95 The Parathyroid Glands
Overview of the Parathyroid Glands
The parathyroid glands are small endocrine glands in the neck that produce parathyroid hormone.
Learning Objectives
Describe the location and function of the parathyroid glands
Key Takeaways
Key Points
- The parathyroid glands are four or more small glands, about the size of a grain of rice, located on the posterior surface of the thyroid gland.
- The major function of the parathyroid glands is to maintain the body’s calcium level within a very narrow range, so the nervous and muscular systems can function properly.
Key Terms
- calcitonin: A hormone that is produced primarily by the parafollicular cells of the thyroid. It acts to reduce blood calcium (Ca2+), opposing the effects of parathyroid hormone.
- parathyroid gland: One of four endocrine glands situated in the neck, usually on the posterior surface of the thyroid gland, that produce parathyroid hormone.
- parathyroid hormone: A hormone produced by the parathyroid gland that acts to increase blood calcium levels by stimulating osteoclasts to release calcium from the bone.
The parathyroid glands are small endocrine glands —approximately the size of a grain of rice—in the neck that produce parathyroid hormone. Humans usually have four parathyroid glands, which are usually located on the posterior surface of the thyroid gland, or, in rare cases, within the thyroid gland itself or in the chest.
Parathyroid gland: The parathyroid gland in relation to the thyroid gland. They are located on the posterior surface of the lobes.
The two parathyroid glands on each side that are positioned higher are called the superior parathyroid glands, while the lower two are called the inferior parathyroid glands. Occasionally, some individuals may have six, eight, or even more parathyroid glands.
Parathyroid glands control the amount of calcium in the blood and within the bones. The major function of the parathyroid glands is to maintain the body’s calcium level within a very narrow range, so that the nervous and muscular systems can function properly. When blood calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland are activated to release
parathyroid hormone (PTH) into the blood.
PTH modulates calcium and phosphate homeostasis, as well as bone physiology. PTH has effects antagonistic to those of calcitonin by increasing blood calcium levels by stimulating osteoclasts to break down bone and release calcium. PTH also increases gastrointestinal calcium absorption by activating vitamin D, and promotes calcium conservation by re-absorption in the kidneys.
Parathyroid Hormone
Parathyroid hormone maintains the body’s calcium levels by increasing the absorption of calcium from the bones, kidneys, and GI tract.
Learning Objectives
Explain the functioning of parathyroid hormone
Key Takeaways
Key Points
- Parathyroid hormone works in concert with another hormone, calcitonin, that is produced by the thyroid to maintain blood calcium levels.
- Parathyroid hormone acts to increase blood calcium levels, while calcitonin acts to decrease blood calcium levels.
- When blood calcium levels drop below a certain point, calcium-sensing receptors in the parathyroid gland are activated, and the parathyroid glands release parathyroid hormone into the blood.
- PTH acts on the bone to increase blood calcium levels by stimulating osteoclasts to break down bone and release calcium into the bloodstream; on the GI tract to increase the activity of the enzyme in the intestines that activates vitamin D; and on the kidneys to promote calcium reabsorption.
Key Terms
- vitamin D: A fat-soluble vitamin required for normal bone development and that prevents rickets; it can be manufactured in the skin on exposure to sunlight.
- osteoclast: A large multinuclear cell associated with the break down and resorption of bone.
- bone remodeling: The resorption by osteoclasts and replacement by osteoblasts in bones.
Parathyroid Glands
The parathyroid glands are small, pea-sized endocrine glands located on the rear side of the thyroid gland. The major function of the parathyroid glands is to maintain the body’s calcium level within a very narrow range, so that the nervous and muscular systems, which depend on calcium to transmit action potentials, can function properly.
When blood calcium levels drop below a certain point, the calcium-sensing receptors in the parathyroid gland are activated, and the parathyroid glands release parathyroid hormone (PTH) into the blood. PTH is a small protein hormone that is integral to the regulation of the level of calcium in the blood via the bone, kidneys, and intestines.
PTH works in concert with another hormone, calcitonin, that is produced by the thyroid to maintain calcium homoeostasis. Parathyroid hormone acts to increase blood calcium levels, while calcitonin acts to decrease blood calcium levels.
This interaction between parathyroid hormone and calcitonin is also an important part of bone remodeling. This is a lifelong process where mature bone tissue is removed from the skeleton and new bone tissue is formed.
Calcium regulation: Parathyroid hormone regulates the levels of calcium in the blood. to the parathyroid glands.
Parathyroid Hormone Action
- Parathyroid hormone acts on a bone to increase its blood calcium levels by stimulating osteoclasts to break down bone and release calcium into the bloodstream.
- Parathyroid hormone acts on the gastrointestinal tract to increase blood calcium by increasing the activity of the enzyme in the intestines that activates vitamin D.
- It acts on the kidneys to increase blood calcium levels by promoting calcium reabsorption in the nephrons.
