[] The thyroid gland, or simply the thyroid, is an in the, consisting of two connected by an. It is found at the front of the, below the. The thyroid gland secretes, which primarily influence the and. The hormones also have many other effects including those on development. The thyroid hormones (T 3) and (T 4) are created from and. The thyroid also produces the, which plays a role in.

Hormonal output from the thyroid is regulated by (TSH) secreted from the gland, which itself is regulated by (TRH) produced by the. The thyroid may be affected by several. Occurs when the gland produces excessive amounts of thyroid hormones, the most common cause being — an. In contrast, is a state of insufficient thyroid hormone production.

Worldwide, the most common cause is. Thyroid hormones are important for development, and hypothyroidism secondary to iodine deficiency remains the leading cause of preventable. In iodine-sufficient regions, the most common cause of hypothyroidism is —also an autoimmune disease. In addition, the thyroid gland may also develop several types of and. The thyroid gland surrounds the and, and consists of two lobes. This image shows a variant thyroid with a pyramidal lobe emerging from the middle of the thyroid.

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The thyroid gland is a butterfly-shaped organ that sits at the front of the neck. It is composed of two lobes, left and right, connected by a narrow isthmus. The thyroid weighs 25 grams in adults, with each lobe being about 5 cm long, 3 cm wide and 2 cm thick, and the isthmus about 1.25 cm in height and width. The gland is usually larger in women, and increases in size in pregnancy. The thyroid sits near the front of the neck, lying against and around the front of the and. The and lie just above the gland, below the. The isthmus extends from the second to third, with the uppermost part of the lobes extending to the thyroid cartilage, and the lowermost around the fourth to sixth tracheal rings.

The thyroid gland is covered by a thin fibrous capsule, which has an inner and an outer layer. The outer layer is continuous with the, attaching the gland to the cricoid and thyroid cartilages, via a thickening of the fascia to form the posterior also known as Berry's ligament. This causes the thyroid to move up and down with swallowing. The inner layer extrudes into the gland and forms the that divides the thyroid tissue into microscopic lobules. Typically four, two on each side, lie on each side between the two layers of the capsule, at the back of the thyroid lobes.

The lie in front of the gland and the to the side. Behind the outer wings of the thyroid lie the two.

The trachea, laryngx, lower pharynx and esophagus all lie behind the thyroid. In this region, the and the inferior thyroid artery pass next to or in the ligament. Blood, lymph and nerve supply [ ] The thyroid is supplied with arterial blood from the, a branch of the, and the, a branch of the, and sometimes by an the, which has a variable origin.

The superior thyroid artery splits into anterior and posterior branches supplying the thyroid, and the inferior thyroid artery splits into superior and inferior branches. The superior and inferior thyroid arteries join together behind the outer part of the thyroid lobes. The venous blood is drained via and, which drain to the, and via the. The inferior thyroid veins originate in a network of veins and drain into the left and right. Both arteries and veins form a plexus between the two layers of the capsule of the thyroid gland.

Drainage frequently passes the (located just above the isthmus), and the and. The gland receives supply from the superior, middle and inferior cervical ganglion of the. The gland receives supply from the and the. Variation [ ] There are many in the size and shape of the thyroid gland, and in the position of the embedded parathyroid glands. Sometimes there is a third lobe present called the pyramidal lobe. When present, this lobe often stretches up the hyoid bone from the thyroid isthmus and may be one to several divided lobes.

The presence of this lobe ranges in reported studies from 18.3% to 44.6%. It was shown to more often arise from the left side and occasionally separated. The pyramidal lobe is also known as pyramid. The pyramidal lobe is a remnant of the which usually wastes away during the thyroid gland's descent. Small accessory thyroid glands may in fact occur anywhere along the thyroglossal duct, from the of the tongue to the position of the thyroid in the adult.

A small horn at the back of the thyroid lobes, usually close to the recurrent laryngeal nerve and the inferior thyroid artery, is called. Other variants include a, connecting the isthmus to the body of the, and the presence of the small.

Microanatomy [ ]. Section of a thyroid gland under the microscope. 1 follicles, 2 follicular cells, 3 cells At the, there are three primary features of the thyroid—follicles,, and, first discovered by Geoffery Websterson in 1664. Follicles Thyroid follicles are small spherical groupings of cells 0.02–0.9mm in diameter that play the main role in thyroid function. They consist of a rim that has a rich blood supply, nerve and lymphatic presence, that surrounds a core of that consists mostly of thyroid hormone precursor proteins called, an. Follicular cells The core of a follicle 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. Follicular cells vary in shape from flat to cuboid to columnar, depending on how active they are.

