出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2016/09/19 23:38:55」(JST)
| Thyroid | |
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The human thyroid as viewed from the front, with arteries visible.
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The thyroid as it may be seen from a posterior view, from behind the trachea
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| Details | |
| Precursor | Thyroid diverticulum (an extension of endoderm into 2nd pharyngeal arch) |
| System | Endocrine system |
| Artery | Superior, Inferior thyroid arteries |
| Vein | Superior, middle, Inferior thyroid veins |
| Identifiers | |
| Latin | Glandula thyreoidea |
| MeSH | A06.407.900 |
| TA | A11.3.00.001 |
| FMA | 9603 |
| Anatomical terminology
[edit on Wikidata]
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The thyroid gland, or simply the thyroid /ˈθaɪrɔɪd/, is an endocrine gland in the body, and consists of two connected lobes. It is found at the front of the neck, below the laryngeal prominence (Adam's apple). The thyroid gland secretes thyroid hormones, which influence the metabolic rate, protein synthesis, and have a wide range of other effects, including on development. The thyroid hormones T3 and T4 are synthesized from iodine and tyrosine. The thyroid also produces calcitonin, which plays a role in calcium homeostasis. [1]
Hormonal output from the thyroid is regulated by thyroid-stimulating hormone (TSH) secreted from the anterior pituitary, which itself is regulated by thyrotropin-releasing hormone (TRH) produced by the hypothalamus.[2]
The thyroid may be affected by several diseases. Hyperthyroidism occurs when the gland produces excessive amounts of thyroid hormones, the most common cause being Graves' disease—an autoimmune disorder. In contrast, hypothyroidism is a state of insufficient thyroid hormone production. Worldwide, the most common cause is iodine deficiency. Thyroid hormones are important for development, and hypothyroidism secondary to iodine deficiency remains the leading cause of preventable intellectual disability.[3] In iodine-sufficient regions, the most common cause of hypothyroidism is Hashimoto's thyroiditis—also an autoimmune disease. In addition, the thyroid gland may also develop several types of nodules and cancer.
The thyroid gland is a butterfly-shaped organ and is composed of two lobes, one on the right and the left as the wings, and the narrow connecting isthmus as the body.[4][5]The thyroid is one of the larger endocrine glands, weighing 2-3 grams in neonates[citation needed] and 25 grams in adults, and is increased in pregnancy.[4] Each lobe is about 5 cm long, 3 cm wide and 2 cm thick.[4] The lobes are asymmetrical, with the right lobe usually larger. The gland itself is usually larger in women.[5]The isthmus of the thyroid gland connects together the lower thirds of the right and left lobes of the thyroid gland. The isthmus measures about 1.25 cm in breadth, and the same in depth,[4] and usually covers the second and third rings of the trachea. There are, however, many variations in its situation and size. The isthmus lies at the front of the neck and is covered by the skin and fascia, and close to the middle line, on either side, by the sternothyroid muscles. Across its upper border runs an anastomotic branch uniting the two superior thyroid arteries; at its lower border are the inferior thyroid veins.
The thyroid sits near the front of the neck, lying against and around the front of the larynx and trachea.[4] The top of the thyroid lies below the thyroid cartilage (just below the laryngeal prominence, or Adam's apple, and extends to the fifth or sixth tracheal ring. It is difficult to demarcate the gland's upper and lower border with vertebral levels because it moves position in relation to these during swallowing.[6] However it usually spans from C5 to C7.[5]
The thyroid gland is covered by a thin fibrous sheath, the capsule of the thyroid.[4] The external layer is anteriorly continuous with the pretracheal fascia and posterolaterally continuous with the carotid sheath.[citation needed] The capsule extrudes into the gland itself and forms the septae that divides the thyroid tissue into microscopic lobules.[4]
The gland is covered anteriorly with infrahyoid muscles and laterally with the sternocleidomastoid muscle. On the posterior side, the gland is fixed to the cricoid and tracheal cartilages and cricopharyngeus muscle by a thickening of the fascia to form the posterior suspensory ligament of thyroid gland also known as Berry's ligament.[7][8] The thyroid gland's firm attachment to the underlying trachea is the reason behind its movement with swallowing.[9]
In this region, the recurrent laryngeal nerve and the inferior thyroid artery pass next to or in the ligament, and where present Zuckerkandl's tubercle.[10]
Two parathyroid glands usually lie on each side between the two layers of the capsule, at the back of the thyroid lobes.[4]
The thyroid is supplied with arterial blood from the superior thyroid artery, a branch of the external carotid artery, and the inferior thyroid artery, a branch of the thyrocervical trunk, and sometimes by an anatomical variant the thyroid ima artery, branching directly from the subclavian artery.[4] The superior thyroid artery splits into anterior and posterior branches supplying the thyroid, and the inferior thyroid artery splits into superior and inferior branches.[4] The venous blood is drained via superior and middle thyroid veins, which drain to the internal jugular vein, and via the inferior thyroid veins. The inferior thyroid veins originate in a network of veins and drain into the left and right brachiocephalic veins.[4]
Lymphatic drainage passes frequently the deep lateral cervical lymph nodes, and the pretracheal and paratracheal lymph nodes. The gland is supplied by parasympathetic nerve input from the superior laryngeal nerve and the recurrent laryngeal nerve.
