Parathyroid gland

Parathyroid glands are small

Hyperparathyroidism and hypoparathyroidism, characterized by alterations in the blood calcium levels and bone metabolism, 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.^[1] 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.^[2] These glands are not visible or able to be felt during examination of the neck.^[3]

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.^[4]

Lymphatic drainage

Lymphatic vessels from the parathyroid glands drain into

The parathyroid glands are variable in number: three or more small glands,[5] and can usually be located on the posterior surface of the thyroid gland.^[5] Occasionally, some individuals may have six, eight, or even more parathyroid glands.^[3] Rarely, the parathyroid glands may be within the thyroid gland itself, the chest, or even the thymus.^[5]

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.^[6] Two unique types of cells are present in the parathyroid gland:

• 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.^[7]
• Oxyphil cells, which are lighter in appearance and increase in number with age,^[7] have an unknown function.^[8]

• 
  Intermediate magnification

  fat cells. Adipose tissue comprises 25–40% of normal parathyroid gland tissue.^[7]
• 
  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

Development

In the

Parathyroid development is regulated by a number of genes, including those coding for several transcription factors.^[10][a]

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 and muscular systems can function properly. The parathyroid glands do this by secreting parathyroid hormone (PTH).^[11]

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.^[12]

• Calcium. PTH increases blood calcium levels by directly stimulating osteoblasts and thereby indirectly stimulating osteoclasts (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 kidneys.^[12]
• 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.^[12]

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.^[13]

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

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.^[15]

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

Hypoparathyroidism may present with symptoms associated with decreased calcium, and is generally treated with Vitamin D analogues.^[16]

History

The parathyroid glands were first discovered in the Indian rhinoceros by Richard Owen in 1852.^[17] In his description of the neck anatomy, Owen referred to the glands as "a small compact yellow glandular body attached to the thyroid at the point where the veins emerged". The glands were first discovered in humans by Ivar Viktor Sandström (1852–1889), a Swedish medical student, in 1880 at Uppsala University.^[18] Unaware of Owen's description, he described the glands in his monograph "On a New Gland in Man and Fellow Animals" as the "glandulae parathyroidae", noting its existence in dogs, cats, rabbits, oxen, horses and humans.^[19][20] For several years, Sandström's description received little attention.^[21]

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.^[20] He noted that "Tetany occurs spontaneously in many forms and may be produced by the destruction of the parathyroid glands".^[22]

The first successful removal of the parathyroid may have been carried out in 1928 by medical doctor Isaac Y Olch, whose

Parathyroid hormone was isolated in 1923 by Adolph M. Hanson and 1925 by James B. Collip. Studies of parathyroid hormone levels by Roger Guillemin, Andrew Schally and Rosalyn Sussman Yalow led to the development of immunoassays capable of measuring body substances and a Nobel Prize in 1977.^[18][20]

Other animals

Parathyroid glands are found in all adult tetrapods; they vary in their number and position. Mammals 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.^[23]

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.^[10][24]

Additional images

• 
  Gross pathology of a parathyroid gland (white arrow), next to the thyroid gland
• 
  Scheme showing development of branchial epithelial bodies. I, II, III, IV.

  Branchial pouches

  .

See also

• Thyroid

Notes

References

1. ISBN 0-443-01505-8
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2. PMID 15790694
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3. ^ ^{a b} Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012, p. 159
4. ^
   ISBN 978-0-8089-2306-0
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5. ^
   ISBN 978-0-443-10373-5.{{cite book}}: CS1 maint: multiple names: authors list (link)^{[page needed}
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6. S2CID 207431255
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7. ^
   ISBN 978-0-443-06850-8
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8. PMID 22585091
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9. ISBN 0-443-06583-7
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10. ^
    S2CID 7971353
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11. ISBN 978-0-443-06850-8
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12. ^
    ISBN 978-0-7216-0240-0.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
13. ^
    ISBN 978-0-7020-3084-0
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14. ISBN 978-0-7020-3084-0
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15. ISBN 978-1-57059-574-5
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16. ^
    ISBN 978-0-7020-3084-0
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17. ^ Cave, A.J.E. (1953). "Richard Owen and the discovery of the parathyroid glands" (PDF). In E. Ashworth Underwood (ed.). Science, Medicine and History. Essays on the Evolution of Scientific Thought and Medical Practice. Vol. 2. Oxford University Press. pp. 217–222. Retrieved 2009-07-20.
18. ^
    PMID 7485136
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19. doi:10.1001/jama.1938.02790280087037
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20. ^
    PMID 15708157
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21. PMID 8764749
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22. PMID 19867238
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23. OCLC 654587300
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24. PMID 15591343
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Further reading

• OCLC 69441047
  .

External links

Wikimedia Commons has media related to Parathyroid glands.

• Endocrine Web: Parathyroid
• University of Iowa Hospitals & Clinics: Parathyroid