Parathyroid hormone diagram

The Parathyroid Glands | Anatomy and Physiology II

Learning Objectives

By the end of this section, you will be able to:

• Describe the location and structure of the parathyroid glands
• Describe the hormonal control of blood calcium levels
• Discuss the physiological response of parathyroid dysfunction

The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland (Figure 1). A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four parathyroid glands, but occasionally there are more in tissues of the neck or chest. The function of one type of parathyroid cells, the oxyphil cells, is not clear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the parathyroid hormone (PTH), the major hormone involved in the regulation of blood calcium levels.

Figure 1. The small parathyroid glands are embedded in the posterior surface of the thyroid gland. LM × 760. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

View the University of Michigan WebScope to explore the tissue sample in greater detail.

The parathyroid glands produce and secrete PTH, a peptide hormone, in response to low blood calcium levels (Figure 2). PTH secretion causes the release of calcium from the bones by stimulating osteoclasts, which secrete enzymes that degrade bone and release calcium into the interstitial fluid. PTH also inhibits osteoblasts, the cells involved in bone deposition, thereby sparing blood calcium. PTH causes increased reabsorption of calcium (and magnesium) in the kidney tubules from the urine filtrate. In addition, PTH initiates the production of the steroid hormone calcitriol (also known as 1,25-dihydroxyvitamin D), which is the active form of vitamin D₃, in the kidneys. Calcitriol then stimulates increased absorption of dietary calcium by the intestines. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH.

Figure 2. Parathyroid hormone increases blood calcium levels when they drop too low. Conversely, calcitonin, which is released from the thyroid gland, decreases blood calcium levels when they become too high. These two mechanisms constantly maintain blood calcium concentration at homeostasis.

Abnormally high activity of the parathyroid gland can cause hyperparathyroidism, a disorder caused by an overproduction of PTH that results in excessive calcium reabsorption from bone. Hyperparathyroidism can significantly decrease bone density, leading to spontaneous fractures or deformities. As blood calcium levels rise, cell membrane permeability to sodium is decreased, and the responsiveness of the nervous system is reduced. At the same time, calcium deposits may collect in the body’s tissues and organs, impairing their functioning.

In contrast, abnormally low blood calcium levels may be caused by parathyroid hormone deficiency, called hypoparathyroidism, which may develop following injury or surgery involving the thyroid gland. Low blood calcium increases membrane permeability to sodium, resulting in muscle twitching, cramping, spasms, or convulsions. Severe deficits can paralyze muscles, including those involved in breathing, and can be fatal.

When blood calcium levels are high, calcitonin is produced and secreted by the parafollicular cells of the thyroid gland. As discussed earlier, calcitonin inhibits the activity of osteoclasts, reduces the absorption of dietary calcium in the intestine, and signals the kidneys to reabsorb less calcium, resulting in larger amounts of calcium excreted in the urine.

Chapter Review

Calcium is required for a variety of important physiologic processes, including neuromuscular functioning; thus, blood calcium levels are closely regulated. The parathyroid glands are small structures located on the posterior thyroid gland that produce parathyroid hormone (PTH), which regulates blood calcium levels. Low blood calcium levels cause the production and secretion of PTH. In contrast, elevated blood calcium levels inhibit secretion of PTH and trigger secretion of the thyroid hormone calcitonin. Underproduction of PTH can result in hypoparathyroidism. In contrast, overproduction of PTH can result in hyperparathyroidism.

Self Check

Answer the question(s) below to see how well you understand the topics covered in the previous section.

Critical Thinking Questions

1. Describe the role of negative feedback in the function of the parathyroid gland.
2. Explain why someone with a parathyroid gland tumor might develop kidney stones.