Parafollicular cells Scattered among follicular cells and in spaces between the spherical follicles are another type of thyroid cell, parafollicular cells. These cells secrete and so are also called C cells. Development [ ]. Floor of pharynx of embryo between 18 and 21 days In the, at 3–4 weeks, the thyroid gland appears as an proliferation in the floor of the pharynx at the base of the tongue between the and the. The copula soon becomes covered over by the at a point later indicated by the. The thyroid then descends in front of the pharyngeal gut as a bilobed through the. 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. The and start to secrete (TRH) and (TSH). TSH is first measurable at 11 weeks. By 18–20 weeks, the production of (T 4) reaches a clinically significant and self-sufficient level. Fetal (T 3) remains low, less than 15 ng/dL until 30 weeks, and increases to 50 ng/dL. The needs to be self-sufficient in thyroid hormones in order to guard against that would arise from maternal hypothyroidism.

The presence of sufficient iodine is essential for healthy neurodevelopment. The, also known as C cells, responsible for the production of, are derived from cells, which migrate to the. This part of the thyroid then first forms as the, which begins in the ventral fourth and joins the primordial thyroid gland during its descent to its final location. Aberrations in can result in various forms of which can cause, and if untreated this can lead to. Function [ ].

Main article: The primary function of the thyroid is the production of the iodine-containing, (T 3) and (T 4) and the. T 3 is so named because it contains three atoms of iodine per molecule and T 4 contains four atoms of iodine per molecule. The thyroid hormones have a wide range of effects on the human body. These include: • Metabolic. The thyroid hormones increase the and have effects on almost all body tissues.

Appetite, the absorption of substances, and gut motility are all influenced by thyroid hormones. They increase the absorption in the gut,,, and of glucose. They stimulate the, and increase the number of. Despite increasing free fatty acids, thyroid hormones decrease levels, perhaps by increasing the rate of secretion of cholesterol in. • Cardiovascular. The hormones increase the rate and strength of the heartbeat. They increase the rate of breathing, intake and consumption of oxygen, and increase the activity of.

Combined, these factors increase blood flow and the body's temperature. • Developmental. Thyroid hormones are important for normal development. They increase the growth rate of young people, and cells of the developing brain are a major target for the thyroid hormones T 3 and T 4. Thyroid hormones play a particularly crucial role in brain maturation during fetal development. • The thyroid hormones also play a role in maintaining normal sexual function, sleep, and thought patterns.

Increased levels are associated with increased speed of thought generation but decreased focus. Sexual function, including libido and the maintenance of a normal, are influenced by thyroid hormones. After secretion, only a very small proportion of the thyroid hormones travel freely in the blood. Most are bound to (about 70%), (10%), and (15%).

Only the 0.03% of T 4 and 0.3% of T 3 traveling freely has hormonal activity. In addition, up to 85% of the T 3 in blood is produced following conversion from T 4 by in organs around the body.

Thyroid hormones act by crossing the and binding to, which bind with and to modulate. In addition to these actions on DNA, the thyroid hormones also act within the cell membrane or within cytoplasm via reactions with, including,, and.

Hormone production [ ]. Synthesis of the, as seen on an individual: - is synthesized in the and follows the to enter the colloid in the lumen of the.

- Meanwhile, a pumps iodide (I −) into the cell, which previously has crossed the by largely unknown mechanisms. - This iodide enters the follicular lumen from the cytoplasm by the transporter, in a purportedly manner. - In the colloid, iodide (I −) is to iodine (I 0) by an enzyme called.

- Iodine (I 0) is very reactive and iodinates the thyroglobulin at 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 various liberates and molecules, which enters the blood by largely unknown mechanisms. The thyroid hormones are created from thyroglobulin. This is a within the follicular space that is originally created within the of follicular cells and then transported into the follicular space. Thyroglobulin contains 123 units of, which reacts with iodine within the follicular space.

