{"id":4835,"date":"2019-06-24T15:01:56","date_gmt":"2019-06-24T15:01:56","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/11-5-thyroid-gland-3\/"},"modified":"2023-11-30T18:52:57","modified_gmt":"2023-11-30T18:52:57","slug":"11-5-thyroid-gland-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/11-5-thyroid-gland-3\/","title":{"raw":"9.5\u00a0Thyroid Gland","rendered":"9.5\u00a0Thyroid Gland"},"content":{"raw":" \r\n\r\n\"Enlarged\r\n\r\n[caption id=\"attachment_3452\" align=\"alignnone\" width=\"457\"]\"Goiter\" Figure 9.5.1 This goiter looks, and is, uncomfortable. It is also a sign that something in the endocrine system isn't quite right.[\/caption]\r\n\r\n
\r\n

Too Much of a Good Thing<\/h1>\r\n<\/div>\r\nThe woman in this photo has a[pb_glossary id=\"3451\"]\u00a0goiter<\/strong>[\/pb_glossary],\u00a0<\/strong>an abnormal enlargement of the thyroid gland, located in the neck. A goiter may form as a result of a number of different thyroid disorders. You\u2019ll learn why in this section.\r\n
\r\n

Thyroid Structure<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_3455\" align=\"alignright\" width=\"381\"]\"Thyroid Figure 9.5.2 The thyroid gland is a two-lobed gland in the front of the neck.<\/em>[\/caption]\r\n\r\nThe\u00a0[pb_glossary id=\"2958\"]thyroid gland[\/pb_glossary]<\/strong> is one of the largest endocrine glands in the body. It is located in the front of the neck below the Adam\u2019s apple (see Figure 9.5.2). The gland is butterfly shaped and composed of two lobes. The lobes are connected by a narrow band of thyroid tissue called an isthmus.\r\n
\r\n\r\nInternally, the thyroid gland is composed mainly of follicles. A <\/span>[pb_glossary id=\"6015\"]follicle[\/pb_glossary]<\/strong>\u00a0is a small cluster of cells surrounding a central cavity, which stores [pb_glossary id=\"5661\"]hormones[\/pb_glossary] and other molecules made by the follicular cells. Thyroid follicular cells are unique in being highly specialized to absorb and use [pb_glossary id=\"3457\"]iodine[\/pb_glossary]. They absorb iodine as iodide ions (I-) from the blood and use the iodide to produce thyroid hormones. The cells also use some of the iodide they absorb to form a protein called\u00a0<\/span>[pb_glossary id=\"3458\"]thyroglobulin[\/pb_glossary]<\/strong>, which serves to store iodide for later hormone synthesis. The outer layer of cells of each follicle secretes thyroid hormones as needed. Scattered among the follicles are another type of thyroid cells, called\u00a0<\/span>[pb_glossary id=\"3459\"]parafollicular cells[\/pb_glossary]<\/strong>\u00a0(or C cells). These cells synthesize and secrete the hormone [pb_glossary id=\"5921\"]calcitonin[\/pb_glossary].<\/span>\r\n\r\n<\/div>\r\n
\r\n

Function of the Thyroid<\/h1>\r\n<\/div>\r\nLike all endocrine glands, the function of the thyroid is to synthesize hormones and secrete them into the bloodstream. Once in the blood, they can travel to cells throughout the body and influence their functions.\r\n

Thyroid Hormones: T4 and T3<\/h2>\r\nThere are two main thyroid hormones produced by the follicles:\u00a0[pb_glossary id=\"3461\"]thyroxine[\/pb_glossary] (T4),<\/strong> which contains four iodide ions and is represented by the structural diagram below (Figure 9.5.3), and [pb_glossary id=\"3463\"]triiodothyronine[\/pb_glossary] (T3),<\/strong> which contains three iodide ions. T3 is much more powerful than T4, but T4 makes up about 90 per cent of circulating thyroid hormone, and T3 makes up only about ten per cent. However, most of the T4 is converted to T3 by target tissues.\r\n\r\n[caption id=\"attachment_3465\" align=\"alignnone\" width=\"500\"]\"Thyroxine\" Figure 9.5.3 This structural model represents a single molecule of thyroxine (T4). The I\u2019s represent the four iodide ions it contains. The rings consist mainly of carbon atoms.<\/em>[\/caption]\r\n\r\nLike steroid hormones, T3 and T4 cross cell membranes everywhere in the body and bind to intracellular receptors to regulate gene expression.\u00a0Unlike steroid hormones, however, thyroid hormones can cross cell membranes only with the help of special transporter proteins. Once inside the nucleus of cells, T3 and T4 turn on genes that control protein synthesis. Thyroid hormones increase the rate of metabolism in cells,\u00a0allowing them to\u00a0absorb more carbohydrates, use more energy, and produce more heat. Thyroid hormones also increase the rate and force of the heartbeat. In addition, they increase the sensitivity of cells to fight-or-flight hormones (that is, catecholamine hormones, such as adrenaline).\r\n\r\nThe production of both [pb_glossary id=\"3462\"]T4[\/pb_glossary] and [pb_glossary id=\"3464\"]T3[\/pb_glossary] is regulated primarily by thyroid stimulating hormone (TSH), which is secreted by the anterior pituitary gland (see Figure 9.5.4). TSH production, in turn, is regulated by thyrotropin releasing hormone (TRH), which is produced by the hypothalamus. The thyroid gland, pituitary gland, and hypothalamus form a negative feedback loop to keep thyroid hormone secretion within a normal range. TRH and TSH production is suppressed when T4 levels start to become too high. The opposite occurs when T4 levels start to become too low.\r\n\r\n[caption id=\"attachment_3466\" align=\"alignnone\" width=\"613\"]\"Thyroid Figure 9.5.4 The thyroid system is a negative feedback loop that includes the hypothalamus, pituitary gland, and thyroid gland. As this diagram shows, thyroid hormones increase the effect of catecholamines such as adrenaline, a fight-or-flight hormone.<\/em>[\/caption]\r\n