Show Answers

Glossary

hyperparathyroidism: disorder caused by overproduction of PTH that results in abnormally elevated blood calcium

hypoparathyroidism: disorder caused by underproduction of PTH that results in abnormally low blood calcium

parathyroid glands: small, round glands embedded in the posterior thyroid gland that produce parathyroid hormone (PTH)

parathyroid hormone (PTH): peptide hormone produced and secreted by the parathyroid glands in response to low blood calcium levels

The Parathyroid Glands – Anatomy & Physiology

The Endocrine System

OpenStaxCollege

Learning Objectives

By the end of this section, you will be able to:

• Describe the location and structure of the parathyroid glands
• Describe the hormonal control of blood calcium levels
• Discuss the physiological response of parathyroid dysfunction

The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland ([link]). A thick connective tissue capsule separates the glands from the thyroid tissue. Most people have four parathyroid glands, but occasionally there are more in tissues of the neck or chest. The function of one type of parathyroid cells, the oxyphil cells, is not clear. The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete the parathyroid hormone (PTH), the major hormone involved in the regulation of blood calcium levels.

Parathyroid Glands

The small parathyroid glands are embedded in the posterior surface of the thyroid gland. LM × 760. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

View the University of Michigan WebScope to explore the tissue sample in greater detail.

The parathyroid glands produce and secrete PTH, a peptide hormone, in response to low blood calcium levels ([link]). PTH secretion causes the release of calcium from the bones by stimulating osteoclasts, which secrete enzymes that degrade bone and release calcium into the interstitial fluid. PTH also inhibits osteoblasts, the cells involved in bone deposition, thereby sparing blood calcium. PTH causes increased reabsorption of calcium (and magnesium) in the kidney tubules from the urine filtrate. In addition, PTH initiates the production of the steroid hormone calcitriol (also known as 1,25-dihydroxyvitamin D), which is the active form of vitamin D₃, in the kidneys. Calcitriol then stimulates increased absorption of dietary calcium by the intestines. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH.

Parathyroid Hormone in Maintaining Blood Calcium Homeostasis

Parathyroid hormone increases blood calcium levels when they drop too low. Conversely, calcitonin, which is released from the thyroid gland, decreases blood calcium levels when they become too high. These two mechanisms constantly maintain blood calcium concentration at homeostasis.

Abnormally high activity of the parathyroid gland can cause hyperparathyroidism, a disorder caused by an overproduction of PTH that results in excessive calcium reabsorption from bone. Hyperparathyroidism can significantly decrease bone density, leading to spontaneous fractures or deformities. As blood calcium levels rise, cell membrane permeability to sodium is decreased, and the responsiveness of the nervous system is reduced. At the same time, calcium deposits may collect in the body’s tissues and organs, impairing their functioning.

In contrast, abnormally low blood calcium levels may be caused by parathyroid hormone deficiency, called hypoparathyroidism, which may develop following injury or surgery involving the thyroid gland. Low blood calcium increases membrane permeability to sodium, resulting in muscle twitching, cramping, spasms, or convulsions. Severe deficits can paralyze muscles, including those involved in breathing, and can be fatal.

When blood calcium levels are high, calcitonin is produced and secreted by the parafollicular cells of the thyroid gland. As discussed earlier, calcitonin inhibits the activity of osteoclasts, reduces the absorption of dietary calcium in the intestine, and signals the kidneys to reabsorb less calcium, resulting in larger amounts of calcium excreted in the urine.

Calcium is required for a variety of important physiologic processes, including neuromuscular functioning; thus, blood calcium levels are closely regulated. The parathyroid glands are small structures located on the posterior thyroid gland that produce parathyroid hormone (PTH), which regulates blood calcium levels. Low blood calcium levels cause the production and secretion of PTH. In contrast, elevated blood calcium levels inhibit secretion of PTH and trigger secretion of the thyroid hormone calcitonin. Underproduction of PTH can result in hypoparathyroidism. In contrast, overproduction of PTH can result in hyperparathyroidism.

When blood calcium levels are low, PTH stimulates ________.