Is essential for the production of the thyroid hormones. Iodine (I 0) travels in the blood as (I −), which is taken up into the follicular cells by a. This is an on the cell membrane which in the same action transports two sodium ions and an iodide ion into the cell. Iodide then travels from within the cell into the follicular space, through the action of, an iodide-chloride. In the follicular space, the iodide is then to iodine. This makes it more reactive, and the iodine is attached to the active tyrosine units in thyroglobulin by the enzyme.

This forms the precursors of thyroid hormones (MIT), and (DIT). When the follicular cells are stimulated by, the follicular cells reabsorb thyroglobulin from the follicular space. The iodinated tyrosines are cleaved, forming the thyroid hormones T 4, T 3, DIT, MIT, and traces of. T 3 and T 4 are released into the blood. The hormones secreted from the gland are about 80–90% T 4 and about 10–20% T 3. In peripheral tissues remove the iodine from MIT and DIT and convert T 4 to T 3 and RT 3. This is a major source of both RT 3 (95%) and T 3 (87%) in peripheral tissues.

Regulation [ ] The production of thyroxine and triiodothyronine is primarily regulated by thyroid-stimulating hormone (TSH), released by the gland. TSH release in turn is stimulated by (TRH), released in a pulsatile manner from the. The thyroid hormones provide to the 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. TRH is secreted at an increased rate in situations such as cold exposure in order to stimulate. In addition to being suppressed by the presence of thyroid hormones, TSH production is blunted by,, and.

Calcitonin [ ]. Main article: The thyroid gland also produces the hormone, which helps regulate blood levels. Parafollicular cells produce calcitonin in response to. Calcitonin decreases the release of calcium from bone, by decreasing the activity of, cells which break down bone. Bone is constantly reabsorbed by osteoclasts and created by, so calcitonin effectively stimulates movement of calcium into. The effects of calcitonin are opposite those of the (PTH) produced in the parathyroid glands. However, calcitonin seems far less essential than PTH, as calcium metabolism remains clinically normal after removal of the thyroid (), but not the.

Gene and protein expression [ ]. Further information: About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal thyroid. Some 250 of these genes are more specifically expressed in the thyroid with about 20 genes being highly thyroid specific. The corresponding specific proteins are mainly involved in thyroid hormone synthesis, such as, and, and expressed in follicular cells. Other proteins elevated in the thyroid are calcitonin related proteins such as and, expressed in the parafollicular c-cells. Clinical significance [ ] Symptoms [ ] Hyperthyroidism [ ]. Main article: Excessive production of the thyroid hormones is called, which is most commonly a result of, a, a solitary, or inflammation.

Other causes include drug-induced excess of iodine, particularly from, an; an excess caused by the preferential uptake of iodine by the thyroid following; or from which may cause an overproduction of thyroid stimulating hormone. Hyperthyroidism often causes a variety of including weight loss, increased appetite, insomnia, decreased tolerance of heat, tremor,, anxiety and nervousness.

In some cases it can cause,, hair loss and muscle weakness. Such symptoms may be managed temporarily with drugs such as. Long-term management of hyperthyroidism may include drugs that suppress thyroid function such as, and. 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 will depend on the individual and on the country where being treated. Can sometimes be performed as a, a. Surgery does however carry a risk of damage to the and the nerves controlling the. If the entire thyroid gland is removed, hypothyroidism will naturally result, and will be needed. Hypothyroidism [ ]. Download Free Sales Distribution Management Pdf Ebook Free. Main article: An underactive thyroid gland results in. Typical symptoms are abnormal weight gain, tiredness,,, hair loss, cold intolerance, and.

Is the most common cause of hypothyroidism worldwide, and the autoimmune disease is the most common cause in the developed world. Other causes include congenital abnormalities, diseases causing transient inflammation, surgical removal or radioablation of the thyroid, the drugs and,, and. Some forms of hypothyroidism can result in and severe cases can result in.

Hypothyroidism is managed with replacement of the hormone. This is usually given daily as an oral supplement, and may take a few weeks to become effective. Some causes of hypothyroidism, such as and may be transient and pass over time, and other causes such as iodine deficiency may be able to be rectified with dietary supplementation. Main article: are often found on the gland, with a of 4–7%. The majority of nodules do not cause any symptoms and are non-cancerous.

Non-cancerous cases include simple,, and. Malignant nodules, which only occur in about 5% of nodules, include,, carcinomas and from other sites Nodules are more likely in females, those who are exposed to radiation, and in those who are iodine deficient. When a nodule is present, are performed and reveal whether a person has a normal amount of thyroid hormones ('euthyroid') or an excess of hormones, usually secreted by the nodule, causing hyperthyroidism. When the thyroid function tests are normal, an 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. A may then be performed, and the sample undergoes, in which the appearance of cells is viewed to determine whether they resemble normal or cancerous cells. There can be many nodules, which is termed a, and this can sometimes be a. Swelling [ ].

Main article: An enlarged thyroid gland is called a. Goitres are present in some form in about 5% of people, 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, and infltrative disease such as and.

Sometimes no cause can be found, a state called 'simple goitre'. Some forms of goitre are associated with pain, whereas many do not cause any symptoms. Enlarged goitres may extend beyond the normal position of the thyroid gland to below the sternum, around the airway or esophagus.

Goitres may be associated with hyperthyoidism or hypothyroidism, relating to the underlying cause of the goitre. Thyroid function tests may be done to investigate the cause and effects of the goitre. The underlying cause of the goitre may be treated, however many goitres with no associated symptoms are simply monitored. Main article: Inflammation of the thyroid is called.

Inflamed thyroids may cause symptoms of hyperthyroidism or hypothyroidism. Two types thyroiditis initially present with hyperthyroidism and are sometimes followed by a period of hypothyroidism – Hashimoto's thyroiditis and. There are other disorders that cause inflammation of the thyroid, and these include,,, and traumatic injury, including. Is an disorder in which the thyroid gland is infltrated by the and. These progressively destroy the thyroid gland.

In this way, Hasimoto's thyroiditis may have occurred insidiously, and only be noticed when thyroid hormone production decreases, causing symptoms of hypothyroidism. Hashimoto's is more common in females than males, much more common after the age of 60, and has known genetic risk factors.

Also more common in individuals with Hashimoto's thyroiditis are,,. Occurs in some females following. After delivery, the gland becomes inflamed and the condition initially presents with a period of hyperthyroidism followed by hypothyroidism and, usually, a return to normal function.

The course of the illness takes place over several months, and is characterised by a painless goitre. Antibodies against thyroid peroxidase can be found on testing.

The inflammation usually resolves without treatment, although thyroid hormone replacement may be needed during the period of hypothyroidism. Main article: The most common affecting the thyroid gland is a benign, usually presenting as a painless mass in the neck. Malignant thyroid cancers are most often, although cancer can occur in any tissue that the thyroid consists of, including cancer of C-cells and lymphomas. Cancers from other sites also rarely lodge in the thyroid. Radiation of the head and neck presents a for thyroid cancer, and cancer is more common in women than men, occurring at a rate of about 2:1.

In most cases, thyroid cancer presents as a painless mass in the neck. It is very unusual for thyroid cancers to present with other symptoms, although in some cases cancer may cause hyperthyroidism. Most malignant thyroid cancers are, followed by,, and. Because of the prominence of the thyroid gland, cancer is often detected earlier in the course of disease as the cause of a nodule, which may undergo. Thyroid function tests will help reveal whether the nodule produces excess thyroid hormones. A can help reveal the activity and location of the cancer and metastases. Thyroid cancers are treated.

Radioactive may be given to the thyroid. Is given to replace the hormones lost and to suppress TSH production, as TSH may stimulate recurrence. With the exception of the rare, which carries a very poor prognosis, most thyroid cancers carry an excellent prognosis and can even be considered curable.

Congenital [ ] A is the most common clinically significant 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. Preterm neonates are at risk of hypothyroidism as their thyroid glands are insufficiently developed to meet their postnatal needs. In order to detect hypothyroidism in newborn babies, to prevent growth and development abnormalities in later life, many countries have programs at birth. Infants with thyroid hormone deficiency () can manifest problems of physical growth and development as well as brain development, termed.

Children with congenital hypothyroidism are treated supplementally with, which facilitates normal growth and development. 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 anterior to the. Segments of the duct and cysts that occur high in the neck are lined by, which is essentially identical to that covering the posterior portion of the 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. May convert these lesions into abscess cavities, and rarely, give rise to cancers.

[ ] Another disorder is that of which can result in various presentations of one or more accessory thyroid glands. These can be asymptomatic. Child affected by, associated with a lack of iodine., most common in inland and mountainous areas, can predispose to goitre – if widespread, known as. Pregnant women deficient of iodine can give birth to infants with thyroid hormone deficiency. The use of used to add iodine to the diet has eliminated endemic cretinism in most developed countries, and over 120 countries have made the iodination of mandatory.