Sometimes there is a third lobe present called the pyramidal lobe of the thyroid gland.[11] One study showed an 18.3% presence of this extra lobe.[12] Another study showed a presence of 44.6%.[13]The pyramidal lobe grows upwards from the isthmus to the hyoid bone. It was shown to more often arise from the left side and also to be mostly attached to the main gland with 9.2% shown to be separated.[12] The pyramidal lobe is a remnant of the thyroglossal duct (fetal thyroid stalk) which usually wastes away during the thyroid gland’s descent.[14]The pyramidal lobe is also known as Lalouette's pyramid.[15]
The thyroid isthmus is variable in its situation and size and can also change in shape. It can encompass the pyramidal lobe (lobus or processus pyramidalis.[citation needed] In variable extent, the pyramidal lobe is present at the most anterior side of the lobe.
At the microscopic level, there are three primary features of the thyroid—follicles, follicular cells, and parafollicular cells, first discovered by Geoffery Websterson in 1664.[16]
Follicles are small spherical groupings of cells 0.02-0.9mm in diameter that play the main role in thyroid function.[4] They consist of a rim that has a rich blood flow, nerve and lymphatic supply, that surrounds a core of colloid that consists mostly of thyroid hormone precursor proteins called thyroglobulin, an iodinated glycoprotein.[4][17]
The core of follicles is surrounded by a single layer of follicular cells. When stimulated by thyroid stimulating hormone (TSH), these secrete the thyroid hormones T3 and T4. They do this by transporting and metabolising the thyroglobulin contained in the colloid.[4] Follicular cells vary in shape from flat to cuboid to columnar, depending on how active they are.[4][17]
Scattered among follicular cells and in spaces between the spherical follicles are another type of thyroid cell, parafollicular cells.[4] These cells secrete calcitonin. Because the cytoplasm of these cells is clear, they are also called "C cells".[4]
In the development of the embryo, at 3–4 weeks gestational age, the thyroid gland appears as an epithelial proliferation in the floor of the pharynx at the base of the tongue between the tuberculum impar and the copula linguae. The copula soon becomes covered over by the hypopharyngeal eminence [18] at a point later indicated by the foramen cecum. The thyroid then descends in front of the pharyngeal gut as a bilobed diverticulum through the thyroglossal duct. Over the next few weeks, it migrates to the base of the neck, passing in front of the hyoid bone. During migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct. At the end of the fifth week the thyroglossal duct degenerates and the detached thyroid continues on to its final position over the following two weeks.[18]
The fetal hypothalamus and pituitary start to secrete thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) at 18-20 weeks, and the production of thyroxine (T4) reaches a clinically significant level at this time.[19] Fetal triiodothyronine (T3) remains low, less than 15 ng/dL (nanograms per decilitre) until 30 weeks, and increases to 50 ng/dL at full-term.[19] The fetus needs to be self-sufficient in thyroid hormones in order to guard against neurodevelopmental disorders that would arise from maternal hypothyroidism.[20] Preterm neonates are at risk of these disorders as their thyroid glands are insufficiently developed to meet their postnatal needs.[21]
The neuroendocrine parafollicular cells, also known as C cells, responsible for the production of calcitonin, are derived from neural crest cells, which migrate to the pharyngeal arches. This part of the thyroid then first forms as the ultimopharyngeal body, which begins in the ventral fourth pharyngeal pouch and joins the primordial thyroid gland during its descent to its final location in the anterior neck.