1. urinary excretion of calcium by the kidneys
2. a reduction in calcium absorption from the intestines
3. the activity of osteoblasts
4. the activity of osteoclasts

D

Which of the following can result from hyperparathyroidism?

1. increased bone deposition
2. fractures
3. convulsions
4. all of the above

B

Describe the role of negative feedback in the function of the parathyroid gland.

The production and secretion of PTH is regulated by a negative feedback loop. Low blood calcium levels initiate the production and secretion of PTH. PTH increases bone resorption, calcium absorption from the intestines, and calcium reabsorption by the kidneys. As a result, blood calcium levels begin to rise. This, in turn, inhibits the further production and secretion of PTH.

Explain why someone with a parathyroid gland tumor might develop kidney stones.

A parathyroid gland tumor can prompt hypersecretion of PTH. This can raise blood calcium levels so excessively that calcium deposits begin to accumulate throughout the body, including in the kidney tubules, where they are referred to as kidney stones.

hyperparathyroidism

disorder caused by overproduction of PTH that results in abnormally elevated blood calcium

hypoparathyroidism

disorder caused by underproduction of PTH that results in abnormally low blood calcium

parathyroid glands

small, round glands embedded in the posterior thyroid gland that produce parathyroid hormone (PTH)

parathyroid hormone (PTH)

peptide hormone produced and secreted by the parathyroid glands in response to low blood calcium levels

Parathyroid glands: structure, function

For the smooth functioning of the human body, the constancy of all vital processes - homeostasis - is important. It is provided by metabolic processes, the "conductors" of which are hormones, enzymes, neurotransmitters. It is difficult to overestimate the importance of the organs that produce these biologically active substances, since a malfunction in the functioning of any of them can lead to an imbalance in the overall balance in the body. One of these key links in the metabolism are the parathyroid (parathyroid) glands.

Where are the parathyroid glands located in humans?

For a long time, doctors did not suspect the existence of this organ. Only at the end of the 19th century, the Swedish scientist Ivar Sandstrom discovered new endocrine glands in the human body.

The parathyroid glands consist of a dense outer shell and glandular cells located inside. Usually, two to six pairs of these small lenticular masses are located on the posterior surface of the thyroid gland. However, the thymus gland, the wall of the esophagus, and the neurovascular bundle, suitable for the thyroid gland, can become their localization site.

This variability in the number and location of these glands complicates the detection of these glands during instrumental diagnostics and surgical intervention.

Why do we need parathyroid glands? What are their functions?

"Small and daring" - this is how one can characterize the significance of these endocrine glands. Despite their modest size (up to 8 mm in length and 3-4 mm in width), their functioning significantly affects the functioning of the nervous, musculoskeletal, and cardiovascular systems.

The main task of the parathyroid glands is the production of a hormone that regulates the amount of calcium in the blood. Due to the presence of special receptors, the glands react to changes in the content of this element in the blood and, in accordance with the values ​​obtained, change the rate of release of the hormone into the blood.

Maintaining calcium balance is extremely important for the body, since calcium ions provide the process of impulse transmission through nerve cells and muscle tissue contraction. In addition, calcium is one of the components of the blood coagulation system, and also contributes to the activation of a number of enzymes.

The parathyroid hormone is called parathyroid, or otherwise, parathormone. It increases the content of calcium ions in the blood in three different ways.

1. Under the influence of parathyroid hormone, vitamin D is activated in the kidneys, followed by the formation of calcitriol. Calcitriol, in turn, improves the absorption of calcium from food in the intestinal lumen. In order for this mechanism of action of parathyroid hormone to be realized, a sufficient level of vitamin D in the body is required.

2. An increase in the concentration of calcium ions in the blood is also due to the activation of its absorption from the primary urine in the kidneys.