Because the thyroid concentrates iodine, it also concentrates the various radioactive of iodine produced. 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, taken in the form of potassium iodide tablets.

One consequence of the was an increase in in children in the years following the accident. 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, resulting in permanent hypothyroidism. Graves' disease [ ]. Main article: is an autoimmune disorder that is the most common cause of hyperthyroidism.

In Graves' disease, for an unknown reason autoantibodies develop against the thyroid stimulating hormone receptor. These antibodies activate the receptor, leading to development of a goitre and symptoms of hyperthyroidism, such as heat intolerance, weight loss, diarrhoea and palpitations. Occasionally such antibodies block but do not activate the receptor, leading to symptoms associated with hypothyroidism. In addition, gradual protrusion of the eyes may occur, called, as may swelling of the front of the shins. Graves' disease can be diagnosed by the presence of features such as involvement of the eyes and shins, or isolation of autoantibodies, or by results of a radiolabelled uptake scan.

Graves' disease is treated with anti-thyroid drugs such as propylthiouracil, which decrease the production of thyroid hormones, but hold a high rate of relapse. If there is no involvement of the eyes, then use of radioactive isotopes to ablate the gland may be considered. Surgical removal of the gland with subsequent thyroid hormone replacement may be considered, however this will not control symptoms associated with the eye or skin.

Examination [ ] Physicians who specialise in the treatment of thyroid disorders are known generally as, thyroid specialists. Thyroid surgeon or may play a role in the surgical management of thyroid disease and and may play a role in monitoring for and identifying symptoms related to thyroid disease. The thyroid itself is examined by observation of the gland and surrounding neck for swelling or enlargement. It is then felt, usually from behind, and a person is often asked to swallow to better feel the gland against the fingers of the examiner. The gland moves up and down with swallowing because of its attachments to the thyroid and cricoid cartilages. In a healthy person the gland is not visible yet is as a soft mass. Examination of the thyroid gland includes the search for abnormal masses and the assessment of overall thyroid size.

The character of the thyroid, swellings, nodules, and their consistency may all be able to be felt. If a goitre is present, an examiner may also feel down the neck consider the upper part of the chest to check for extension. Further tests may include raising the arms (), listening to the gland with a for, testing of reflexes, and palpation of the lymph nodes in the head and neck. A medical examination of the thyroid will also include observation of the person as a whole, to look for systemic signs such as weight gain or loss, hair loss, and signs in other locations – such as protrusion of the eyes or swelling of the calves in Grave's disease. Tests [ ] A number of tests that can be used to test the function of the thyroid, for the presence of diseases, and for the success or failure of treatment. Blood tests in general aim to measure thyroid function or determine the cause of thyroid dysfunction.

Include a battery of blood tests including the measurement of the thyroid hormones T3 and T4, as well as the measurement of TSH. They may reveal hyperthyroidism (high T3 and T4), hypothyroidism (low T3, T4), or subclinical hyperthyroidism (normal T3 and T4 with a low TSH). TSH levels are considered the most sensitive marker of thyroid dysfunction.

They are however not always accurate, particularly if the cause of hypothyroidism is thought to be related to insufficient TRH secretion, in which case it may be low or falsely normal. In such a case a TRH stimulation test, in which TRH is given and TSH levels are measured at 30 and 60-minutes after, may be conducted. T3 and T4 can be measured directly. However, as the two thyroid hormones travel bound to other molecules, and it is the 'free' component that is biologically active, free T3 and free T4 levels can be measured.

T4 is preferred, because in hypothyroidism T3 levels may be normal. The ratio of bound to unbound thyroid hormones is known as the thyroid hormone binding ratio (THBR). It is also possible to measure directly the main carriers of the thyroid hormones, thryoglobulin and throxine-binding globulin. Thyroglobulin will also be measurable in a healthy thyroid, and will increase with inflammation, and may also be used to measure the success of thyroid removal or ablation. If successful, thyroglobulin should be undetectable. Lastly, antibodies against components of the thyroid, particularly anti-TPO and anti-thyroglobulin, can be measured. These may be present in normal individuals but are highly for autoimmune-related disease.

