTH increases the amount of active vitamin D which is produced, thereby increasing the amount of calcium which is absorbed and enters the bloodstream.
The kidneys determine how much calcium and phosphate are excreted in the urine (amongst many other substances in our blood). In the kidney, PTH acts to increase the amount of calcium that is reabsorbed into the bloodstream, and to increase the amount of phosphate that enters the urine.
Osteoporosis and PTH
The development of osteoporosis is a near certainty in patients with untreated hyperparathyroidism. Osteoporosis is a condition characterised by a loss of bone density and an increased risk of fracture. Bone density is lost due to the leeching of calcium from the bones at a rate faster than the body can re-deposit new minerals into the bone. In patients with hyperparathyroidism, the parathyroid glands are over-active and produce excessive amounts of PTH. This means that calcium and phosphate are constantly being released from the bone, making the bone weaker. In some cases, hyperparathyroidism may be diagnosed following a fracture.
It is important to treat hyperparathyroidism to prevent losses to bone density. The treatment of hyperparathyroidism depends on the exact cause of the problem. In some cases, surgical removal of the overactive parathyroid gland is performed. Soon after surgery, circulating levels of blood calcium fall, and there is a gradual improvement in the bone density over time. In other cases, osteoporosis medications may be prescribed to reduce the losses in bone density. Some cases of hyperparathyroidism are caused by a deficiency in vitamin D. In such cases, treatment with vitamin D supplements is indicated.
If you have questions about osteoporosis and the parathyroid glands or hyperparathyroidism contact your local doctor, who will arrange for you to see a thyroid surgeon.