3. Under the action of parathyroid hormone, the activity of osteoclasts, cells located in the bone tissue and destroying it, increases. The calcium released during this enters the bloodstream. As a result of this process, the concentration of calcium ions in the blood increases. If parathyroid hormone is produced more than normal, the process of destruction of bone tissue begins to prevail over its formation. Because of this, the bone loses strength and becomes brittle, and this can lead to fractures.

To regulate phosphorus-calcium metabolism in the body, another hormone is formed that acts opposite to parathyroid hormone. This is calcitonin. It is produced by the thyroid gland and several other organs. Thanks to its production, the level of calcium in the blood decreases and the activity of osteoblasts is stimulated - another type of bone tissue cells, which, unlike osteoclasts, do not destroy, but form new bone tissue.

How does dysfunction of the parathyroid glands manifest itself?

Gland dysfunction leads to excess or deficiency of calcium ions in the blood. If the production of parathyroid hormone decreases, a person has signs such as brittleness and hair loss, delamination of nails and damage to tooth enamel, peeling and dry skin, and muscle pain. Patients may complain of irritability, headaches, mood swings, and seizures. The syndrome (complex of symptoms), which develops due to insufficient function of the parathyroid glands, is called "hypoparathyroidism".

An increase in the production of parathyroid hormone is accompanied by problems with the kidneys (formation of stones, the development of nephrocalcinosis), high blood pressure (hypertension), pain in the joints and bones. The skeleton becomes fragile: bones can break with any movement. Patients complain of memory impairment, general weakness, unstable emotional state, as well as gastrointestinal disorders - loss of appetite, vomiting, heartburn, colic. The pathological condition that occurs due to excessive production of parathyroid hormone is called "hyperparathyroidism".

Primary hyperparathyroidism may develop due to neoplasms of the parathyroid gland - adenoma or cancer. Secondary hyperparathyroidism develops as a result of prolonged stimulation of the parathyroid glands, observed in chronic vitamin D deficiency (for example, in residents of the northern regions), unbalanced nutrition (insufficient intake of calcium from food), malabsorption syndrome (impaired absorption in the intestine, including calcium) and after resection of the stomach.

The presence of the above symptoms, as well as doubts about the normal functioning of the parathyroid glands, is a good reason to seek advice from a general practitioner or endocrinologist and undergo an examination.

Parathormone

• Parathormone analysis
• Parathormone and calcium tests
• Parathormone blood test
• Get tested for parthormone in St. Petersburg
• Parathyroid hormone test elevated
• Treatment of primary hyperparathyroidism
• Parathyroid hormone test lowered

Parathyroid hormone - parathyroid hormone (it is more correct to say “parathyroid glands”, but many patients are already very accustomed to the term “parathyroid glands”, although it is not entirely correct in terms of word formation).

The production of parathyroid hormone is produced by the cells of the parathyroid gland in response to a decrease in the level of ionized calcium in the blood. On the surface of the cells of the parathyroid glands there are special receptors that are able to assess the concentration of ionized calcium in the blood and, in accordance with its level, produce parathyroid hormone in larger or smaller quantities.

Very often, the term " parathyroid hormone " (parathyroid hormone - from parathyroid hormone) is spelled incorrectly, since it can be difficult for a layman to hear all the features of the correct spelling. Often on the Internet you can find terms such as " hormone parade ", " hormone parade " and even " hormone parade ". The correct term, of course, is one - parathormone (spelled together and without a hyphen).

Parathyroid hormone is a polypeptide hormone (i.e. consisting of amino acids). There are 84 amino acid residues in the parathyroid hormone molecule. Currently, the structure of parathyroid hormone is completely deciphered by scientists. It was found that in the parathyroid hormone molecule, the first 34 amino acid residues are responsible for biological activity, and the rest are responsible for the binding of the hormone to receptors and the stability of the molecule as a whole.

The main action of parathyroid hormone is aimed at increasing the level of ionized calcium in the blood. This action is implemented through three different effects.