Aberrations in prenatal development can result in various forms of thyroid dysgenesis which can cause congenital hypothyroidism, and if untreated this can lead to cretinism.
The primary function of the thyroid is the production of iodine-containing hormones, triiodothyronine (T3) and thyroxine (T4) and peptide hormone calcitonin.[22] T3 is so named beceause it contains three atoms of iodine per molecule and T4 contains four atoms of iodine per molecule.[23] The thyroid hormones have a wide range of effects on the human body. These include:
After secretion, only a very small proportion of the thyroid hormones travel freely in the blood. Most are bound to thyroxine-binding globulin, transthyretin, and albumin. Only the 0.03% of T4 and 0.3% of T3 traveling freely has hormonal activity.[28] In addition, up to 85% of the T3 in blood is produced following conversion from T4 by iodothyronine deiodinases in organs around the body.[22]
Thyroid hormones act by crossing the cell membrane and binding to intracellular nuclear thyroid hormone receptors TR-α1,TR-α2,TR-β1 and TR-β2, which act alone, in pairs or together with the retinoid X-receptor as transcription factors to modulate DNA transcription.[28]
In addition to these actions on DNA, the thyroid hormones also act within the cell membrane or within cytoplasm. Cell membrane-initiated actions begin at a receptor on the alpha-v beta-3 integrin that activates MAPK3 and MAPK1. This binding culminates in local membrane actions on ion transport systems such as the Na+/H+ exchanger or complex cellular events including cell proliferation. These integrins are concentrated on cells of the vasculature and on some types of tumor cells, which in part explains the proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on some cancers including gliomas. T4 also acts on the mitochondrial genome via imported isoforms of nuclear thyroid receptors to affect several mitochondrial transcription factors. Regulation of actin polymerization by T4 is critical to cell migration in neurons and glial cells and is important to brain development.
T3 can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin alpha-V beta-3.
Synthesis of the thyroid hormones, as seen on an individual thyroid follicular cell:[30]
- Thyroglobulin is synthesized in the rough endoplasmic reticulum and follows the secretory pathway to enter the colloid in the lumen of the thyroid follicle by exocytosis.
- Meanwhile, a sodium-iodide (Na/I) symporter pumps iodide (I−) actively into the cell, which previously has crossed the endothelium by largely unknown mechanisms.
- This iodide enters the follicular lumen from the cytoplasm by the transporter pendrin, in a purportedly passive manner.[31]
- In the colloid, iodide (I−) is oxidized to iodine (I0) by an enzyme called thyroid peroxidase.
- Iodine (I0) is very reactive and iodinates the thyroglobulin at tyrosyl residues in its protein chain (in total containing approximately 120 tyrosyl residues).
- In conjugation, adjacent tyrosyl residues are paired together.
- The entire complex re-enters the follicular cell by endocytosis.
The thyroid hormones are formed from the addition of succesive iodide ions to thyroglobulin, a precursor molecule,
Iodide from the circulation is taken up by follicular cells through the sodium-iodide symporter. This is an ion channel on the cell membrane which transports two sodium ions and an iodide ion into the cell.[32] From within the cell iodide is then transported to the follicular space from the cell via the iodide-chloride antiporter pendrin. In the follicular space, the iodide is then oxidized to iodine, following which it is attached to thyroglobulin by the enzyme thyroid peroxidase. This forms the precursors of thyroid hormones monoiodotyrosine (MIT), and diiodotyrosine (DIT)).[2]
Thyrogobulin is a protein within the follicular space that has 123 tyrosine residues, only 4-6 of which are active. When the follicular cells are stimulated by thyroid-stimulating hormone (TSH), the follicular cells reabsorb thyroglobulin and cleave the iodinated tyrosines, forming free T4, (fT4), DIT, MIT, T3 and traces of reverse triiodothyronine (rT3), and releasing T3 and T4 into the blood. Deiodinase enzymes releases the iodine from MIT and DIT and convert T4 to T3 and RT3, [33] which is a major source of both RT3 (95%) and T3 (87%) in peripheral tissues.[34] Thyroid hormone secreted from the gland is about 80-90% T4 and about 10-20% T3.[35][36]
The production of thyroxine and triiodothyronine is primarily regulated by thyroid-stimulating hormone (TSH), released by the anterior pituitary gland. TSH release in turn is stimulated by thyrotropin releasing hormone (TRH) from the hypothalamus. The thyroid hormones provide negative feedback to the thyrotropes TSH and TRH: when the thyroid hormones are high, TSH production is suppressed. This negative feedback also occurs when levels of TSH are high, causing TRH production to be suppressed.[37]
TRH is secreted at an increased rate in situations such as cold exposure (to stimulate thermogenesis) which is prominent in case of infants. TSH production is blunted by dopamine and somatostatin which act as local regulators at the level of the pituitary, in response to rising levels of glucocorticoids and sex hormones (estrogen and testosterone), and excessively high blood iodide concentration.
