Editing Thyroid (section)

== Structure ==
===Features===
[[File:Illu08 thyroid.jpg|thumb| The thyroid gland surrounds the [[cricoid cartilage|cricoid]] and [[trachea|tracheal cartilages]] and consists of two lobes. This image shows a variant thyroid with a pyramidal lobe emerging from the middle of the thyroid.|alt=Image showing the thyroid gland surrounding the cricoid cartilage]]

The thyroid gland is a butterfly-shaped organ composed of two lobes, left and right, connected by a narrow tissue band, called an "isthmus".{{sfn|Gray's Anatomy|2008|pp=462–4}} It weighs 25 grams in adults, with each lobe being about 5&nbsp;cm long, 3&nbsp;cm wide, and 2&nbsp;cm thick and the isthmus about 1.25&nbsp;cm in height and width.{{sfn|Gray's Anatomy|2008|pp=462–4}} The gland is usually larger in women than in men, and increases in size during pregnancy.{{sfn|Gray's Anatomy|2008|pp=462–4}}{{sfn|Elsevier's|2007|p=342}}

The thyroid is near the front of the neck, lying against and around the front of the [[larynx]] and [[trachea]].{{sfn|Gray's Anatomy|2008|pp=462–4}} The [[thyroid cartilage]] and [[cricoid cartilage]] lie just above the gland, below the [[Adam's apple]]. The isthmus extends from the second to third [[rings of the trachea]], with the uppermost part of the lobes extending to the thyroid cartilage and the lowermost around the fourth to sixth tracheal rings.{{sfn|Elsevier's|2007|pp=342–3}} The [[infrahyoid muscles]] lie in front of the gland and the [[sternocleidomastoid muscle]] to the side.<ref name="isbn0-443-07168-3">{{cite book | last1 = Ellis | first1 = Harold | first2 = Susan | last2 = Standring | last3 = Gray | first3 = Henry David | name-list-style = vanc |title=Gray's anatomy: the anatomical basis of clinical practice |publisher=Elsevier Churchill Livingstone |location=St. Louis, Mo |year=2005 |pages=[https://archive.org/details/graysanatomyanat0000unse/page/538 538–539] |isbn=978-0-443-07168-3 |url=https://archive.org/details/graysanatomyanat0000unse/page/538 }}</ref> Behind the outer wings of the thyroid lie the two [[carotid artery|carotid arteries]]. The trachea, larynx, lower pharynx and esophagus all lie behind the thyroid.{{sfn|Elsevier's|2007|p=342}} In this region, the [[recurrent laryngeal nerve]]<ref name="Netter">{{cite book|last1=Netter|first1=Frank H. | name-list-style = vanc |title=Atlas of Human Anatomy Including Student Consult Interactive Ancillaries and Guides.|date=2014|publisher=W B Saunders Co|location=Philadelphia, Penn.|isbn=978-1-4557-0418-7|page=27|edition=6th}}</ref> and the inferior thyroid artery pass next to or in the ligament.<ref name=PAGE2009>{{cite journal | vauthors = Page C, Cuvelier P, Biet A, Boute P, Laude M, Strunski V | title = Thyroid tubercle of Zuckerkandl: anatomical and surgical experience from 79 thyroidectomies | journal = The Journal of Laryngology and Otology | volume = 123 | issue = 7 | pages = 768–71 | date = July 2009 | pmid = 19000342 | doi = 10.1017/s0022215108004003 | s2cid = 22063700 }}</ref> Typically, four [[parathyroid gland]]s, two on each side, lie on each side between the two layers of the thyroid capsule, at the back of the thyroid lobes.{{sfn|Gray's Anatomy|2008|pp=462–4}}

The thyroid gland is covered by a thin fibrous capsule,{{sfn|Gray's Anatomy|2008|pp=462–4}} which has an inner and an outer layer. The inner layer extrudes into the gland and forms the [[septum|septa]] that divide the thyroid tissue into microscopic lobules.{{sfn|Gray's Anatomy|2008|pp=462–4}} The outer layer is continuous with the [[pretracheal fascia]], attaching the gland to the cricoid and thyroid cartilages{{sfn|Elsevier's|2007|p=342}} via a thickening of the fascia to form the posterior [[suspensory ligament of thyroid gland]], also known as Berry's ligament.{{sfn|Elsevier's|2007|p=342}} This causes the thyroid to move up and down with the movement of these cartilages when swallowing occurs.{{sfn|Elsevier's|2007|p=342}}

