Thyroid
| Thyroid | |
|---|---|
The human thyroid as viewed from the front, with arteries visible.
Note the superior thyroid artery, which supplies the thyroid with much of its blood. | |
The thyroid as it may be seen from a posterior view, from behind the trachea.
| |
| Latin | Glandula thyreoidea |
| Gray's | p.1269 |
| System | Endocrine system |
| Precursor | Thyroid diverticulum (an extension of endoderm into 2nd Branchial arch) |
The thyroid gland or simply, the thyroid /ˈθaɪərɔɪd/, in vertebrate anatomy, is one of the largest endocrine glands and consists of two connected lobes. The thyroid gland is found in the neck, below the thyroid cartilage (which forms the laryngeal prominence, or "Adam's apple"). The thyroid gland controls how quickly the body uses energy, makes proteins, and controls how sensitive the body is to other hormones. It participates in these processes by producing thyroid hormones, the principal ones being triiodothyronine (T3) and thyroxine (sometimes referred to as tetraiodothyronine (T4)). These hormones regulate the growth and rate of function of many other systems in the body. T3 and T4 are synthesized from iodine and tyrosine. The thyroid also produces calcitonin, which plays a role in calcium homeostasis.
Hormonal output from the thyroid is regulated by thyroid-stimulating hormone (TSH) produced by the anterior pituitary, which itself is regulated by thyrotropin-releasing hormone (TRH) produced by the hypothalamus.
Structure
The thyroid gland is a butterfly-shaped organ and is composed of two cone-like lobes or wings,lobus dexter (right lobe) and lobus sinister (left lobe), connected via the isthmus. Each lobe is about 5 cm long, 3 cm wide and 2 cm thick. The organ is situated on the anterior side of the neck, lying against and around the larynx and trachea, reaching posteriorly the oesophagus andcarotid sheath. It starts cranially at the oblique line on the thyroid cartilage (just below the laryngeal prominence, or 'Adam's Apple'), and extends inferiorly to approximately the fifth or sixthtracheal ring.[1] 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. There is occasionally a third lobe present called the pyramidal lobe of the thyroid gland. It is of conical shape and extends from the upper part of the isthmus, up across the thyroid cartilage to the hyoid bone. The pyramidal lobe is a remnant of the fetal thyroid stalk, or thyroglossal duct.[2] It is occasionally quite detached, or may be divided into two or more parts. The pyramidal lobe is also known asLalouette's pyramid.[3]
The thyroid gland is covered by a thin fibrous sheath, the capsula glandulae thyreoideae, composed of an internal and external layer. The external layer is anteriorly continuous with thepretracheal fascia and posteriorolaterally continuous with the carotid sheath. The gland is covered anteriorly with infrahyoid muscles and laterally with the sternocleidomastoid muscle also known as sternomastoid muscle. On the posterior side, the gland is fixed to the cricoid and tracheal cartilage and cricopharyngeus muscle by a thickening of the fascia to form the posterior suspensory ligament of thyroid gland also known as Berry's ligament.[4][5] The thyroid gland's firm attachment to the underlying trachea is the reason behind its movement with swallowing.[6] In variable extent, the pyramidal lobe is present at the most anterior side of the lobe. In this region, the recurrent laryngeal nerveand the inferior thyroid artery pass next to or in the ligament and tubercle.
Between the two layers of the capsule and on the posterior side of the lobes, there are on each side two parathyroid glands.
The thyroid isthmus is variable in presence and size, can change shape and size, and can encompass the pyramidal lobe (lobus or processus pyramidalis.[citation needed] The thyroid is one of the larger endocrine glands, weighing 2-3 grams in neonates and 18-60 grams in adults, and is increased in pregnancy.