The thyroid gland also produces the hormone calcitonin, which helps regulate blood calcium levels. Parafollicular cells produce calcitonin in response to high blood calcium. Calcitonin decreases the release of calcium from bone, by decreasing the activity of osteoclasts, cells which break bone down. Bone is constantly reabsorbed by osteoclasts and created by osteoblasts, so calcitonin effectively stimulates movement of calcium into bone. The effects of calcitonin are opposite those of the parathyroid hormone, produced in the parathyroid glands. However, calcitonin seems far less essential than PTH, as calcium metabolism remains clinically normal after removal of the thyroid (thyroidectomy), but not the parathyroid glands.[38]
Excessive production of the thyroid hormone due to an overactive thyroid is called hyperthyroidism, which is most commonly a result of Graves' disease, a toxic multinodular goitre, a solitary thyroid adenoma, and inflammation. Other causes include drug-induced excess of iodine, particularly from amiodarone an antiarrhythmic medication; an excess caused by the preferential uptake of iodine by the thyroid following iodinated contrast imaging, or from pituitary adenomas which may cause an overproduction of thyroid stimulating hormone.[39] Hyperthyroidism often causes a variety of non-specific symptoms including weight loss, increased appetite, insomnia, decreased tolerance of heat, tremor, palpitations, anxiety and nervousness. In some cases it can cause chest pain, diarrhoea, hair loss and muscle weakness.[40] Such symptoms may be managed temporarily with drugs such as beta blockers.[41]
Long-term management of hyperthyroidism may include drugs that suppress thyroid function such as propylthiouracil, carbimazole and methimazole.[42] Radioactive iodine-131 can be used to destroy thyroid tissue. Radioactive iodine is selectively taken up by the thyroid, which over time destroys the cells involved in its uptake. The chosen first-line treatment will depend on the individual and on the country where being treated. Surgery to remove the thyroid can sometimes be performed as a transoral thyroidectomy, a minimally-invasive procedure.[43] Surgery does however carry a risk of damage to the parathyroid glands and the nerves controlling the vocal cords. If the entire thyroid gland is removed, hypothyroidism will naturally result, and hormone therapy will be needed.[44][41]
An underactive thyroid gland results in hypothyroidism. Typical symptoms are abnormal weight gain, tiredness, constipation, heavy menstrual bleeding, baldness, cold intolerance, and a slow heart rate.[40] Hypothyroid disorders may occur as a result of autoimmune disease such as Hashimoto's thyroiditis; iodine deficiency; as a result of medical treatments such as surgical removal or radioablation of the thyroid, amiodarone and lithium; as a result of congenital thyroid abnormalities; or as a result of diseases such as amyloidosis or sarcoidosis or because of transient inflammation of the thyroid.[45] Some forms of hypothyroidism can result in myxedema and severe cases can result in myxedema coma.[46]
Hypothyroidism is managed with replacement of the hormone thyroxine. This is usually given daily as an oral supplement, and may take a few weeks to become effective.[46] Some causes of hypothyroidism, such as Postpartum thyroiditis and Subacute thyroiditis may be transient and pass over time, and other causes such as iodine deficiency may be able to be rectified with dietary supplementation.[47]
Thyroid nodules are often found on the gland, with a prevalence of 4-7%.[48] The majority of nodules do not cause any symptoms and are non-cancerous.[49] Non-cancerous cases include simple cysts, colloid nodules, and thyroid adenomas. Malignant nodules, which only occur in about 5% of nodules, include follicular, papillary, medullary carcinomas and metastases from other sites [50] Nodules are more likely in females, those who are exposed to radiation, and in those who are iodine deficient.[48]
When a nodule is present, thyroid function tests are performed and reveal whether a person has a normal about of thyroid hormones ("euthyroid") or an excess of hormones, usually secreted by the nodule, causing hyperthyroidism.[49] When the thyroid function tests are normal, an ultrasound is often used to investigate the nodule, and provide information such as whether the nodule is fluid-filled or a solid mass, and whether the appearance is suggestive of a benign or malignant cancer.[48] A needle aspiration biopsy may then be performed, and the sample undergoes cytology, in which the appearance of cells is viewed to determine whether they resemble normal or cancerous cells.[50] Investigations of a malignant nodule, or when hyperthyroidism is present, is discussed in the "Cancer" section below.