===Blood, lymph and nerve supply===
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 [[Human variability|anatomical variant]] the [[thyroid ima artery]],{{sfn|Gray's Anatomy|2008|pp=462–4}} which has a variable origin.{{sfn|Elsevier's|2007|p=343}} The superior thyroid artery splits into anterior and posterior branches supplying the thyroid, and the inferior thyroid artery splits into superior and inferior branches.{{sfn|Gray's Anatomy|2008|pp=462–4}} The superior and inferior thyroid arteries join behind the outer part of the thyroid lobes.{{sfn|Elsevier's|2007|p=343}} The venous blood is drained via [[superior thyroid vein|superior]] and [[middle thyroid vein]]s, 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 vein]]s.{{sfn|Gray's Anatomy|2008|pp=462–4}} Both arteries and veins form a plexus between the two layers of the capsule of the thyroid gland.{{sfn|Elsevier's|2007|p=343}}

[[Lymph]]atic drainage frequently passes the [[prelaryngeal lymph nodes]] (located just above the isthmus) and the [[pretracheal lymph nodes|pretracheal]] and [[paratracheal lymph nodes]].{{sfn|Gray's Anatomy|2008|pp=462–4}} The gland receives [[sympathetic nervous system|sympathetic nerve]] supply from the superior, middle and inferior cervical ganglion of the [[sympathetic trunk]].{{sfn|Gray's Anatomy|2008|pp=462–4}} The gland receives [[parasympathetic nerve]] supply from the [[superior laryngeal nerve]] and the [[recurrent laryngeal nerve]].{{sfn|Gray's Anatomy|2008|pp=462–4}}

===Variation===
[[File:Sobo 1906 444.png|thumb|right|upright=0.85|Clear ''pyramidal lobe'' (center) as viewed from the front]]
There are many [[anatomical variation|variants]] in the size and shape of the thyroid gland, and in the position of the embedded parathyroid glands.{{sfn|Elsevier's|2007|p=342}}

Sometimes there is a third lobe present called the ''pyramidal lobe''.{{sfn|Elsevier's|2007|p=342}} When present, this lobe often stretches up to the hyoid bone from the thyroid isthmus and may be one to several divided lobes.{{sfn|Gray's Anatomy|2008|pp=462–4}} The presence of this lobe ranges in reported studies from 18.3%<ref name="MedJ">{{cite journal |vauthors=Cicekcibasi AE, Salbacak A, Seker M, Ziylan T, Tuncer I, Buyukmumcu M |date=April 2007 |title=Developmental variations and clinical importance of the fetal thyroid gland. A morphometric study |url=https://pubmed.ncbi.nlm.nih.gov/17457471/ |url-status=live |journal=Saudi Medical Journal |volume=28 |issue=4 |pages=524–8 |pmid=17457471 |archive-url=https://web.archive.org/web/20240605020835if_/https://pubmed.ncbi.nlm.nih.gov/17457471/ |archive-date=5 June 2024 |access-date=6 October 2024}}</ref> to 44.6%.<ref>{{cite journal | vauthors = Kim DW, Jung SL, Baek JH, Kim J, Ryu JH, Na DG, Park SW, Kim JH, Sung JY, Lee Y, Rho MH | display-authors = 6 | title = The prevalence and features of thyroid pyramidal lobe, accessory thyroid, and ectopic thyroid as assessed by computed tomography: a multicenter study | journal = Thyroid | volume = 23 | issue = 1 | pages = 84–91 | date = January 2013 | pmid = 23031220 | doi = 10.1089/thy.2012.0253 }}</ref> It was shown to more often arise from the left side and occasionally separated.<ref name=MedJ/> The pyramidal lobe is also known as [[Pierre Lalouette|Lalouette's]] pyramid.<ref>{{cite book| first = William Alexander Newman | last = Dorland | veditors = Anderson DM | title = Dorland'sIllustrated Medical Dictionary | edition = 32nd |date=2012 |publisher=Elsevier Saunders |isbn=978-1-4160-6257-8 |pages=999 redirect to 1562}}</ref> The pyramidal lobe is a remnant of the [[thyroglossal duct]], which usually wastes away during the thyroid gland's descent.{{sfn|Elsevier's|2007|p=342}} Small accessory thyroid glands may in fact occur anywhere along the thyroglossal duct, from the [[foramen cecum (tongue)|foramen cecum]] of the tongue to the position of the thyroid in the adult.{{sfn|Gray's Anatomy|2008|pp=462–4}} A small horn at the back of the thyroid lobes, usually close to the recurrent laryngeal nerve and the inferior thyroid artery, is called [[Zuckerkandl's tubercle (thyroid gland)|Zuckerkandl's tubercle]].<ref name=PAGE2009 />