In a healthy patient the gland is not visible yet can be palpated as a soft mass. Examination of the thyroid gland is carried out by locating the thyroid cartilage and passing the fingers up and down, examining for abnormal masses and overall thyroid size. Then, place one hand on each of the trachea and gently displace the thyroid tissue to the contralateral side of the neck for both sides while the other hand manually palpates the displaced gland tissue; having the patient flex the neck slightly to the side when being palpated may help in this examination. Next, the two lobes of the gland should be compared for size and texture using visual inspection, as well as manual or bimanual palpation. Finally, ask the patient to swallow to check for mobility of the gland; many clinicians find that having the patient swallow water helps this part of the examination. In a healthy state, the gland is mobile when swallowing occurs due its fascial encasement. Thus when the patient swallows, the gland moves superiorly, as does the whole larynx.[7]
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 the thyroid ima artery, branching directly from the subclavian artery. The venous blood is drained via superior thyroid veins, draining in the internal jugular vein, and via inferior thyroid veins, draining via the plexus thyreoideus impar in the left brachiocephalic vein.
Lymphatic drainage passes frequently the lateral deep cervical lymph nodes and the pre- and parathracheal lymph nodes. The gland is supplied by parasympathetic nerve input from the superior laryngeal nerve and the recurrent laryngeal nerve.
Embryological and fetal development
In the embryo, at 3–4 weeks of gestation, 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 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 anterior to the hyoid bone. During migration, the thyroid remains connected to the tongue by a narrow canal, the thyroglossal duct.
Thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) start being secreted from the fetal hypothalamus andpituitary at 18-20 weeks of gestation, and fetal production of thyroxine (T4) reach a clinically significant level at 18–20 weeks.[8] Fetaltriiodothyronine (T3) remains low (less than 15 ng/dL) until 30 weeks of gestation, and increases to 50 ng/dL at term.[8] Fetal self-sufficiency of thyroid hormones protects the fetus against e.g. brain development abnormalities caused by maternal hypothyroidism.[9]However, preterm births can suffer neurodevelopmental disorders due to lack of maternal thyroid hormones due their own thyroid being insufficiently developed to meet their postnatal needs.[10]
The portion of the thyroid containing the parafollicular cells, also known as C cells, responsible for the production of calcitonin, are derived from the neural crest. This is first seen as the ultimobranchial body, which joins the primordial thyroid gland during its descent to its final location in the anterior neck.
Aberrations in embryological development can cause various forms of thyroid dysgenesis.
Histology
At the microscopic level, there are three primary features of the thyroid:(first discovered by Geoffary Websterson in 1664)[11]
| Feature | Description |
| Follicles | The thyroid is composed of spherical follicles that selectively absorb iodine (as iodide ions, I-) from the blood for production of thyroid hormones, and also for storage of iodine in thyroglobulin. Twenty-five percent of the body's iodide ions are in the thyroid gland. Inside the follicles, in a region called the follicular lumen, colloid serves as a reservoir of materials for thyroid hormone production and, to a lesser extent, acts as a reservoir for the hormones themselves. Colloid is rich in a protein called thyroglobulin. |
| Thyroid epithelial cells (or "follicular cells") | The follicles are surrounded by a single layer of thyroid epithelial cells, which secrete T3 and T4. When the gland is not secreting T3 and T4 (inactive), the epithelial cells range from low columnar to cuboidal cells. When active, the epithelial cells become tall columnar cells. |
| Parafollicular cells (or "C cells") | Scattered among follicular cells and in spaces between the spherical follicles are another type of thyroid cell, parafollicular cells, which secrete calcitonin. |
Physiology