Calcitonin<\/h2>\r\nThe\u00a0[pb_glossary id=\"5921\"]calcitonin[\/pb_glossary]<\/strong>\u00a0produced by the [pb_glossary id=\"3459\"]parafollicular cells[\/pb_glossary] of the thyroid gland has the role of helping to regulate blood calcium levels by stimulating the movement of calcium into bone. Calcitonin is secreted in response to rising blood calcium levels. It decreases blood calcium levels by enhancing calcium absorption and deposition in bone. Calcitonin works hand-in-hand with [pb_glossary id=\"3467\"]parathyroid hormone[\/pb_glossary], which is secreted by the parathyroid glands and has the opposite effects as calcitonin. Together, these two hormones maintain calcium [pb_glossary id=\"5761\"]homeostasis[\/pb_glossary].\r\n
\r\n

Thyroid Disorders<\/h1>\r\n<\/div>\r\nAs with other endocrine disorders, thyroid disorders are generally associated with either over- or under-secretion of hormones. Abnormal secretion of thyroid hormones may occur for a variety of reasons.\r\n

Hyperthyroidism<\/h2>\r\n[caption id=\"attachment_3470\" align=\"alignright\" width=\"353\"]\"Grave's Figure 9.5.5 Protruding eyes are one sign of hyperthyroidism, such as Graves\u2019 disease.<\/em>[\/caption]\r\n\r\n[pb_glossary id=\"3468\"]Hyperthyroidism[\/pb_glossary]<\/strong>\u00a0occurs when the thyroid gland produces excessive amounts of thyroid hormones. The most common cause of hyperthyroidism is Graves\u2019 disease.\u00a0[pb_glossary id=\"3469\"]Graves\u2019 disease[\/pb_glossary] <\/strong>is an autoimmune disorder in which abnormal antibodies produced by the immune system stimulate the thyroid to secrete excessive quantities of its hormones. This stimulation overrides the usual [pb_glossary id=\"2955\"]negative feedback[\/pb_glossary] mechanism that normally controls thyroid hormone output. Graves\u2019 disease often results in the formation of an enlarged thyroid ([pb_glossary id=\"3451\"]goiter[\/pb_glossary]) because of the continued stimulation to produce more hormones.\r\n\r\nBesides a goiter, other signs and symptoms of hyperthyroidism may include protruding eyes (see Figure 9.5.5), heart palpitations, excessive sweating, diarrhea, weight loss despite increased appetite, muscle weakness, and unusual sensitivity to heat. Medications can be prescribed to mitigate the symptoms of the disease. Anti-thyroid drugs can also be given to decrease the production of thyroid hormones. If the drugs are ineffective, the gland can be partially or entirely removed. This can be done surgically or with the administration of radioactive iodine. Removal of the thyroid produces hypothyroidism.\r\n\r\n \r\n
\r\n\r\nHypothyroidism<\/span>\r\n\r\n<\/div>\r\n[pb_glossary id=\"3471\"]Hypothyroidism[\/pb_glossary]<\/strong>\u00a0occurs when the thyroid gland produces insufficient amounts of thyroid hormones. It can result from surgical removal of the thyroid. However, worldwide, the most common cause of hypothyroidism is dietary iodine deficiency. In cases of iodine deficiency, the negative feedback loop controlling the release of thyroid hormone causes repeated stimulation of the thyroid, resulting in the thyroid gland growing in size and producing a goiter. Although the gland gets larger, it cannot increase hormone output because of the lack of iodine in the diet.\r\n\r\nIodine deficiency is uncommon in the Western world because iodine is added to salt. In places like this where iodine deficiency isn't\u00a0a problem, the most common cause of hypothyroidism is\u00a0Hashimoto\u2019s thyroiditis.<\/strong>\u00a0This is another autoimmune disease, but in this case, the immune system destroys the thyroid gland, producing hypothyroidism. Hashimoto\u2019s thyroiditis tends to run in families, so it is likely to have a genetic component. It usually appears after the age of 30, and is more common in females than males.\r\n\r\nHypothyroidism produces many signs and symptoms, as shown in Figure 9.5.6. These may include abnormal weight gain, tiredness, baldness, cold intolerance, and slow heart rate. Hypothyroidism is generally treated with thyroid hormone replacement therapy. This may be needed for the rest of a person\u2019s life.\r\n\r\n[caption id=\"attachment_3472\" align=\"aligncenter\" width=\"808\"]\"\" Figure 9.5.6 Hypothyroidism generally causes symptoms that are the opposite of those caused by hyperthyroidism.<\/em>[\/caption]\r\n\r\nHypothyroidism in a pregnant woman can have serious adverse consequences for the fetus. During the fetal period, cells of the developing brain are a major target for thyroid hormones, which play a crucial role in brain maturation. When levels of thyroid hormones are too low, the fetus may suffer permanent deficits in cognitive abilities. Deafness is also a potential outcome of hypothyroidism\u00a0in utero<\/em>.\r\n
\r\n