First, parathyroid hormone enhances the activation of vitamin D in the kidneys , which leads to the formation of an important hormone-like substance, calcitriol, from vitamin D. Calcitriol stimulates the absorption of calcium in the intestine, which leads to an increased intake of calcium from food into the blood. A prerequisite for the implementation of this effect of parathyroid hormone is the presence of a sufficient amount of vitamin D in the body. Without sufficient intake of vitamin D in the blood, parathyroid hormone is not able to enhance the absorption of calcium in the intestine.

Secondly, parathyroid hormone enhances the reabsorption of calcium ions from primary urine . This effect is realized at the level of the renal tubules.

Thirdly, parathyroid hormone enhances the activity of osteoclasts - cells that destroy bone tissue. Osteoclasts, like bulldozers or excavators, begin to actively destroy bone beams and release the resulting calcium into the blood. As a result, the concentration of calcium in the blood increases, but the strength of the bone tissue decreases, which increases the likelihood of fractures.

Parathormone is a very interesting hormone, since the effect of parathyroid hormone on the bone directly depends on the mode of its production . Everything that we said above about the negative effect of parathyroid hormone on bone tissue is true only for cases where parathyroid hormone is constantly and continuously elevated. At the same time, the periodic and short-term intake of parathyroid hormone into the blood has a positive effect on bone tissue, leading to increased formation of bone beams and strengthening of the bone. Now this effect is used in the treatment of osteoporosis - even a medicinal analogue of parathyroid hormone (teriparatide) has been synthesized, the periodic introduction of which into the body can increase the strength of bone tissue and reduce the likelihood of fractures.

The production of parathyroid hormone is regulated by the level of ionized calcium in the blood. If calcium in the blood decreases, parathyroid hormone begins to be released more actively.

On the surface of the cells of the parathyroid glands is a calcium-binding receptor, which is directly able to "feel" the concentration of calcium in the blood and regulate the rate at which parathyroid hormone is produced. This is the only receptor currently known to science that is "controlled" not by peptides or hormones, but by the substance itself - or rather, by its ions. Be that as it may, normally parathyroid hormone is produced by the parathyroid glands only when the concentration of calcium in the blood decreases.

There are two "friends" in the body, two substances that are inextricably linked - parathyroid hormone, calcium . At the same time, there are relationships between them, which in endocrinology are described as “double feedback”. They sort of regulate each other. With a decrease in the level of calcium in the blood, parathyroid hormone begins to be released more strongly, as a result of which calcium in the blood rises and acts on the cells of the parathyroid glands through the receptor, forcing them to stop secreting parathyroid hormone. After the cessation of the release of parathyroid hormone, calcium begins to gradually decrease until it reaches a level at which the cells of the parathyroid glands become activated with the release of parathyroid hormone - and the cycle repeats itself. Calcium is the main thing that parathyroid hormone affects, and at the same time parathyroid hormone is one of the most important substances that calcium affects .

Unlike a substance such as calcium, parathyroid hormone and calcitonin are "enemies", antagonists of . Parathyroid hormone aims to increase the level of calcium in the blood, and calcitonin - to reduce it. Parathyroid hormone stimulates the destruction of bone beams with a prolonged increase, and calcitonin, on the contrary, causes new bone tissue to form and thereby strengthens the bone. The relationship between hormones, if you "dig" deep, even deeper - for example, with some hereditary syndromes (multiple endocrine neoplasia syndrome, MEN), tumors develop simultaneously that produce both hormones - parathyroid hormone, calcitonin. Therefore when examining for elevated parathyroid hormone, calcitonin must be given .