The primary function of the thyroid is production of the hormones T3, T4 and calcitonin. Up to 80% of the T4 is converted to T3 by organs such as the liver, kidney and spleen. T3is several times more powerful than T4, which is largely a prohormone, perhaps four[12] or even ten times more active.[13]
T3 and T4 production and action
Thyroxine (T4) is synthesised by the follicular cells from free tyrosine and on the tyrosine residues of the protein calledthyroglobulin (Tg). Iodine is captured with the "iodine trap" by the hydrogen peroxide generated by the enzyme thyroid peroxidase (TPO)[17] and linked to the 3' and 5' sites of the benzene ring of the tyrosine residues on Tg, and on free tyrosine. Upon stimulation by the thyroid-stimulating hormone (TSH), the follicular cells reabsorb Tg and cleave the iodinated tyrosines from Tg in lysosomes, forming T4 and T3 (in T3, one iodine atom is absent compared to T4), and releasing them into the blood. Deiodinase enzymes convert T4 to T3.[18] Thyroid hormone secreted from the gland is about 80-90% T4 and about 10-20% T3.[12][13]
Cells of the developing brain are a major target for the thyroid hormones T3 and T4. Thyroid hormones play a particularly crucial role in brain maturation during fetal development.[19] A transport protein that seems to be important for T4 transport across the blood–brain barrier (OATP1C1) has been identified.[20] A second transport protein (MCT8) is important for T3transport across brain cell membranes.[20]
Non-genomic actions of T4 are those that are not initiated by liganding of the hormone to intranuclear thyroid receptor. These may begin at the plasma membrane or within cytoplasm. Plasma membrane-initiated actions begin at a receptor on the integrin alphaV beta3 that activates ERK1/2. 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 beta3.
In the blood, T4 and T3 are partially bound to thyroxine-binding globulin (TBG), transthyretin, and albumin. Only a very small fraction of the circulating hormone is free (unbound) - T4 0.03% and T3 0.3%. Only the free fraction has hormonal activity. As with the steroid hormones and retinoic acid, thyroid hormones cross the cell membrane and bind to intracellular receptors (α1, α2, β1 and β2), which act alone, in pairs or together with theretinoid X-receptor as transcription factors to modulate DNA transcription[1].
T3 and T4 regulation
The production of thyroxine and triiodothyronine is regulated by thyroid-stimulating hormone (TSH), released by the anterior pituitary. The thyroid and thyrotropes form a negative feedback loop: TSH production is suppressed when the T4 levels are high.[21] The TSH production itself is modulated by thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus and secreted at an increased rate in situations such as cold exposure (to stimulate thermogenesis). TSH production is blunted by somatostatin (SRIH), rising levels of glucocorticoids and sex hormones (estrogen and testosterone), and excessively high blood iodide concentration.
An additional hormone produced by the thyroid contributes to the regulation of blood calcium levels.Parafollicular cells produce calcitonin in response to hypercalcemia. Calcitonin stimulates movement of calcium into bone, in opposition to the effects of parathyroid hormone (PTH). However, calcitonin seems far less essential than PTH, as calcium metabolism remains clinically normal after removal of the thyroid (thyroidectomy), but not the parathyroids.
Clinical significance
Main article: Thyroid disease
Thyroid disorders include
- hyperthyroidism (abnormally increased activity),
- hypothyroidism (abnormally decreased activity)
- thyroiditis, inflammation of the thyroid
- thyroid nodules, which are generally benign thyroid neoplasms (tumours), but may be thyroid cancers.
All these disorders may give rise to a goiter, that is, an enlarged thyroid.
Hyperthyroidism
Hyperthyroidism, or overactive thyroid, is due to the overproduction of the thyroid hormones T3 and T4, which is most commonly caused by the development of Graves' disease, an autoimmune disease in which antibodies are produced which stimulate the thyroid to secrete excessive quantities of thyroid hormones.[22] The disease can result in the formation of a toxic goiter as a result of thyroid growth in response to a lack of negative feedback mechanisms. It presents with symptoms such as a thyroid goiter, protruding eyes (exopthalmos), palpitations, excess sweating, diarrhea, weight loss, muscle weakness and unusual sensitivity to heat. The appetite is often increased.
