Chapter 17. The Endocrine System
17.5 The Parathyroid Glands
Learning Objectives
By the end of this section, you will be able to:
- describe the location and structure of the parathyroid glands;
- indicate the general functions of parathyroid hormone; and
- discuss how PTH release is regulated.
The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland (Figure 17.5.1). 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 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. The gland also contains oxyphil cells but their function is not clear.
Regulation of Release and Functions of PTH
The parathyroid glands produce and secrete PTH, a peptide hormone, in response to low blood calcium levels (Figure 17.5.2). 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. This is considered an indirect effect of PTH. A negative feedback loop regulates the levels of PTH, with rising blood calcium levels inhibiting further release of PTH.
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 phosphate deposits may collect in the body’s tissues and organs (extraosseous calcification), 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.
Section 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. PTH increases bone resorption, calcium absorption from the intestines, and calcium reabsorption by the kidneys 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
Critical Thinking Questions
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
Glossary Flashcards
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