Feature: Myth vs. Reality<\/h1>\r\n<\/div>\r\nThyroid disorders are relatively common, affecting as many as 20 million people in the United States. According to recent studies, one in ten Canadians have some type of thyroid condition and up to 50 per cent may be undiagnosed! Because thyroid disorders are common, there are also many common myths about them.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
\r\n

Myth<\/span><\/h2>\r\n<\/td>\r\n

\r\n

Reality<\/span><\/h2>\r\n<\/td>\r\n<\/tr>\r\n

\"If you have a thyroid problem, you will know something is wrong because you will have obvious symptoms.\"<\/em><\/td>\r\nThe majority of people with a thyroid disorder are not aware they have it, because the symptoms are often mild, nonspecific, and easy to ignore. Generally, blood tests of thyroid hormone levels are needed to make a conclusive diagnosis.<\/strong><\/td>\r\n<\/tr>\r\n
\"If you are diagnosed with a thyroid disorder, you will have to take medication for the rest of your life.\"<\/em><\/td>\r\nWhether you need to continue thyroid medication for life depends on the cause of the disorder. For example, some women develop hypothyroidism during pregnancy but no longer need medication after the pregnancy is over and hormone levels return to normal.<\/strong><\/td>\r\n<\/tr>\r\n
\"As soon as you start taking thyroid medication, your symptoms will resolve.\"<\/em><\/td>\r\nIt often takes weeks \u2014 or even months \u2014 for thyroid hormone levels to return to normal and for symptoms to disappear.<\/strong><\/td>\r\n<\/tr>\r\n
\"You can take an over-the-counter iodine supplement to correct hypothyroidism.\"<\/em><\/td>\r\nIn the United States, where dietary iodine is almost always adequate, iodine deficiency is unlikely to be the cause of hypothyroidism. Therefore, taking supplemental iodine is not likely to correct the problem.<\/strong><\/td>\r\n<\/tr>\r\n
\"If thyroid symptoms are mild, you don\u2019t need to take medication.\"<\/em><\/td>\r\nBecause thyroid hormones are responsible for so many vital body functions, failing to treat even a mild thyroid disorder may lead to a range of other problems, such as osteoporosis or infertility.<\/strong><\/td>\r\n<\/tr>\r\n
\"Goiter may be caused by eating \u201cgoitrogenic\u201d vegetables, such as broccoli, Brussels sprouts, and spinach.\"<\/em><\/td>\r\nAlthough these foods can interfere with the thyroid\u2019s ability to process iodide, you would have to eat\u00a0huge<\/em>\u00a0amounts of them to cause a goiter.<\/strong><\/td>\r\n<\/tr>\r\n
\"Thyroid disorders occur only after middle age and only in women.\"<\/em><\/td>\r\nThyroid disorders may occur at any age and in any sex. Hypothyroidism occurs more commonly in older adults, but hyperthyroidism occurs more commonly in younger adults. Although women are more likely to develop thyroid disorders, about 20 per cent of cases occur in men.<\/strong><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n \r\n
\r\n
\r\n

9.5 Summary<\/span><\/h1>\r\n<\/header>\r\n
\r\n