Parathyroid glands are small in the neck of humans and other . Humans usually have four parathyroid glands, located on the back of the thyroid gland in variable locations. The parathyroid gland produces and secretes parathyroid hormone in response to low blood calcium, which plays a key role in regulating the amount of calcium in the blood and within the bones.
Parathyroid glands share a similar blood supply, venous drainage, and lymphatic drainage to the thyroid glands. Parathyroid glands are derived from the epithelial lining of the third and fourth pharyngeal pouches, with the superior glands arising from the fourth pouch and the inferior glands arising from the higher third pouch. The relative position of the inferior and superior glands, which are named according to their final location, changes because of the migration of embryological tissues.
Hyperparathyroidism and hypoparathyroidism, characterized by alterations in the blood calcium levels and bone remodeling, are states of either surplus or deficient parathyroid function.
Structure
The parathyroid glands are two pairs of glands usually positioned behind the left and right lobes of the
thyroid. Each gland is a yellowish-brown flat ovoid that resembles a
lentil seed, usually about 6 mm long and 3 to 4 mm wide, and 1 to 2 mm anteroposteriorly.
There are typically four parathyroid glands. The two parathyroid glands on each side which are positioned higher are called the superior parathyroid glands, while the lower two are called the inferior parathyroid glands. Healthy parathyroid glands generally weigh about 30 mg in men and 35 mg in women.
These glands are not visible or able to be felt during examination of the neck.
[Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012, p. 159]
Each parathyroid vein drains into the superior, middle and inferior thyroid veins. The superior and middle thyroid veins drain into the internal jugular vein, and the inferior thyroid vein drains into the brachiocephalic vein.
Lymphatic drainage
Lymphatic vessels from the parathyroid glands drain into deep cervical lymph nodes and paratracheal lymph nodes.
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Variation
The parathyroid glands are variable in number: three or more small glands,
Microanatomy
The parathyroid glands are named for their proximity to the thyroid—and 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 follicular structure of the thyroid.
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Chief cells, which synthesize and release parathyroid hormone. These cells are small, and appear dark when loaded with parathyroid hormone, and clear when the hormone has been secreted, or in their resting state.
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Oxyphil cells, which are lighter in appearance and increase in number with age,
[ have an unknown function.]
File:Parathyroid gland intermed mag.jpg|Intermediate magnification micrograph. H&E stain. The white round structures are . Adipose tissue comprises 25–40% of normal parathyroid gland tissue.[
File:Parathyroid oxyphil and chief cells - annotated.jpg|High magnification micrograph. H&E stain. The small, dark cells are chief cells, which are responsible for secreting parathyroid hormone. The cells with orange/pink staining cytoplasm are oxyphil cells
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Development
In the early development of the human embryo, a series of five and four pharyngeal pouches form that give rise to the face, neck, and surrounding structures. The pouches are numbered such that the first pouch is the closest to the top of the embryo's head and the fourth is the farthest from it. The parathyroid glands originate from the interaction of the endoderm of the third and fourth pouch and neural crest mesenchyme.
Parathyroid development is regulated by a number of , including those coding for several transcription factors.
Function
The major function of the parathyroid glands is to maintain the body's calcium and phosphate levels within a very narrow range, so that the nervous system and can function properly. The parathyroid glands do this by secreting parathyroid hormone (PTH).
Parathyroid hormone (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.
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Calcium. PTH increases blood calcium levels by directly stimulating osteoblasts and thereby indirectly stimulating (through RANK/RANKL mechanism) to break down bone and release calcium. PTH increases gastrointestinal calcium absorption by activating vitamin D, and promotes calcium conservation (reabsorption) by the .
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Phosphate. PTH is the major regulator of serum phosphate concentrations via actions on the kidney. It is an inhibitor of proximal tubular reabsorption of phosphorus. Through activation of vitamin D the absorption (intestinal) of phosphate is increased.
Disorders
Parathyroid disease is conventionally divided into states where the parathyroid is overactive (hyperparathyroidism), and states where the parathyroid is under- or hypoactive (hypoparathyroidism). Both states are characterised by their symptoms, which relate to the excess or deficiency of parathyroid hormone in the blood.
Hyperparathyroidism
Primary
Hyperparathyroidism is the state in which there is excess parathyroid hormone circulating in the blood. This may cause bone pain and tenderness, due to increased bone resorption. With increased circulating calcium, there may be other symptoms associated with hypercalcemia, most commonly dehydration. Hyperparathyroidism is most commonly caused by a benign proliferation of chief cells in one parathyroid gland, and rarely in MEN syndrome. This is known as primary hyperparathyroidism,[ which is generally managed by surgical removal of the abnormal parathyroid gland.]
Secondary
Renal disease may lead to hyperparathyroidism. When too much calcium is lost from the blood via urination, there is a compensation by the parathyroid, and parathyroid hormone is released. The glands enlarge (hypertrophy) to synthesize more parathyroid hormone. This is known as secondary hyperparathyroidism.
Tertiary
If secondary hyperparathyroidism persists over months, the parathyroid tissue may become unresponsive to the blood calcium levels, and begin to autonomously release parathyroid hormone. This is known as tertiary hyperparathyroidism.
Hypoparathyroidism
The state of decreased parathyroid activity is known as hypoparathyroidism. This is most commonly associated with damage to the glands or their blood supply during thyroidectomy may be associated with rarer genetic syndromes such as DiGeorge syndrome, which is inherited as an autosomal dominant syndrome. Hypoparathyroidism will occur after surgical removal of the parathyroid glands.
Occasionally, an individual's tissues are resistant to the effects of parathyroid hormone. This is known as pseudohypoparathyroidism. In this case the parathyroid glands are fully functional, and the hormone itself is not able to function, resulting in a decrease in blood calcium levels. Pseudohypoparathyroidism is often associated with the genetic condition Albright's hereditary osteodystrophy. Pseudo-pseudohypoparathyroidism, one of the longest words in the English language, is used to describe an individual with Albright's hereditary osteodystrophy with normal parathyroid hormone and serum calcium levels.[
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Hypoparathyroidism may present with symptoms associated with Hypocalcemia, and is generally treated with Vitamin D analogues.[
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History
The parathyroid glands were first discovered in the Indian rhinoceros by Richard Owen in 1852.
Eugene Gley, Giulio Vassale, and others documented the putative function of the glands in 1891, noting the connection between their removal and the development of muscular tetany. William G. MacCallum in 1908, investigating tumours of the parathyroid, proposed their role in calcium metabolism.
The first successful removal of the parathyroid may have been carried out in 1928 by medical doctor Isaac Y Olch, whose intern had noticed elevated calcium levels in an elderly patient with muscle weakness. Prior to this surgery, patients with removed parathyroid glands typically died from muscular tetany.
Parathyroid hormone was isolated in 1923 by Adolph M. Hanson and 1925 by James Collip. Studies of parathyroid hormone levels by Roger Guillemin, Andrew Schally and Rosalyn Sussman Yalow led to the development of capable of measuring body substances and a Nobel Prize in 1977.
Other animals
Parathyroid glands are found in all adult ; they vary in their number and position. typically have four parathyroid glands, while other types of animals typically have six. The removal of parathyroid glands in animals produces a condition resembling acute poisoning with irregular muscle contractions.
Fish do not possess parathyroid glands; several species have been found to express parathyroid hormone. Developmental genes and calcium-sensing receptors in fish gills are similar to those within the parathyroid glands of birds and mammals. It has been suggested that the tetrapod glands may have been evolutionarily derived from these fish gills.
==Additional images==
See also
Notes
Further reading
External links