Other variants include a [[levator muscle of thyroid gland]], connecting the isthmus to the body of the [[hyoid bone]],{{sfn|Elsevier's|2007|p=342}} and the presence of the small [[thyroid ima artery]].{{sfn|Elsevier's|2007|p=342}}

===Microanatomy===
[[File:Thyroid-histology.jpg|thumb|Section of a thyroid gland under the microscope. 1 colloid, 2 follicular cells, 3 [[Endothelium|endothelial]] cells]] 
At the [[histology|microscopic level]], there are three primary features of the thyroid—[[thyroid follicle]]s, [[thyroid follicular cell]]s, and [[parafollicular cell]]s, first discovered by Geoffery Websterson in 1664.<ref>{{Cite book|last1 = Fawcett|first1 = Don| last2 = Jensh | first2 = Ronald | name-list-style = vanc |title = Bloom & Fawcett's Concise Histology|publisher = Arnold Publishers|year = 2002|location = New York|pages = 257–258|isbn = 978-0-340-80677-7}}</ref>

;Follicles
[[Thyroid follicular cell#Location|Thyroid follicles]] are small spherical groupings of cells 0.02–0.9mm in diameter that play the main role in thyroid function.{{sfn|Gray's Anatomy|2008|pp=462–4}} They consist of a rim that has a rich blood supply, nerve and lymphatic presence, that surrounds a core of [[colloid]] that consists mostly of thyroid hormone precursor proteins called [[thyroglobulin]], an [[iodine|iodinated]] [[glycoprotein]].{{sfn|Gray's Anatomy|2008|pp=462–4}}<ref name="Wheaters2006">{{cite book| first1 = Paul R | last1 = Wheater | first2 = Barbara | last2 = Young | name-list-style = vanc |title=Wheater's functional histology : a text and colour atlas|url=https://archive.org/details/wheatersfunction00youn|url-access=limited|date=2006|publisher=Churchill Livingstone|location=Oxford|isbn=978-0-443-06850-8|pages=[https://archive.org/details/wheatersfunction00youn/page/n741 333]–335|edition=5th}}</ref>

;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 T<sub>3</sub> and T<sub>4</sub>. They do this by transporting and metabolising the thyroglobulin contained in the colloid.{{sfn|Gray's Anatomy|2008|pp=462–4}} Follicular cells vary in shape from flat to cuboid to columnar, depending on how active they are.{{sfn|Gray's Anatomy|2008|pp=462–4}}<ref name=Wheaters2006 />

;Follicular lumen
The ''follicular lumen'' is the fluid-filled space within a follicle of the thyroid gland. There are hundreds of follicles within the thyroid gland. A follicle is formed by a spherical arrangement of [[thyroid follicular cell|follicular cells]]. The follicular lumen is filled with ''colloid'', a concentrated solution of [[thyroglobulin]] and is the site of synthesis of the thyroid hormones [[thyroxine]] (T4) and [[triiodothyronine]] (T3).<ref>[http://www.expertconsultbook.com/expertconsult/ob/book.do?method=display&type=bookPage&decorator=none&eid=4-u1.0-B978-1-4160-5583-9..00072-1--s0040&isbn=9781416055839 The Thyroid Follicle] {{Webarchive|url=https://archive.today/20130123031837/http://www.expertconsultbook.com/expertconsult/ob/book.do?method=display&type=bookPage&decorator=none&eid=4-u1.0-B978-1-4160-5583-9..00072-1--s0040&isbn=9781416055839 |date=2013-01-23 }}, ''Endocrinology'' by J. Larry Jameson, MD, PhD and Leslie J. De Groot, MD, chapter 72</ref>

;Parafollicular cells
Scattered among follicular cells and in spaces between the spherical follicles are another type of thyroid cell, parafollicular cells.{{sfn|Gray's Anatomy|2008|pp=462–4}} These cells secrete [[calcitonin]] and so are also called C cells.<ref>{{cite journal | vauthors = Hazard JB | title = The C cells (parafollicular cells) of the thyroid gland and medullary thyroid carcinoma. A review | journal = The American Journal of Pathology | volume = 88 | issue = 1 | pages = 213–50 | date = July 1977 | pmid = 18012 | pmc = 2032150 }}</ref>