The parathyroid glands are embedded in the posterior surface of the thyroid gland. There are usually four parathyroid glands—two pairs—on each side of the thyroid. The parathyroid glands are tiny, pea-shaped glands weighing only a total of 0.06 ounces (1.6 grams) together. Each measures approximately 0.1 to 0.3 inches (3 to 8 millimeters) in length, 0.7 to 0.2 inches (2 to 5 millimeters) in width, and 0.05 inches (1.5 millimeters) in depth.
The parathyroid glands secrete parathyroid hormone (PTH). The main function of PTH is to regulate the levels of calcium and phosphate in the blood.
There are four ways that parathyroid hormone PTH increases the level of calcium in the blood.
1. PTH stimulates osteoclasts to break down bone tissue and release calcium ions from the bone.
2. PTH inhibits osteoblasts to reduce the rate of calcium deposition in bone.
3. PTH enhances the absorption of calcium and phosphate from the small intestine in conjunction with the secretion of cacitriol by the kidneys.
4. PTH promotes the reabsorption of calcium at the kidneys, reducitng the amount of calcium excreted in the urine.
The thyroid gland secretes calcitonin when the calcium level in the blood is elevated when the blood calcium level drops, the parathyroid glands increase the secretion of parathyroid hormone until the blood calcium level increases to the normal value. Homeostasis of blood calcium levels is maintained through the interactions of calcitonin and PTH.
Not all of the parathyroid glands are needed to maintain hoemostaisis. The secretion of even a proiton of one gland can maintain normal calcium cocnetnrations. When calcium concentration levels are abnormally high, through a condition called hyperparathyroidm. Results. This is typically caused by a tumor, and surgical removal of the overactive tissue will often correct the imbalcane.
Hypoparathyrodism is most often the result of damage to or removal of the parathyroid glands during surgery or when a tumor is on the parathyroid gland. It is suspected that prior to the discovery of the parathyroid glands, many patients died following thyroid surgery since the parathyroid glads were accidently removed, resulting in abnormally low levels of calcium.
The parathyroid glands are small endocrine glands in the neck that produce parathyroid hormone. Humans usually have four parathyroid glands, which are usually located on the rear surface of the thyroid gland, or, in rare cases, within the thyroid gland itself or in the chest. Parathyroid glands control the amount of calcium in the blood and within the bones.
The parathyroid glands are four or more small glands, about the size of a grain of rice, located on the posterior surface (back side) of the thyroid gland. The parathyroid glands usually weigh between 25mg and 40mg in humans. There are typically two, one above the other, on the left lobe of the thyroid and similarly on the right. The two parathyroid glands on each side which are positioned higher (closer to the head) 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.
The parathyroid glands are named for their proximity to the thyroid but serve a completely different role than the thyroid gland. The parathyroid glands are quite easily recognizable from the thyroid as they have densely packed cells, in contrast with the follicle structure of the thyroid. [1] However, at surgery, they are harder to differentiate from the thyroid or fat.
In the histological sense, they distinguish themselves from the thyroid gland, as they contain two types of cells:
| Name | Staining | Quantity | Size | Function |
| darker | many | smaller | manufacture PTH (see below). | |
| lighter | few | larger | function unknown.[3] |
[edit]History
The parathyroid glands were first discovered in the Indian Rhinoceros by Richard Owen in 1850.[4] The glands were first discovered in humans by Ivar Viktor Sandström (1852-1889), a Swedish medical student, in 1880.[5] It is the last major organ to be recognized in humans so far.
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 hormone into the blood.
Parathyroid hormone (PTH, also known as parathormone) is a small protein that takes part in the control of calcium and phosphate homeostasis, as well as bone physiology. Parathyroid hormone has effects antagonistic to those of calcitonin. PTH increases 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 (reabsorption) by the kidneys. PTH affects the perception of well being and absence of PTH can be associated with feeling of fatigue and anxiety.
[edit]Role in disease
Main article: Parathyroid disease
Many conditions are associated with disorders of parathyroid function. These can be divided into those causing hyperparathyroidism, and those causing hypoparathyroidism.
[edit]Embryology and evolution
The parathyroid glands originate from the interaction of neural crest mesenchyme and third and fourth branchial pouch endoderm.
Eya-1 (transcripitonal co-activator), Six-1 (a homeobox transcription factor), and Gcm-2 (a transcription factor) have been associated with the development of the parathyroid gland, and alterations in these genes alters parathyroid gland development.
The superior parathyroids arise from the fourth pharyngeal pouch, and the inferior parathyroids arise from the third pharyngeal pouch. They are vertically transposed during embryogenesis. This is significant in function-preserving parathyroidectomy, because both the superior and the inferior parathyroids are supplied by the inferior thyroid artery. If the surgeon is to leave a single functional parathyroid for the patient, he/she must preserve the appropriate blood supply.
[edit]In other animals
Parathyroid glands are found in all adult tetrapods, although they vary in their number, and in their exact position. Mammals typically have four parathyroids, while other groups typically have six.
Fish do not possess parathyroid glands, although the ultimobranchial glands, which are found close to the oesophagus, may have a similar function and could even be homologous with the tetrapod parathyroids. Even these glands are absent in the most primitive vertebrates, the jawless fish, but as these species have no bone in their skeletons, only cartilage, it may be that they have less need to regulate calcium metabolism.
The conserved homology of genes and calcium-sensing receptors in fish gills with those in the parathryroid glands of birds and mammals is recognized by evolutionary developmental biology as evolution-using genes and gene networks in novel ways to generate new structures with some similar functions and novel functions.
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