There can be many nodules, which is termed a multinodular goiter, and this can sometimes be a toxic multinodular goiter.[50]
An enlarged thyroid gland is called a goiter.[51] Goiters are present in some form in about 5% of people,[50] and are the result of a large number of causes, including iodine deficiency, autoimmune disease (both Grave's disease and Hashimoto's thyroiditis), infection, inflammation, infltrative disease such as sarcoidosis and amyloidosis. Sometimes no cause can be found, a state called "simple goiter".[52]
Some forms of goiter are associated with pain, whereas many do not cause any symptoms. Enlarged goiters may extend beyond the normal position of the thyroid gland to below the sternum, around the airway or esophagus.[50] Goiters may be associated with causes or hyperthyoidism, hypothyroidism, relating to the underlying cause.[50] Thyroid function tests may be done to investigate the cause and effects of the goiter. The underlying cause of the goiter may be treated, however many goiters with no associated symptoms are simply monitored.[50]
Disorders of the thyroid are functional–caused by dysfunction in the production of hormones, and nodes and tumors either benign or malignant. Functional disorders can cause inflammation as can some other forms of thyroiditis. Functional disorders can result in the overproduction or underproduction of hormones. Any of the functional thyroid disorders can result in the gland's enlargement and cause a swollen neck termed a goiter.
Inflammation of the thyroid is called thyroiditis. There are two types of thyroiditis where initially hyperthyroidism presents which is followed by a period of hypothyroidism; (the overproduction of T3 and T4 followed by the underproduction of T3 and T4). These are Hashimoto's thyroiditis and postpartum thyroiditis.
Hashimoto's thyroiditis or Hashimoto's disease is an autoimmune disorder whereby the body's own immune system reacts with the thyroid tissues in an attempt to destroy it. At the beginning, the gland may be overactive, and then becomes underactive as the gland is damaged resulting in too little thyroid hormone production or hypothyroidism. Some patients may experience "swings" in hormone levels that can progress rapidly from hyper-to-hypothyroid (sometimes mistaken as severe mood swings, or even being bipolar, before the proper clinical diagnosis is made). Some patients may experience these "swings" over a longer period of time, over days or weeks or even months.[citation needed]Hashimoto's is more common in females than males, usually appearing after the age of 30, and tends to run in families, meaning it can be seen as a genetic disease. Also more common in individuals with Hashimoto's thyroiditis are type 1 diabetes and celiac disease.[53]
Postpartum thyroiditis occurs in some females following childbirth due to the development of immune tolerance in pregnancy. After delivery, the gland becomes inflamed and the condition initially presents with overactivity of the gland followed by underactivity. In some cases, the gland may recover with time and resume its functions. In others it may not. The etiology is not always known, but can sometimes be attributed to an autoimmune disorder, such as Hashimoto's thyroiditis or Graves' disease.