Beta blockers are used to decrease symptoms of hyperthyroidism such as increased heart rate, tremors, anxiety and heart palpitations, and anti-thyroid drugs are used to decrease the production of thyroid hormones, in particular, in the case of Graves' disease. These medications take several months to take full effect and have side-effects such as skin rash or a drop in white blood cell count, which decreases the ability of the body to fight off infections. These drugs involve frequent dosing (often one pill every 8 hours) and often require frequent doctor visits and blood tests to monitor the treatment, and may sometimes lose effectiveness over time. Due to the side-effects[clarification needed] and inconvenience of such drug regimens, some patients choose to undergo radioactive iodine-131 treatment. Radioactive iodine is administered in order to destroy a portion of or the entire thyroid gland, since the radioactive iodine is selectively taken up by the gland and gradually destroys the cells of the gland. Alternatively, the gland may be partially or entirely removed surgically, though iodine treatment is usually preferred since the surgery is invasive and carries a risk of damage to the parathyroid glands or the nerves controlling the vocal cords. If the entire thyroid gland is removed, hypothyroidism results.[23]
Hypothyroidism
Hypothyroidism is the underproduction of the thyroid hormones T3 and T4.
Hypothyroid disorders may occur as a result of
- congenital thyroid abnormalities (Thyroid deficiency at birth. See congenital hypothyroidism),
- autoimmune disorders such as Hashimoto's thyroiditis,
- iodine deficiency (more likely in poorer countries) or
- the removal of the thyroid following surgery to treat severe hyperthyroidism and/or thyroid cancer.
Typical symptoms are abnormal weight gain, tiredness, baldness, cold intolerance, and bradycardia. Hypothyroidism is treated with hormone replacement therapy, such aslevothyroxine, which is typically required for the rest of the patient's life. Thyroid hormone treatment is given under the care of a physician and may take a few weeks to become effective.[24]
Negative feedback mechanisms result in growth of the thyroid gland when thyroid hormones are being produced in sufficiently low quantities, as a means of increasing the thyroid output; however, where hypothyroidism is caused by iodine insufficiency, the thyroid is unable to produce T3 and T4 and as a result, the thyroid may continue to grow to form a non-toxic goiter. It is termed non-toxic as it does not produce toxic quantities of thyroid hormones, despite its size.
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 moodswings, 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. 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.[25]
Postpartum thyroiditis occurs in some females following the birth of a child. 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 autoimmunity, 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 and Riedel's thyroiditis.[26]
Cancers
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. One may be able to feel a hard nodule in the neck. Diagnosis is made using a needle biopsy and various radiological studies.[27]
Non-cancerous nodules
Many individuals may find the presence of thyroid nodules on the gland. The majority of these thyroid nodules are benign (non cancerous), and their presence does not necessarily indicate disease. Most thyroid nodules do not cause any symptoms, and most are discovered on an incidental examination. Often there can be many nodules, which is termed a multinodular goiter. Doctors usually perform a needle aspiration biopsy of the thyroid to determine the status of the nodules. If the nodule is found to be non-cancerous, no other treatment is required. If the nodule is suspicious then surgery is recommended.
Congenital anomalies
A persistent thyroglossal duct or cyst is the most common clinically significant congenital anomaly 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 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 foreamen cecum. The anomalies 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 anomaly is that of thyroid dysgenesis which can result in various presentations of one or more accessory thyroid glands. These can be asymptomatic.
Other disorders
- Limited research shows that seasonal allergies may trigger episodes of hypo- or hyperthyroidism.[28][29]
- Some rapid-cycling versions of bipolar disorder seem to have a complex relationship with thyroid dysfunction, however the specifics of the relationship are poorly understood.[30]
Thyroid function tests
Main article: Thyroid function tests
| Test | Abbreviation | Normal ranges[31] |
|---|---|---|
| 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 |
- μU/ml = mU/l, microunit per milliliter
- ng/dl, nanograms per deciliter
- μg, micrograms
- pg/d, picograms per day
- μIU/ml = mIU/l, micro-international unit per milliliter
- See [2] for more information on medical units of measure
As of early 2014, 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 0.3 to 3.0. Many testing facilities and doctors are still unaware of the change.[citation needed]
Significance of iodine
In areas of the world where iodine is lacking in the diet, the thyroid gland can become considerably enlarged, a condition called 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 ofnewborn screening. Children with congenital hypothyroidism are treated supplementally with levothyroxine, which facilitates normal growth and development.
