Editing Thyroid (section)

==Development==
[[Image:Gray42.png|thumb|Floor of pharynx of embryo between 35 and 37 days after fertilization.]]
In the [[human embryogenesis|development of the embryo]], at 3–4 weeks [[Gestational age (obstetrics)|gestational age]], the thyroid gland appears as an [[epithelium|epithelial]] proliferation in the floor of the pharynx at the base of the tongue between the [[Median tongue bud|''tuberculum impar'']] and the ''[[copula linguae]]''. The copula soon becomes covered over by the [[hypopharyngeal eminence]]<ref name="Larsen">{{cite book |last1=Larsen |first1=William J. |name-list-style = vanc |title=Human embryology |url=https://archive.org/details/humanembryology0003lars |url-access=registration |date=2001 |publisher=Churchill Livingstone |location=Philadelphia, Pa. |isbn=978-0-443-06583-5 |pages=[https://archive.org/details/humanembryology0003lars/page/372 372]–374 |edition=3.}}</ref> at a point later indicated by the [[Tongue|foramen cecum]]. The thyroid then descends in front of the pharyngeal gut as a bilobed [[thyroid diverticulum|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 over the following two weeks the detached thyroid migrates to its final position.<ref name=Larsen/>

The [[prenatal development|fetal]] [[hypothalamus]] and [[pituitary]] start to secrete [[thyrotropin-releasing hormone]] (TRH) and [[thyroid-stimulating hormone]] (TSH). TSH is first measurable at 11 weeks.{{sfn|Greenspan's|2011|p=179}} By 18–20 weeks, the production of [[thyroxine]] (T<sub>4</sub>) reaches a clinically significant and self-sufficient level.{{sfn|Greenspan's|2011|p=179}}<ref name=Eugster>{{Cite book |last1 = Eugster |first1 = Erica A. |last2 = Pescovitz |first2 = Ora Hirsch |name-list-style = vanc |title=Pediatric endocrinology: mechanisms, manifestations and management |publisher=Lippincott Williams & Wilkins |location=Hagerstwon, MD |year=2004 |page=493 (Table 33–3) |isbn=978-0-7817-4059-3 }}</ref> Fetal [[triiodothyronine]] (T<sub>3</sub>) remains low, less than 15&nbsp;ng/dL until 30 weeks, and increases to 50&nbsp;ng/dL at [[pregnancy#Childbirth maturity stages|full-term]].<ref name=Eugster/> The [[fetus]] needs to be self-sufficient in thyroid hormones in order to guard against [[neurodevelopmental disorder]]s that would arise from [[maternal hypothyroidism]].<ref>{{cite journal |vauthors = Zoeller RT |title = Transplacental thyroxine and fetal brain development |journal = The Journal of Clinical Investigation |volume = 111 |issue = 7 |pages = 954–7 |date = April 2003 |pmid = 12671044 |pmc = 152596 |doi = 10.1172/JCI18236 }}</ref> The presence of sufficient iodine is essential for healthy neurodevelopment.<ref name=":1">{{cite web |url=https://www.who.int/elena/titles/iodine_pregnancy/en/ |archive-url=https://web.archive.org/web/20140104171410/http://www.who.int/elena/titles/iodine_pregnancy/en/ |url-status=dead |archive-date=January 4, 2014 |title=Iodine supplementation in pregnant and lactating women |website=World Health Organization |language=en-GB |access-date=2016-11-13}}</ref>

The [[neuroendocrine cell|neuroendocrine]] [[parafollicular cell]]s, also known as C cells, responsible for the production of [[calcitonin]], are derived from foregut endoderm. This part of the thyroid then first forms as the [[ultimopharyngeal body]], which begins in the ventral fourth [[Pharyngeal pouch (embryology)|pharyngeal pouch]] and joins the primordial thyroid gland during its descent to its final location.<ref name=LANGMAN>{{cite book |first1 = Jan |last1 = Langman |first2 = T W |last2 = Sadler |first3 = Susan L |last3 = Sadler-Redmond |first4 = Kathryn |last4 = Tosney |first5 = Jennifer |last5 = Byrne |first6 = Hytham |last6 = Imseis |name-list-style = vanc |title=Langman's Medical Embryology |isbn=978-1-4511-9164-6 |pages=285–6,293 |edition=13th |year=2015}}</ref>

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]].{{sfn|Greenspan's|2011|p=179}}