There are other disorders that cause inflammation of the thyroid, and these include subacute thyroiditis, acute thyroiditis, silent thyroiditis, Riedel's thyroiditis.[54] and palpation thyroiditis.[55]
In most cases, thyroid cancer presents as a painless mass in the neck. It is very unusual for thyroid cancers to present with symptoms, unless they have been neglected. A hard nodule may be felt in the neck. Diagnosis is made using a needle biopsy and various radiological studies.[56]
| This section needs expansion. You can help by adding to it. (September 2016) |
A persistent thyroglossal duct is the most common clinically significant congenital disorder of the thyroid gland. A persistent sinus tract may remain as a vestigial remnant of the tubular development of the thyroid gland. Parts of this tube may be obliterated, leaving small segments to form thyroglossal cysts. These occur at any age and might not become evident until adult life. Mucinous, clear secretions may collect within these cysts to form either spherical masses or fusiform swellings, rarely larger than 2 to 3 cm in diameter. These are present in the midline of the neck anterior to the trachea. Segments of the duct and cysts that occur high in the neck are lined by stratified squamous epithelium, which is essentially identical to that covering the posterior portion of the tongue in the region of the foramen cecum. The disorders that occur in the lower neck more proximal to the thyroid gland are lined by epithelium resembling the thyroidal acinar epithelium. Characteristically, next to the lining epithelium, there is an intense lymphocytic infiltrate. Superimposed infection may convert these lesions into abscess cavities, and rarely, give rise to cancers.[citation needed]
Another disorder is that of thyroid dysgenesis which can result in various presentations of one or more ectopic accessory thyroid glands. These can be asymptomatic.
In areas of the world where iodine is lacking in the diet, iodine deficiency can cause the thyroid gland can to become considerably enlarged, a condition called an endemic goiter. Pregnant women on a diet that is severely deficient of iodine can give birth to infants with thyroid hormone deficiency (congenital hypothyroidism), manifesting in problems of physical growth and development as well as brain development (a condition referred to as endemic cretinism). In many developed countries, newborns are routinely tested for congenital hypothyroidism as part of newborn screening. Children with congenital hypothyroidism are treated supplementally with levothyroxine, which facilitates normal growth and development.
Because the thyroid concentrates iodine, it also concentrates the various radioactive isotopes of iodine produced by nuclear fission. In the event of large accidental releases of such material into the environment, the uptake of radioactive iodine isotopes by the thyroid can, in theory, be blocked by saturating the uptake mechanism with a large surplus of non-radioactive iodine, taken in the form of potassium iodide tablets. One consequence of the Chernobyl disaster was an increase in thyroid cancers in children in the years following the accident.[58]
The use of iodised salt is an efficient way to add iodine to the diet. It has eliminated endemic cretinism in most developed countries, and some governments have made the iodination of flour, cooking oil, and salt mandatory. Potassium iodide and sodium iodide are typically used forms of supplemental iodine.
As with most substances, either too much or too little can cause problems. Recent studies on some populations are showing that excess iodine intake could cause an increased prevalence of autoimmune thyroid disease, resulting in permanent hypothyroidism.[59]
| This section needs expansion. You can help by adding to it. (September 2016) |
An important sign in the diagnosis of Graves' disease is that of the collected symptoms of hyperthyroidism.
In a healthy person the gland is not visible yet is palpable as a soft mass. Examination of the thyroid gland includes the search for abnormal masses and the assessment of overall thyroid size.[60]
There are a number of blood tests that can be used to test the function of the thyroid:
| Test | Abbreviation | Normal ranges[61] |
|---|---|---|
| Serum thyrotropin/thyroid-stimulating hormone | TSH | 0.5–6.0 μU/ml |
| Free thyroxine | fT4 | 7–18 ng/l = 0.7–1.8 ng/dl |
| Serum triiodothyronine | T3 | 0.8–1.8 μg/l = 80–180 ng/dl |
| Radioactive iodine-123 uptake | RAIU | 10–30% |
| Radioiodine scan (gamma camera) | N/A | N/A - thyroid contrasted images |
| Free thyroxine fraction | fT4F | 0.03–0.005% |
| Serum thyroxine | T4 | 46–120 μg/l = 4.6–12.0 μg/dl |
| Thyroid hormone binding ratio | THBR | 0.9–1.1 |
| Free thyroxine index | fT4I | 4–11 |
| Free triiodothyronine l | fT3 | 230–619 pg/d |
| Free T3 Index | fT3I | 80–180 |
| Thyroxine-binding globulin | TBG | 12–20 ug/dl T4 +1.8 μg |
| TRH stimulation test | Peak TSH | 9–30 μIU/ml at 20–30 min. |
| Serum thyroglobulin l | Tg | 0-30 ng/m |
| Thyroid microsomal antibody titer | TMAb | Varies with method |
