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15.5: Parathyroid Glands

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    22358
  • 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

    The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland (Figure \(\PageIndex{1}\)). 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.

    Diagram of the four parathyroid glands embedded in the posterior thyroid gland.Figure \(\PageIndex{1}\): The Parathyroid Glands. Typically four parathyroid glands are embedded in the posterior surface of the thyroid gland alongside the superior portion of the trachea, two on the left and two on the right, with the superior set just inferior to the cricoid cartilage of the larynx. Also labeled in the image are the hyoid bone and the thyroid cartilage of the larynx for orientation, as well as the left and right inferior thyroid arteries that branch from the left and right subclavian arteries, respectively, and supply the posterior thyroid and parathyroid glands with blood. (Image credit: "Posterior Thyroid" by OpenStax is licensed under CC BY-SA 3.0)

    The primary functional cells of the parathyroid glands are the chief cells. These epithelial cells produce and secrete parathyroid hormone (PTH), the major hormone involved in the regulation of blood calcium levels (Figure \(\PageIndex{2}\)). When blood calcium levels drop too low, PTH is released from the parathyroid glands into the bloodstream. Parathyroid hormone targets the bones and kidneys. In bones, PTH inhibits osteoblast activity while stimulating osteoclast activity, resulting in stored calcium being released from bone matrix into the bloodstream. In the kidneys, PTH stimulates an increase in calcium reabsorption into the bloodstream and also stimulates the release of calcitriol. Calcitriol targets the intestines to stimulate increased absorption of calcium from food.

    When blood calcium levels are high, calcitonin is produced and secreted by the parafollicular cells of the thyroid gland. As discussed in the previous section, 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.

    Diagram of parathyroid hormone and calcitonin release for complementary affects in the hormonal regulation of calcium homeostasis.
    Figure \(\PageIndex{2}\): Hormonal Control of Blood Calcium Homeostasis. When the blood calcium concentration drops too low, chief cells within the superior and inferior pairs of parathyroid glands release parathyroid hormone (PTH) into the bloodstream. PTH effects on bone include inhibition of osteoblasts and stimulation of osteoclasts to break down bone, releasing calcium ions into the bloodstream. PTH effects on the kidneys include stimulating kidney tubule cells to recover waste calcium from the urine, reabsorbing it back into the bloodstream, and stimulating kidney tubule cells to release calcitriol into the bloodstream. The effect of calcitriol on the intestine is to stimulate the absorption of calcium ions from digested food. All of these mechanisms increase blood calcium levels. High concentrations of calcium in the blood stimulate parafollicular cells in the thyroid gland to release calcitonin. Calcitonin targets bone, stimulating osteoblasts and inhibiting osteoclasts so calcium is removed from the bloodstream and used to build bone. These two mechanisms constantly maintain homeostasis of blood calcium levels. (Image credit: "The Role of Parathyroid Hormone in Maintaining Blood Calcium Homeostasis" by OpenStax is licensed under CC BY 3.0)

    DISORDERS OF THE...

    Endocrine System: Hyperparathyroidism and Hypoparathyroidism

    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.

    Concept 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 of the 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.

    Review Questions

    Q. When blood calcium levels are low, PTH stimulates ________.

    A. urinary excretion of calcium by the kidneys

    B. a reduction in calcium absorption from the intestines

    C. the activity of osteoblasts

    D. the activity of osteoclasts

    Answer

    Answer: D

    Q. Which of the following can result from hyperparathyroidism?

    A. increased bone deposition

    B. fractures

    C. convulsions

    D. all of the above

    Answer

    Answer: B

    Critical Thinking Questions

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

    Answer

    A. 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.

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

    Answer

    A. 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.

    Glossary

    calcitriol
    hormone released by kidney tubule cells in response to parathyroid hormone that stimulates increased absorption of calcium from digested food in the intestines
    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 that targets bone and the kidneys to raise blood calcium levels and targets the kidneys to release calcitriol

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