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15.4: Thyroid Gland

  • Page ID
    22357
  • By the end of this section, you will be able to:

    • Describe the location and anatomy of the thyroid gland
    • Explain the role of thyroid hormones in the regulation of basal metabolism
    • Identify the hormone produced by the parafollicular cells of the thyroid

    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. 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.

    Gross and Histological Anatomy of the Thyroid Gland
    Figure \(\PageIndex{1}\): Thyroid Gland. The thyroid gland is located in the neck where it wraps around the anterior of the trachea. (a) Anterior view of the thyroid gland showing its location below the hyoid bone and thyroid cartilage of the larynx and between the common carotid arteries. The bilateral lobes are connected across the midline by the thinner isthmus of the thyroid, giving the gland a butterfly shape. The anterior of the thyroid gland is supplied blood by the superior thyroid artery. (b) Posterior view of the thyroid gland showing the bilateral pair of parathyroid glands on the thyroid gland's posterior surface just lateral to the trachea at its superior connection to the cricoid cartilage of the larynx. The posterior of the thyroid and the parathyroid glands are supplied blood by the right and left inferior thyroid arteries that branch from the right and left subclavian arteries, respectively. (c) A histological view show the glandular tissue is composed primarily of thyroid follicles lined with cuboidal follicle cells that surround a central cavity filled with colloid. The larger parafollicular cells often appear within the matrix of follicle cells. LM × 1332. (Image credit: "The Thyroid Gland" by OpenStax is licensed under CC BY 3.0 / Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    Regulation of TH Synthesis

    Thyroglobulin is the precursor to two thyroid hormones: triiodothyronine (T3) with three iodines and thyroxine (T4) with four iodines. Thyroglobulin is produced by the follicle cells and secreted into the colloid where the iodines are attached to form T3 and T4. Ninety-nine percent of circulating T3 and T4 is bound to specialized transport proteins called thyroxine-binding globulins (TBGs), to albumin, or to other plasma proteins. This “packaging” prevents their free diffusion into body cells. When blood levels of T3 and T4 begin to decline, bound T3 and T4 are released from these plasma proteins, now referred to as "unbound", and may readily cross the membrane of target cells. T3 is more potent than T4, and many cells convert T4 to T3 through the removal of an iodine atom.

    The release of T3 and T4 from the thyroid gland is regulated by thyroid-stimulating hormone (TSH). As shown in Figure \(\PageIndex{2}\), low blood levels of T3 and T4 stimulate the release of thyrotropin-releasing hormone (TRH) from the hypothalamus, which triggers secretion of TSH from the anterior pituitary. In turn, TSH stimulates the thyroid gland to secrete T3 and T4. The levels of TRH, TSH, T3, and T4 are regulated by a negative feedback system in which increasing levels of T3 and T4 decrease the production and secretion of TSH.

    Negative Feedback Loop Regulation of T3 and T4 Hormone Levels
    Figure \(\PageIndex{2}\): Regulation of Thyroid Hormone Release. A classic negative feedback loop controls the regulation of thyroid hormone levels. If metabolic rate and/or T3 and T4 levels are low, the hypothalamus releases TRH, which triggers TSH release by the anterior pituitary gland. TSH release triggers the thryoid gland to release T3 and T4 from thyroid follicles. T3 and T4 release cause an increased balal metabolic rate of body cells and rise in body temperature (the calorigenic effect). If metabolic rate and/or T3 and T4 levels are high, the hypothalamus stops TRH release, which stops the release of TSH from the anterior pituitary. This inhibition is referred to as negative feedback. (Image credit: "A Classic Negative Feedback Loop" by OpenStax is licensed under CC BY 3.0)

    Functions of Thyroid Hormones

    The thyroid hormones, T3 and T4, are often referred to as metabolic hormones because their levels influence the body’s basal metabolic rate, the amount of energy used by the body at rest. When T3 and T4 bind to intracellular receptors located on the mitochondria, they cause an increase in nutrient breakdown and the use of oxygen to produce ATP. In addition, T3 and T4 initiate the transcription of genes involved in glucose oxidation. Although these mechanisms prompt cells to produce more ATP, the process is inefficient, and an abnormally increased level of heat is released as a byproduct of these reactions. This so-called calorigenic effect (calor- = “heat”) raises body temperature.

    Adequate levels of thyroid hormones are also required for protein synthesis and for fetal and childhood tissue development and growth. They are especially critical for normal development of the nervous system both in utero and in early childhood, and they continue to support neurological function in adults. As noted earlier, these thyroid hormones have a complex interrelationship with reproductive hormones, and deficiencies can influence libido, fertility, and other aspects of reproductive function. Finally, thyroid hormones increase the body’s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T3 and T4 hormones are excessive, this effect accelerates the heart rate, strengthens the heartbeat, and increases blood pressure. Because thyroid hormones regulate metabolism, heat production, protein synthesis, and many other body functions, thyroid disorders can have severe and widespread consequences.

    DISORDERS OF THE...

    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 \(\PageIndex{3}\)). 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.