Vitamin D and parathyroid hormone are substances whose effects are similar and largely dependent on each other. Both substances - and vitamin D, and parathyroid hormone - have their main effect on increasing the level of calcium in the blood . As with calcium, parathyroid hormone and vitamin D can interfere with each other. This effect is very interesting and is implemented in general terms as follows. With a decrease in the level of calcium in the blood, the cells of the parathyroid glands begin to actively produce parathyroid hormone, which enhances the hydroxylation of vitamin D in the kidneys and the formation of calcitriol, the active form of vitamin D, which, by the strength of its action, can be confidently recognized as a hormone. Calcitriol, on the one hand, enhances the secretion of a special transport protein in the intestinal wall - calmodulin, which "drags" calcium from the intestinal lumen into the blood, and on the other hand, directly acts on a special receptor on the surface of the cells of the parathyroid glands (it is called the vitamin receptor D or VDR, vitamin D receptor). Activation of the vitamin D receptor leads to suppression of the proliferation of parathyroid gland cells, i.e. indirectly acts in the direction of lowering the level of parathyroid hormone.

It is important to understand that a decrease in the intake of vitamin D in the human body leads to a “disinhibition” of parathyroid cell division and, at the same time, to stimulation of the production of parathyroid hormone by these cells. This occurs when there is little sunlight on the skin, since vitamin D is produced in human skin. The second cause of vitamin D deficiency is insufficient intake of vitamin D from food. Low vitamin D in the blood leads to a low intake of calcium into the blood, which activates the production of parathyroid hormone by the cells of the parathyroid glands.

Vitamin D deficiency has been shown to increase the incidence of benign tumors - adenomas of the parathyroid glands (probably due to the elimination of the inhibitory effect of vitamin D on cell division of the parathyroid glands in case of its deficiency).

The second common situation with which patients turn to the North-West Endocrinology Center is the so-called secondary hyperparathyroidism, i.e. a condition in which parathyroid hormone is elevated in the blood, and calcium is normal . The detection of normal or reduced calcium along with an increase in the level of parathyroid hormone usually indicates a low level of vitamin D in the blood. You can, of course, conduct a blood test for vitamin D, but you can do otherwise - prescribe vitamin D and calcium preparations to the patient, and after 1-2 months, conduct a second blood test for parathyroid hormone and ionized calcium. If the repeated analysis reveals a decrease or normalization of parathyroid hormone, and the calcium level is normal, this will indicate with a high degree of certainty that the patient simply needs to use calcium and vitamin D supplements longer. is still high, and calcium has risen above the norm - this will indicate that the patient has primary hyperparathyroidism, a tumor of the parathyroid gland.

Analysis for parathyroid hormone is one of the most important examinations in the list of examinations prescribed for suspected calcium metabolism disorders, including the development of osteoporosis. Blood for parathyroid hormone is usually taken simultaneously with an analysis for ionized calcium, phosphorus, calcitonin , since such a block of studies allows the endocrinologist to most fully assess the state of metabolism. It is also highly desirable to immediately perform densitometry - a study of bone density, which shows the likelihood of developing bone fractures.

Parathormone - analysis , the quality of which varies greatly between different laboratories. Currently, the most common methods for performing a blood test for parathyroid hormone are enzyme immunoassay (the so-called 2nd generation method) and immunochemiluminescent (3rd generation method).

Most laboratories perform parathyroid hormone testing using the 2nd generation method, since ELISA equipment and reagents are inexpensive - even domestically produced reagents can be used. At the same time, the use of the ELISA method leads to a decrease in the accuracy of the analysis of parathyroid hormone in the blood and an increase in the error.

The specialized laboratory of the North-West Center of Endocrinology uses the 3rd generation automatic immunochemiluminescent analyzer DiaSorin Liaison XL (Italy) to perform analysis for parathyroid hormone - a device with exceptionally high accuracy of analysis. In the work of endocrinologists of our center, the accuracy of such a study as a blood test for parathyroid hormone is the main diagnostic, so we take the quality of the study very seriously. The specialized laboratory of the center NEVER performs the analysis of parathyroid hormone by the 2nd generation method and NEVER uses either domestic or Chinese reagents - only reagents made in Italy by DiaSorin.