Editing Thyroid (section)

===Hormone production===
[[File:Thyroid hormone synthesis.png|thumb|upright=2|Synthesis of the [[thyroid hormone]]s, as seen on an individual [[thyroid follicular cell]]:<ref name="Elsevier/Saunders">{{cite book|title=Medical Physiology: A Cellular And Molecular Approaoch|vauthors=Boron WF, Boulpaep E|publisher=Elsevier/Saunders|year=2003|isbn=978-1-4160-2328-9|page=1300|chapter=Chapter 48: "synthesis of thyroid hormones"}}</ref>
<br>- [[Thyroglobulin]] is synthesized in the [[rough endoplasmic reticulum]] and follows the [[secretory pathway]] to enter the colloid in the lumen of the [[thyroid follicle]] by [[exocytosis]].
<br>- Meanwhile, a [[sodium-iodide symporter|sodium-iodide (Na/I) symporter]] pumps iodide (I<sup>−</sup>) [[active transport|actively]] into the cell, which previously has crossed the [[endothelium]] by largely unknown mechanisms. 
<br>- This iodide enters the follicular lumen from the cytoplasm by the transporter [[pendrin]], in a purportedly [[passive transport|passive]] manner.
<br>- In the colloid, iodide (I<sup>−</sup>) is [[Redox|oxidized]] to iodine (I<sup>0</sup>) by an enzyme called [[thyroid peroxidase]].
<br>- Iodine (I<sup>0</sup>) is very reactive and iodinates the thyroglobulin at [[Tyrosine|tyrosyl]] residues in its protein chain (in total containing approximately 120 tyrosyl residues).
<br>- In ''conjugation'', adjacent tyrosyl residues are paired together.
<br>- The entire complex re-enters the follicular cell by [[endocytosis]].
<br>- [[Proteolysis]] by various [[protease]]s liberates [[thyroxine]] and [[triiodothyronine]] molecules, which enters the blood by largely unknown mechanisms.
]]

The thyroid hormones are created from [[thyroglobulin]]. This is a [[protein]] within the colloid in the [[follicular lumen]] that is originally created within the [[rough endoplasmic reticulum]] of follicular cells and then transported into the follicular lumen. Thyroglobulin contains 123 units of [[tyrosine]], which reacts with iodine within the follicular lumen.<ref name="ACBianco">{{cite journal | vauthors = Bianco AC, Salvatore D, Gereben B, Berry MJ, Larsen PR | title = Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases | journal = Endocrine Reviews | volume = 23 | issue = 1 | pages = 38–89 | date = February 2002 | pmid = 11844744 | doi = 10.1210/edrv.23.1.0455 | doi-access = free }}</ref>

[[Iodine]] is essential for the production of the thyroid hormones. Iodine (I<sup>0</sup>) travels in the blood as [[iodide]] (I<sup>−</sup>), which is taken up into the follicular cells by a [[sodium-iodide symporter]]. This is an [[ion channel]] on the cell membrane which in the same action transports two sodium ions and an iodide ion into the cell.<ref name=williams>{{cite book| vauthors = Melmed S, Polonsky KS, Larsen PR, Kronenberg HM |title=Williams Textbook of Endocrinology |url= https://archive.org/details/williamstextbook00melm_993 |url-access=limited|date=2011|publisher=Saunders|page=[https://archive.org/details/williamstextbook00melm_993/page/n336 331]|isbn=978-1-4377-0324-5|edition=12th}}</ref> Iodide then travels from within the cell into the lumen, through the action of [[pendrin]], an iodide-chloride [[antiporter]]. In the follicular lumen, the iodide is then [[oxidation|oxidized]] to iodine. This makes it more reactive,<ref name="Elsevier/Saunders"/> and the iodine is attached to the active tyrosine units in thyroglobulin by the enzyme [[thyroid peroxidase]]. This forms the precursors of thyroid hormones [[monoiodotyrosine]] (MIT), and [[diiodotyrosine]] (DIT).<ref name=boron />

When the follicular cells are stimulated by [[thyroid-stimulating hormone]], the follicular cells reabsorb thyroglobulin from the follicular lumen. The iodinated tyrosines are cleaved, forming the thyroid hormones T<sub>4</sub>, T<sub>3</sub>, DIT, MIT, and traces of [[reverse triiodothyronine]]. T<sub>3</sub> and T<sub>4</sub> are released into the blood. The hormones secreted from the gland are about 80–90% T<sub>4</sub> and about 10–20% T<sub>3</sub>.<ref name="endocrine">[http://www.endocrineweb.com/thyfunction.html How Your Thyroid Works: A Delicate Feedback Mechanism]. Updated 2009-05-21.</ref><ref name="percent">[https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=endocrin.chapter.235 The thyroid gland] in ''Endocrinology: An Integrated Approach'' by Stephen Nussey and Saffron Whitehead (2001) Published by BIOS Scientific Publishers Ltd. {{ISBN|1-85996-252-1}}</ref> [[Deiodinase|Deiodinase enzymes]] in peripheral tissues remove the iodine from MIT and DIT and convert T<sub>4</sub> to T<sub>3</sub> and RT<sub>3.</sub> <ref name="ACBianco" /> This is a major source of both RT<sub>3</sub> (95%) and T<sub>3</sub> (87%) in peripheral tissues.<ref>{{Cite book|title=Ganong's review of medical physiology Edition 25}}</ref>