    A goiter from an extremely enlarged thyroid is visible in a photograph of a person's neck.
    Figure \(\PageIndex{3}\): Goiter. A goiter is the abnormal enlargement of the thyroid gland. (Image credit: "Goiter" by Almazi is licensed under CC BY 4.0)

    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 thyrocytes) that stud the tissue between distinct follicles. Calcitonin is released in response to a rise in blood calcium levels. It appears to have a function in decreasing blood calcium concentrations by:

    • Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix
    • Increasing osteoblastic activity
    • Decreasing calcium absorption in the intestines
    • Decreasing calcium reabsorption in the kidneys, which increases calcium loss in the urine

    However, these functions are usually not significant in maintaining calcium homeostasis, so the importance of calcitonin is not entirely understood. Pharmaceutical preparations of calcitonin are sometimes prescribed to reduce osteoclast activity in people with osteoporosis and to reduce the degradation of cartilage in people with osteoarthritis. The hormones secreted by thyroid are summarized in Table \(\PageIndex{1}\).

    Table \(\PageIndex{1}\): Thyroid Hormones
    Associated hormones Effect
    Thyroxine (T4), triiodothyronine (T3) Stimulate basal metabolic rate
    Calcitonin Reduces blood Ca2+ levels

    Of course, calcium is critical for many other biological processes. It is a second messenger in many signaling pathways, and is essential for muscle contraction, nerve impulse transmission, and blood clotting. Given these roles, it is not surprising that blood calcium levels are tightly regulated by the endocrine system. The parathyroid glands, which will be discussed in more detail in the next section, are also involved in the regulation of calcium.

    Concept Review

    The thyroid gland is a butterfly-shaped organ located in the neck anterior to the trachea. Its hormones regulate basal metabolism, oxygen use, nutrient metabolism, the production of ATP, and calcium homeostasis. They also contribute to protein synthesis and the normal growth and development of body tissues, including maturation of the nervous system, and they increase the body’s sensitivity to catecholamines. The thyroid hormones triiodothyronine (T3) and thyroxine (T4) are produced and secreted by the thyroid gland 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.

    Review Questions

    Q. Which of the following statements about the thyroid gland is true?

    A. It is located anterior to the trachea and inferior to the larynx.

    B. The parathyroid glands are embedded within it.

    C. It manufactures three hormones.

    D. all of the above

    Answer

    Answer: D

    Q. The secretion of thyroid hormones is controlled by ________.

    A. TSH from the hypothalamus

    B. TSH from the anterior pituitary

    C. thyroxine from the anterior pituitary

    D. thyroglobulin from the thyroid’s parafollicular cells

    Answer

    Answer: B

    Q. The development of a goiter indicates that ________.

    A. the anterior pituitary is abnormally enlarged

    B. there is hypertrophy of the thyroid’s follicle cells

    C. there is an excessive accumulation of colloid in the thyroid follicles

    D. the anterior pituitary is secreting excessive growth hormone

    Answer

    Answer: C

    Critical Thinking Questions

    Q. Explain why maternal iodine deficiency might lead to neurological impairment in the fetus.

    Answer

    A. Iodine deficiency in a pregnant woman would also deprive the fetus. Iodine is required for the synthesis of thyroid hormones, which contribute to fetal growth and development, including maturation of the nervous system. Insufficient amounts would impair these functions.

    Q. Define hyperthyroidism and explain why one of its symptoms is weight loss.

    Answer

    A. Hyperthyroidism is an abnormally elevated blood level of thyroid hormones due to an overproduction of T3 and T4. An individual with hyperthyroidism is likely to lose weight because one of the primary roles of thyroid hormones is to increase the body’s basal metabolic rate, increasing the breakdown of nutrients and the production of ATP.

    Glossary

    calcitonin
    peptide hormone produced and secreted by the parafollicular cells (C cells) of the thyroid gland that functions to decrease blood calcium levels
    colloid
    viscous fluid in the central cavity of thyroid follicles, containing the glycoprotein thyroglobulin
    goiter
    enlargement of the thyroid gland either as a result of iodine deficiency or hyperthyroidism
    hyperthyroidism
    clinically abnormal, elevated level of thyroid hormone in the blood; characterized by an increased metabolic rate, excess body heat, sweating, diarrhea, weight loss, and increased heart rate
    hypothyroidism
    clinically abnormal, low level of thyroid hormone in the blood; characterized by low metabolic rate, weight gain, cold extremities, constipation, and reduced mental activity
    neonatal hypothyroidism
    condition characterized by cognitive deficits, short stature, and other signs and symptoms in people born to women who were iodine-deficient during pregnancy
    thyroid gland
    large endocrine gland responsible for the synthesis of thyroid hormones
    thyroxine
    (also, tetraiodothyronine, T4) amino acid–derived thyroid hormone that is more abundant but less potent than T3 and often converted to T3 by target cells
    triiodothyronine
    (also, T3) amino acid–derived thyroid hormone that is less abundant but more potent than T4

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