15.4: Thyroid Gland and Parathyroid Glands
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- 63461
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Describe the location and anatomy of the thyroid gland
- Identify the hormones produced by the follicular cells and the parafollicular cells of the thyroid
- Describe the location and structure of the parathyroid glands
- Describe the hormonal control of blood calcium levels
Thyroid Gland
The thyroid gland, a butterfly-shaped organ, is located anterior to the trachea, just inferior to the larynx (Figure \(\PageIndex{1}\)). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes has a pair of parathyroid glands embedded on its posterior surface. The tissue of the thyroid gland is composed mostly of thyroid follicles lined with simple cuboidal epithelium (Figure \(\PageIndex{2}\)). The follicles are made up of a central cavity filled with a sticky fluid called colloid. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the essential and unique component of thyroid hormones: iodine.
Endocrine System: Iodine Deficiency, Hypothyroidism, and Hyperthyroidism
As discussed above, dietary iodine is required for the synthesis of T3 and T4. But for much of the world’s population, foods do not provide adequate levels of this mineral, because the amount varies according to the level in the soil in which the food was grown, as well as the irrigation and fertilizers used. Marine fish and shrimp tend to have high levels because they concentrate iodine from seawater, but many people in landlocked regions lack access to seafood. Thus, the primary source of dietary iodine in many countries is iodized salt. Fortification of salt with iodine began in the United States in 1924, and international efforts to iodize salt in the world’s poorest nations continue today.
Dietary iodine deficiency can result in the impaired ability to synthesize T3 and T4, leading to a variety of severe disorders. When T3 and T4 cannot be produced, TSH is secreted in increasing amounts. As a result of this hyperstimulation, thyroglobulin accumulates in the thyroid gland follicles, increasing their deposits of colloid. The accumulation of colloid increases the overall size of the thyroid gland, a condition called a goiter (Figure \(\PageIndex{4}\)). A goiter is only a visible indication of the deficiency. Other iodine deficiency disorders include impaired growth and development, decreased fertility, and prenatal and infant death. Moreover, iodine deficiency is the primary cause of preventable intellectual disability worldwide. Neonatal hypothyroidism (cretinism) is characterized by cognitive deficits, short stature, and sometimes deafness and muteness in children and adults born to mothers who were iodine-deficient during pregnancy.
In the absence of iodine deficiency, inflammation of the thyroid gland is the more common cause of low blood levels of thyroid hormones. Called hypothyroidism, the condition is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and intellectual disability.
In contrast, hyperthyroidism—an abnormally elevated blood level of thyroid hormones—is often caused by a pituitary or thyroid tumor. In Graves’ disease, the hyperthyroid state results from an autoimmune reaction in which antibodies overstimulate the follicle cells of the thyroid gland. Hyperthyroidism can lead to an increased metabolic rate, excessive body heat and sweating, diarrhea, weight loss, tremors, and increased heart rate. The person’s eyes may bulge (called exophthalmos) as antibodies produce inflammation in the soft tissues of the orbits.
Calcitonin
The thyroid gland also secretes a hormone called calcitonin that is produced by the parafollicular cells (also called clear cells or C cells) that stud the tissue between distinct follicles (Figure \(\PageIndex{2}\) and Figure \(\PageIndex{3}\)). Calcitonin is released in response to a rise in blood calcium levels.
The hormones secreted by the thyroid gland are summarized in Table \(\PageIndex{1}\).
Parathyroid Glands
The parathyroid glands are tiny, round structures usually found embedded in the posterior surface of the thyroid gland (Figure \(\PageIndex{5}\)). 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 parathyroid hormone (PTH), a major hormone involved in the regulation of blood calcium levels (Figure \(\PageIndex{6}\)). When blood calcium levels drop too low, PTH is released from the parathyroid glands into the bloodstream. Parathyroid hormone encourages the release of stored calcium from the bones and stimulates the kidneys to reabsorb calcium. In bones, PTH inhibits osteoblast activity while stimulating osteoclast activity, resulting in 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. The hormone secreted by the parathyroid glands is summarized in Table \(\PageIndex{1}\).
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
The thyroid gland is a butterfly-shaped organ located in the neck anterior to the trachea. The parathyroid glands are four small nodules on the posterior surface of the thyroid gland.
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 trigger secretion of the thyroid hormone calcitonin.
Thyroid hormones regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP, and calcium homeostasis. The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced and secreted by the follicular cells in response to thyroid-stimulating hormone (TSH) from the anterior pituitary. Synthesis of the amino acid–derived T3 and T4 hormones requires iodine. Insufficient amounts of iodine in the diet can lead to goiter and many other disorders.
The 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.
| Associated hormones | Target(s) | Effect(s) | Associated Disorders |
|---|---|---|---|
| Thyroid Hormone: Thyroxine (T4), triiodothyronine (T3) | Most body cells | Stimulate basal metabolic rate |
Hypothyroidism: goiter Hyperthyroidism: Graves disease |
| Calcitonin | Bones, kidneys | Reduces blood Ca+2 levels; increases Ca+2 deposition in bone | Hypersecretion: increased risk of some cancers; renal failure |
| Parathyroid Hormone | Bones, kidneys | Increases blood Ca+2 levels; decreases Ca+2 deposition in bone | Hypersecretion: increased risk of fractures, kidney stones, constipation; decreased mental activity and coma. |
Review Questions
Query \(\PageIndex{1}\)
Query \(\PageIndex{2}\)
Critical Thinking Questions
Query \(\PageIndex{3}\)
Query \(\PageIndex{4}\)
Glossary
Query \(\PageIndex{5}\)
Contributors and Attributions
OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology