{"id":4095,"date":"2019-05-31T20:26:30","date_gmt":"2019-05-31T20:26:30","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-dutchess-anatomy-physiology\/chapter\/the-thyroid-gland\/"},"modified":"2019-08-11T14:08:22","modified_gmt":"2019-08-11T14:08:22","slug":"the-thyroid-gland","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-dutchess-anatomy-physiology\/chapter\/the-thyroid-gland\/","title":{"raw":"The Thyroid Gland","rendered":"The Thyroid Gland"},"content":{"raw":"
\r\n\r\n \r\n\r\n<\/div>\r\nA butterfly-shaped organ, the thyroid gland<\/strong> is located anterior to the trachea, just inferior to the larynx (Figure 1). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces. The tissue of the thyroid gland is composed mostly of thyroid follicles. The follicles are made up of a central cavity filled with a sticky fluid called colloid<\/strong>. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the hormones\u2019 essential and unique component: iodine.\r\n\r\n[caption id=\"attachment_1913\" align=\"aligncenter\" width=\"1024\"]\"Part Figure 1. The thyroid gland is located in the neck where it wraps around the trachea. (a) Anterior view of the thyroid gland. (b) Posterior view of the thyroid gland. (c) The glandular tissue is composed primarily of thyroid follicles. The larger parafollicular cells often appear within the matrix of follicle cells. LM \u00d7 1332. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]\r\n\r\n \r\n

Functions of Thyroid Hormones<\/h2>\r\nThe thyroid hormones, T3<\/sub> and T4<\/sub>, are often referred to as metabolic hormones because their levels influence the body\u2019s basal metabolic rate, the amount of energy used by the body at rest. When T3<\/sub> and T4<\/sub> 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<\/sub> and T4<\/sub> 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- = \u201cheat\u201d) raises body temperature.\r\n\r\nAdequate 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\u2019s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T3<\/sub> and T4<\/sub> 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.\r\n
\r\n

Disorders of the\u00a0Endocrine System<\/h3>\r\n

Iodine Deficiency, Hypothyroidism, and Hyperthyroidism<\/h4>\r\nAs discussed above, dietary iodine is required for the synthesis of T3<\/sub> and T4<\/sub>. But for much of the world\u2019s 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\u2019s poorest nations continue today.\r\n\r\nDietary iodine deficiency can result in the impaired ability to synthesize T3<\/sub> and T4<\/sub>, leading to a variety of severe disorders. When T3<\/sub> and T4<\/sub> 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<\/strong> (Figure 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 mental retardation worldwide. Neonatal hypothyroidism<\/strong> (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.\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"300\"]\"This Figure 3. Goiter (credit: \u201cAlmazi\u201d\/Wikimedia Commons)[\/caption]\r\n\r\nIn areas of the world with access to iodized salt, dietary deficiency is rare. Instead, inflammation of the thyroid gland is the more common cause of low blood levels of thyroid hormones. Called hypothyroidism<\/strong>, the condition is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and reduced mental activity.\r\n\r\nIn contrast, hyperthyroidism<\/strong>\u2014an abnormally elevated blood level of thyroid hormones\u2014is often caused by a pituitary or thyroid tumor. In Graves\u2019 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\u2019s eyes may bulge (called exophthalmos) as antibodies produce inflammation in the soft tissues of the orbits. The person may also develop a goiter.\r\n\r\n<\/div>\r\n

Calcitonin<\/h2>\r\nThe thyroid gland also secretes a hormone called calcitonin<\/strong> that is produced by the parafollicular cells (also called C cells) 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:\r\n
    \r\n \t
  • Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix<\/li>\r\n \t
  • Increasing osteoblastic activity<\/li>\r\n \t
  • Decreasing calcium absorption in the intestines<\/li>\r\n \t
  • Increasing calcium loss in the urine<\/li>\r\n<\/ul>\r\nHowever, 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 1.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
    Table 1. Thyroid Hormones<\/th>\r\n<\/tr>\r\n
    Associated hormones<\/th>\r\nChemical class<\/th>\r\nEffect<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
    Thyroxine (T4<\/sub>), triiodothyronine (T3<\/sub>)<\/td>\r\nAmine<\/td>\r\nStimulate basal metabolic rate<\/td>\r\n<\/tr>\r\n
    Calcitonin<\/td>\r\nPeptide<\/td>\r\nReduces blood Ca2+<\/sup> levels<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nOf 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 organs involved in the regulation are the parathyroid glands.\r\n

    Chapter Review<\/h2>\r\nThe 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\u2019s sensitivity to catecholamines. The thyroid hormones triiodothyronine (T3<\/sub>) and thyroxine (T4<\/sub>) are produced and secreted by the thyroid gland in response to thyroid-stimulating hormone (TSH) from the anterior pituitary. Synthesis of the amino acid\u2013derived T3<\/sub> and T4<\/sub> hormones requires iodine. Insufficient amounts of iodine in the diet can lead to goiter, cretinism, and many other disorders.\r\n

    Self Check<\/h2>\r\nAnswer the question(s) below to see how well you understand the topics covered in the previous section.\r\n\r\nhttps:\/\/oea.herokuapp.com\/assessments\/233\r\n
    \r\n

    Critical Thinking Questions<\/h3>\r\n
      \r\n \t
    1. Explain why maternal iodine deficiency might lead to neurological impairment in the fetus.<\/li>\r\n \t
    2. Define hyperthyroidism and explain why one of its symptoms is weight loss.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"244886\"]Show Answers[\/reveal-answer]\r\n[hidden-answer a=\"244886\"]\r\n
        \r\n \t
      1. 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.<\/li>\r\n \t
      2. Hyperthyroidism is an abnormally elevated blood level of thyroid hormones due to an overproduction of T3<\/sub> and T4<\/sub>. An individual with hyperthyroidism is likely to lose weight because one of the primary roles of thyroid hormones is to increase the body\u2019s basal metabolic rate, increasing the breakdown of nutrients and the production of ATP.<\/li>\r\n<\/ol>\r\n[\/hidden-answer]\r\n\r\n<\/div>\r\n

        Glossary<\/h2>\r\ncalcitonin:\u00a0<\/strong>peptide hormone produced and secreted by the parafollicular cells (C cells) of the thyroid gland that functions to decrease blood calcium levels\r\n\r\ncolloid:\u00a0<\/strong>viscous fluid in the central cavity of thyroid follicles, containing the glycoprotein thyroglobulin\r\n\r\ngoiter:\u00a0<\/strong>enlargement of the thyroid gland either as a result of iodine deficiency or hyperthyroidism\r\n\r\nhyperthyroidism:\u00a0<\/strong>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\r\n\r\nhypothyroidism:\u00a0<\/strong>clinically abnormal, low level of thyroid hormone in the blood; characterized by low metabolic rate, weight gain, cold extremities, constipation, and reduced mental activity\r\n\r\nneonatal hypothyroidism:\u00a0<\/strong>condition characterized by cognitive deficits, short stature, and other signs and symptoms in people born to women who were iodine-deficient during pregnancy\r\n\r\nthyroid gland:\u00a0<\/strong>large endocrine gland responsible for the synthesis of thyroid hormones\r\n\r\nthyroxine:\u00a0<\/strong>(also, tetraiodothyronine, T4<\/sub>) amino acid\u2013derived thyroid hormone that is more abundant but less potent than T3 <\/sub>and often converted to T3<\/sub> by target cells\r\n\r\ntriiodothyronine:\u00a0<\/strong>(also, T3<\/sub>) amino acid\u2013derived thyroid hormone that is less abundant but more potent than T4<\/sub>","rendered":"
        \n

         <\/p>\n<\/div>\n

        A butterfly-shaped organ, the thyroid gland<\/strong> is located anterior to the trachea, just inferior to the larynx (Figure 1). The medial region, called the isthmus, is flanked by wing-shaped left and right lobes. Each of the thyroid lobes are embedded with parathyroid glands, primarily on their posterior surfaces. The tissue of the thyroid gland is composed mostly of thyroid follicles. The follicles are made up of a central cavity filled with a sticky fluid called colloid<\/strong>. Surrounded by a wall of epithelial follicle cells, the colloid is the center of thyroid hormone production, and that production is dependent on the hormones\u2019 essential and unique component: iodine.<\/p>\n

