a risk factor 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 thyroid cancers are papillary, followed by follicular, medullary, and thyroid lymphoma. 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 fine-needle aspiration. thyroid function tests will help reveal whether the nodule produces excess thyroid hormones. a radioactive iodine uptake test can help reveal the activity and location of the cancer and metastases. thyroid cancers are treated by removing the whole or part of thyroid gland. radioactive iodine-131 may be given to radioablate the thyroid. thyroxine is given to replace the hormones lost and to suppress tsh production, as tsh may stimulate recurrence. with the exception of the rare anaplastic thyroid cancer, which carries a very poor prognosis, most thyroid cancers carry an excellent prognosis and can even be considered curable. congenital a persistent thyroglossal duct is the most common clinically significant birth defect 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. 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 newborn screening programs at birth. infants with thyroid hormone deficiency (congenital hypothyroidism) can manifest problems of physical growth and development as well as brain development, termed cretinism. children with congenital hypothyroidism are treated supplementally with levothyroxine, 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 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. another disorder is that of thyroid dysgenesis which can result in various presentations of one or more misplaced accessory thyroid glands. these can be asymptomatic. iodine further information: iodine in biology child affected by congenital iodine deficiency syndrome, associated with a lack of iodine. iodine deficiency, most common in inland and mountainous areas, can predispose to goitre – if widespread, known as endemic goitre. pregnant women deficient of iodine can give birth to infants with thyroid hormone deficiency. the use of iodised salt to add iodine to the diet has eliminated endemic cretinism in most developed countries, and over 120 countries have made the iodination of salt mandatory. 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. excessive iodine intake is uncommon and usually has no effect on the thyroid function. sometimes though it may cause hyperthyroidism, and sometimes hypothyroidism with a resulting goitre. evaluation the thyroid 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 palpable 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 tapping the upper part of the chest to check for extension. further tests may include raising the arms (pemberton's sign), listening to the gland with a stethoscope for bruits, testing of reflexes, and palpation of the lymph nodes in the head and neck. an 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 graves' disease. tests thyroid function tests include a battery of blood tests, including the measurement of the thyroid hormones, as well as the measurement of thyroid stimulating hormone (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 thyrotropin releasing hormone (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. t3 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 sensitive for autoimmune-related disease. imaging 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. when further imaging is required, a radiolabelled iodine-123 or technetium-99 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 outside the thyroid. a fine needle aspiration of a sample of thyroid tissue may be taken in order to evaluate a lesion seen on ultrasound which is then sent for histopathology and cytology. computed tomography of the thyroid plays an important role in the evaluation of thyroid cancer. ct scans often incidentally find thyroid abnormalities, and thereby practically becomes the first investigation modality. history the thyroid was named by thomas wharton after the ancient greek shield of a similar pronunciation. shown is an example of such a shield, as engraved on a coin dating from 431 to 424 bce. the thyroid gland received its modern name in the 1600s, when the anatomist thomas wharton likened its shape to that of an ancient greek shield or thyos. however, the existence of the gland, and of the diseases associated with it, was known long before then. antiquity the presence and diseases of the thyroid have been noted and treated for thousands of years. in 1600 bce burnt sponge and seaweed (which contain iodine) were used within china for the treatment of goitres, a practice which has developed in many parts of the world. in ayurvedic medicine, the book sushruta samhita written about 1400 bce described hyperthyroidism, hypothyroidism and goitre. aristotle and xenophon in the fifth century bce describe cases of diffuse toxic goitre. 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. pliny the elder in the first century bce referred to epidemics of goitre in the alps and proposed treatment with burnt seaweed, a practice also referred to by galen in the second century, referred to burnt sponge for the treatment of goitre. the chinese pharmacology text shennong ben cao jing, written ca. 200–250, also refers to goitre. scientific era in 1500 polymath leonardo da vinci provided the first illustration of the thyroid. in 1543 anatomist andreas vesalius gave the first anatomic description and illustration of the gland. in 1656 the thyroid received its modern name, by the anatomist thomas wharton. 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 medical latin used by wharton – glandula thyreoidea. glandula means 'gland' in latin, and thyreoidea can be traced back to the ancient greek word θυρεοειδής, meaning 'shield-like/shield-shaped'. french chemist bernard courtois discovered iodine in 1811, and in 1896 eugen baumann documented it as the central ingredient in the thyroid gland. he did this by boiling the thyroid glands of a thousand sheep, and named the precipitate, a combination of the thyroid hormones, 'iodothyrin'. david marine in 1907 proved that iodine is necessary for thyroid function. graves' disease was described by robert james graves in 1834. the role of the thyroid gland in metabolism was demonstrated in 1895 by adolf magnus-levy. thyroxine was first isolated in 1914 and synthesized in 1927, and triiodothyroxine 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 andrew schally in 1970, contributing in part to his nobel prize in medicine in 1977. in the nineteenth century numerous authors described both cretinism and myxedema, and their relationship to the thyroid. charles mayo coined the term hyperthyroidism in 1910. hakaru hashimoto documented a case of hashimoto's thyroiditis in 1912, antibodies in this disease 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. surgery either aetius in the sixth century ce or persian ali ibn abbas al-magusi 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 theodor billroth, swiss surgeon and physiologist theodor kocher, american physician charles mayo, american surgeons william halsted and george crile. these descriptions provided the basis for modern thyroid surgery. theodor kocher went on to win the nobel prize in physiology or medicine in 1909 "for his work on the physiology, pathology and surgery of the thyroid gland". other animals goat affected by a goitre 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. 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