Ultrasound of the thyroid may be used to reveal whether structures are solid or filled with fluid, helping to differentiate between nodules and goitres and cysts. It may also help differentiate between malignant and benign lesions. A may be taken concurrently of thyroid tissue to determine the nature of a lesion. These biopsies are then sent for and. When further imaging is required, a radiolabelled or uptake scan may take place. This can determine the size and shape of lesions, reveal whether nodules or goitres are metabolically active, and reveal and monitor sites of thyroid disease or cancer deposits.

The thyroid was named by after the ancient Greek shield of a similar pronunciation. Shown is an example, as engraved on an dating from 431–424 BCE. The presence and diseases of the thyroid have been noted and treated for thousands of years, although the gland itself has only been described and named since the.

The first recorded mention of the thyroid is in terms of goitre in texts circa 2700 BCE, of which there is general agreement. In 1600 BCE burnt sponge and seaweed were used within China for the treatment of goitres, a practice which has developed in many parts of the world. In, the book written about 1400 BCE describes hyperthyroidism, hypothyroidism and goitre. And in the fifth century BCE describe cases of Grave's disease, which receives its name over two millennia later owing to descriptions provided by in 1834, Hippocrates and Plato in the fourth century BCE provided some of the first descriptions of the gland itself, proposing its function as a salivary gland.

In the first century BCE referred to epidemics of goitre in the and proposed treatment with burnt seaweed, a practice also referred to by in the second century, referred to burnt sponge for the treatment of goitre. In 1500 polymath provides the first illustration of the thyroid. In 1543 Anatomist gave the first anatomic description and illustration of the gland. In 1656 the thyroid received its name, by the anatomist. The gland was named thyroid, meaning shield, as its shape resembled the shields commonly used in Ancient Greece. The English name thyroid gland is derived from the used by Wharton – glandula thyreoidea.

Glandula means gland in Latin, and thyreoidea can be traced back to the word θυρεοειδής, meaning shield-like/ shield-shaped. French chemist discovered Iodine in 1811, and in 1896 documented it as the central ingredient in the thryoid gland. He did this by boiling the thyroid glands of a thousand sheep, and named the precipitate, a combination of the thyroid hormones, 'iodothyrin'. In 1907 provided iodine is necessary for thyroid function. Thyroxine itself was first isolated in 1914 and synthesized in 1927, and trirodothyroxine in 1952. The conversion of T4 to T3 was discovered in 1970.

The process of discovering TSH took place over the early to mid twentieth century. TRH was discovered by Polish endocrinologist in 1970, contributing in part to his Nobel Prize in Medicine in 1977. Either in the sixth century CE or Persian in 990 CE conducted the first recorded thyroidectomy as a treatment for goitre. Operations remained risky and generally were not successful until the 19th century, when descriptions emerged from a number of authors including Prussian surgeon, Swiss surgeon and physiologist, American physician, and. These descriptions provided the basis for modern thyroid surgery.

Went on to win the in 1909 'for his work on the physiology, pathology and surgery of the thyroid gland'. Numerous authors described cretinism, myxoedema their relationship with the thyroid in the nineteenth century. Charles Mayo coined the term hyperthyroidism in 1910, documented a case of Hashimoto's thyroiditis in 1912, and autoantibodies were demonstrated in 1956. Knowledge of the thyroid and its conditions developed throughout the late nineteenth and twentieth centuries, with many modern treatments and investigative modalities evolving throughout the mid twentieth century, including the use of radioactive iodine, thiouracil and fine needle aspiration.

Other animals [ ]. Goat affected by a The thyroid gland is found in all. In fish, it is usually located below the gills and is not always divided into distinct lobes. However, in some, patches of thyroid tissue are found elsewhere in the body, associated with the kidneys, spleen, heart, or eyes. In, 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, and the shape found in humans is common to many other species.

In larval, the thyroid originates as an 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 and, have a structure very similar to that of larval lampreys (the ), and this also secretes iodine-containing compounds (albeit not thyroxine). Thyroxine is critical to the regulation of and growth throughout the animal kingdom. For example, Iodine and T4 trigger the from a plant-eating water-dwelling into a meat-eating land-dwelling, with better neurological, visuospatial, smell and cognitive abilities for hunting, as seen in other predatory animals. A similar phenomenon happens in the amphibian, which, without introducing iodine, don't transform into land-dwelling adults, and live and reproduce in the larval form of aquatic. Among, administering a thyroid-blocking agent such as (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 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 of the human brain. See also [ ] • • References [ ].