        \"Part<\/p>\n

        Figure 1. The thyroid gland is located in the neck where it wraps around the trachea. (a) Anterior view of the thyroid gland. (b) Posterior view of the thyroid gland. (c) The glandular tissue is composed primarily of thyroid follicles. The larger parafollicular cells often appear within the matrix of follicle cells. LM \u00d7 1332. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/p>\n<\/div>\n

         <\/p>\n

        Functions of Thyroid Hormones<\/h2>\n

        The thyroid hormones, T3<\/sub> and T4<\/sub>, are often referred to as metabolic hormones because their levels influence the body\u2019s basal metabolic rate, the amount of energy used by the body at rest. When T3<\/sub> and T4<\/sub> 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<\/sub> and T4<\/sub> 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- = \u201cheat\u201d) raises body temperature.<\/p>\n

        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\u2019s sensitivity to catecholamines (epinephrine and norepinephrine) from the adrenal medulla by upregulation of receptors in the blood vessels. When levels of T3<\/sub> and T4<\/sub> 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.<\/p>\n

        \n

        Disorders of the\u00a0Endocrine System<\/h3>\n

        Iodine Deficiency, Hypothyroidism, and Hyperthyroidism<\/h4>\n

        As discussed above, dietary iodine is required for the synthesis of T3<\/sub> and T4<\/sub>. But for much of the world\u2019s 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\u2019s poorest nations continue today.<\/p>\n

        Dietary iodine deficiency can result in the impaired ability to synthesize T3<\/sub> and T4<\/sub>, leading to a variety of severe disorders. When T3<\/sub> and T4<\/sub> 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<\/strong> (Figure 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 mental retardation worldwide. Neonatal hypothyroidism<\/strong> (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.<\/p>\n

        \"This<\/p>\n

        Figure 3. Goiter (credit: \u201cAlmazi\u201d\/Wikimedia Commons)<\/p>\n<\/div>\n

        In areas of the world with access to iodized salt, dietary deficiency is rare. Instead, inflammation of the thyroid gland is the more common cause of low blood levels of thyroid hormones. Called hypothyroidism<\/strong>, the condition is characterized by a low metabolic rate, weight gain, cold extremities, constipation, reduced libido, menstrual irregularities, and reduced mental activity.<\/p>\n

        In contrast, hyperthyroidism<\/strong>\u2014an abnormally elevated blood level of thyroid hormones\u2014is often caused by a pituitary or thyroid tumor. In Graves\u2019 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\u2019s eyes may bulge (called exophthalmos) as antibodies produce inflammation in the soft tissues of the orbits. The person may also develop a goiter.<\/p>\n<\/div>\n

        Calcitonin<\/h2>\n

        The thyroid gland also secretes a hormone called calcitonin<\/strong> that is produced by the parafollicular cells (also called C cells) 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:<\/p>\n

          \n
        • Inhibiting the activity of osteoclasts, bone cells that release calcium into the circulation by degrading bone matrix<\/li>\n
        • Increasing osteoblastic activity<\/li>\n
        • Decreasing calcium absorption in the intestines<\/li>\n
        • Increasing calcium loss in the urine<\/li>\n<\/ul>\n

          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 1.<\/p>\n\n\n\n\n\n\n\n
          Table 1. Thyroid Hormones<\/th>\n<\/tr>\n
          Associated hormones<\/th>\nChemical class<\/th>\nEffect<\/th>\n<\/tr>\n<\/thead>\n
          Thyroxine (T4<\/sub>), triiodothyronine (T3<\/sub>)<\/td>\nAmine<\/td>\nStimulate basal metabolic rate<\/td>\n<\/tr>\n
          Calcitonin<\/td>\nPeptide<\/td>\nReduces blood Ca2+<\/sup> levels<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

          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 organs involved in the regulation are the parathyroid glands.<\/p>\n

          Chapter Review<\/h2>\n

          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\u2019s sensitivity to catecholamines. The thyroid hormones triiodothyronine (T3<\/sub>) and thyroxine (T4<\/sub>) are produced and secreted by the thyroid gland in response to thyroid-stimulating hormone (TSH) from the anterior pituitary. Synthesis of the amino acid\u2013derived T3<\/sub> and T4<\/sub> hormones requires iodine. Insufficient amounts of iodine in the diet can lead to goiter, cretinism, and many other disorders.<\/p>\n

          Self Check<\/h2>\n

          Answer the question(s) below to see how well you understand the topics covered in the previous section.<\/p>\n