| Thyroglobulin antibody titer | TgAb | Varies with method |
As of early 2015, in the United States, new guidelines for TSH levels have been implemented as endorsed by the American Association of Clinical Endocrinologists. The new range is a TSH of 0.45 to 4.12.[62]
The English name thyroid gland[63] is derived from Latin glandula thyreoidea.[64] Glandula means gland in Latin,[65] and thyreoidea can be traced back to the Ancient Greek word θυρεοειδής, meaning shield-like/shield-shaped.[66] The thyroid was so named by the anatomist Thomas Wharton.[67] Thomas Wharton named the gland the thyroid, meaning shield, as its shape resembled the shields commonly used in Ancient Greece.[68]
Historical references to what we now know as the thyroid gland arise early in medical history. In Ayurvedic medicine, the book Sushruta Samhita written about 1500 BC mentions the disease goitre as 'Galaganda' along with its treatment. In 1600 BC the Chinese were using burnt sponge and seaweed for the treatment of goitres. Celsus first described a bronchoceole (a tumour of the neck) in 15 AD. Around this time Pliny referred to epidemics of goitre in the Alps and also mentioned the use of burnt seaweed in their treatment, in the same way as the Chinese had done 1600 years earlier. In 150 AD Galen, an instrumental figure in the transition from ancient to modern medicine, referred to 'spongia usta' (burnt sponge) for the treatment of goitre. He also suggested (incorrectly, as it turns out) that the role of the thyroid was to lubricate the larynx.
There are several findings that evidence a great interest for thyroid disorders just in the Medieval Medical School of Salerno (12th century). Rogerius Salernitanus, the Salernitan surgeon and author of "Post mundi fabricam" (around 1180) was considered at that time the surgical text par excellence all over Europe. In the chapter "De bocio" of his magnum opus, he describes several pharmacological and surgical cures, some of which nowadays are reappraised as scientifically effective.[69]
It was not until 1475 that Wang Hei anatomically described the thyroid gland and recommended that the treatment of goitre should be dried thyroid. Paracelsus, some fifty years later, attributed goitre to mineral impurities in the water.
In 1909, Theodor Kocher from Switzerland won the Nobel Prize in Medicine "for his work on the physiology, pathology and surgery of the thyroid gland".[70]
The thyroid gland is found in all vertebrates. In fish, it is usually located below the gills and is not always divided into distinct lobes. However, in some teleosts, patches of thyroid tissue are found elsewhere in the body, associated with the kidneys, spleen, heart, or eyes.[71]
In tetrapods, the thyroid is always found somewhere in the neck region. In most tetrapod species, there are two paired thyroid glands - that is, the right and left lobes are not joined together. However, there is only ever a single thyroid gland in most mammals, and the shape found in humans is common to many other species.[71]
In larval lampreys, the thyroid originates as an exocrine gland, secreting its hormones into the gut, and associated with the larva's filter-feeding apparatus. In the adult lamprey, the gland separates from the gut, and becomes endocrine, but this path of development may reflect the evolutionary origin of the thyroid. For instance, the closest living relatives of vertebrates, the tunicates and Amphioxus, have a structure very similar to that of larval lampreys (the endostyle), and this also secretes iodine-containing compounds (albeit not thyroxine).[71]
Thyroxine is critical to the regulation of metabolism and growth throughout the animal kingdom. Among amphibians, for example, administering a thyroid-blocking agent such as propylthiouracil (PTU) can prevent tadpoles from metamorphosing into frogs; in contrast, administering thyroxine will trigger metamorphosis. In amphibian metamorphosis, thyroxine and iodine also exert a well-studied experimental model of apoptosis on the cells of gills, tail, and fins of tadpoles. Iodine, via iodolipids, has favored the evolution of terrestrial animal species and has likely played a crucial role in the evolution of the human brain. Iodine (and T4) trigger the amphibian metamorphosis that transforms the vegetarian aquatic tadpole into a carnivorous terrestrial adult frog, with better neurological, visuospatial, olfactory and cognitive abilities for hunting, as seen in other predatory animals. A similar phenomenon happens in the neotenic amphibian salamanders, which, without introducing iodine, don't transform into terrestrial adults, and live and reproduce in the larval form of aquatic axolotl.[72][73]
Thyroid and parathyroid.
Clearly visible pyramidal lobe of the thyroid
Thyroid gland
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Anatomy of the endocrine system
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| Pituitary gland |
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| Thyroid gland |
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| Pineal gland |
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