Thyroxine is critical to the regulation of metabolism and growth throughout the animal kingdom. Among amphibians, for example, administering a thyroid-blocking agent such aspropylthiouracil (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.[32]
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 ofnon-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.[33]
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.[34]
History
Historical references to what we now know as the thyroid gland arise early in medical history. In Ayurvedic System Of medicine,the book Susrut 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 (enlarged thyroid glands). 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.[35]
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 modern times, the thyroid was first identified in 1656 by the anatomist Thomas Wharton (whose name is also eponymised in Wharton's duct of the submandibular gland).[36] In 1656 Thomas Wharton named the gland the thyroid, meaning shield, as its shape resembled the shields commonly used in Ancient Greece.
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".[37]
Other animals
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.[38]
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.[38]
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).[38]
Etymology
The English name thyroid gland[39] is derived from Latin glandula thyreoidea.[40] Glandula means gland in Latin,[41] and thyreoidea can be traced back to the Ancient Greek word θυρεοειδής, meaning shield-like/shield-shaped.[42] The English anatomist Thomas Wharton was the first to coin the Latin expression for the thyroid gland.[43] However, he introduced the incorrect spelling glandula thyroidaea [44] as the adjective thyroidaea is a faulty rendering in Latin of Ancient Greek θυρεοειδής.[45] The Latin ending aea does not correspond to Ancient Greek ής and more importantly the e after thyr is missing, creating a resemblance between thyroidaea and Ancient Greek θυροειδής, that actually meanslike a door[42] instead of the intended shield-like.
The first edition[40] of the official Latin nomenclature (Nomina Anatomica) of 1895 rightfully included the e after thyr in the expression glandula thyreoidea as well as the subsequent editions from 1936[46][47] and 1955.[48] Subsequent editions[49][50][51][52][53] as well as the most recent list (Terminologia Anatomica)[54] replicated the mistake originally made by Wharton, i.e. incorrect elision of e after thyr, and printed glandula thyroidea (= door-like gland).
The decision to elide the e from thyreo- is a gesture of the nomenclature committee of the Nomina Anatomica to English-speaking anatomists, as they have difficulties in pronouncing this specific sequence of letters,[55] thereby forcing a greater resemblance between Latin and English orthography. Although the spelling thyroid is nowadays accepted in English, earlier English medical dictionaries[56] remarked that thyroid was the less correct form of thyreoid.
The adjective thyreoidea in the expression glandula thyreoidea seems actually quite odd, as this gland isn’t clearly shield-like at all.[43] This name is however based on the fact that the thyroid gland is in the close vicinity of the thyroid cartilage.[43] The English expression thyroid cartilage is derived from the Latin expression cartilago thyreoides.[46][57] The latter is a translation of Ancient Greek χόνδρος θυρεοειδής [42] attested in the writings of the Greek physician Galen.[42] In this case θυρεοειδής in χόνδρος θυρεοειδής refers to the shield-like appearance of the thyroid cartilage.[43] As the thyroid gland itself is not shield-like at all, but refers to another structure that is shield-like in appearance, using the same adjective to refer to both structures might be confusing.[43] The proper rendering of Ancient Greek θυρεοειδής might be thyreoides in Latin.[57] The edition of the Nomina Anatomica from 1936[46] uses the adjective thyreoides to refer to shield-like structures, but uses the (additional) ending eus/ea/eum to refer to structures that are related or in the close vicinity of shield-like structures, but not shield-like themselves.[58] Others[57] use the ending icus/ica/icum as in glandula thyreoidica as the Latin ending eus/ea/eum does not correspond to a possible Greek form.
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