Editing Thyroid (section)

===Thyroid hormones===
{{Main|Thyroid hormones}}

The primary function of the thyroid is the production of the iodine-containing [[thyroid hormones]], [[triiodothyronine]] (T<sub>3</sub>) and [[thyroxine]] or tetraiodothyronine (T<sub>4</sub>) and the [[peptide hormone]] [[calcitonin]].{{sfn|Davidson's|2010|p=736}} The thyroid hormones are created from [[iodine]] and [[tyrosine]]. T<sub>3</sub> is so named because it contains three atoms of iodine per molecule and T<sub>4</sub> contains four atoms of iodine per molecule.{{sfn|Guyton & Hall|2011|p=909}} The thyroid hormones have a wide range of effects on the human body. These include:

* '''Metabolic.'''<!--Metabolic--> The thyroid hormones increase the [[basal metabolic rate]] and have effects on almost all body tissues.{{sfn|Guyton & Hall|2011|p=934}} Appetite, the absorption of substances, and gut motility are all influenced by thyroid hormones.{{sfn|Guyton & Hall|2011|p=937}} They increase the absorption in the gut, [[gluconeogenesis|generation]], [[glucose uptake|uptake by cells]], and [[glycolysis|breakdown]] of glucose.{{sfn|Guyton & Hall|2011|p=936}} They stimulate the [[lipolysis|breakdown of fats]], and increase the number of [[free fatty acid]]s.{{sfn|Guyton & Hall|2011|p=936}} Despite increasing free fatty acids, thyroid hormones decrease [[cholesterol]] levels, perhaps by increasing the rate of secretion of cholesterol in [[bile]].{{sfn|Guyton & Hall|2011|p=936}} 
* '''Cardiovascular.''' <!--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 [[mitochondria]].{{sfn|Guyton & Hall|2011|p=937}} Combined, these factors increase blood flow and the body's temperature.{{sfn|Guyton & Hall|2011|p=937}} 
* '''Developmental.'''<!--In development--> Thyroid hormones are important for normal development.{{sfn|Guyton & Hall|2011|p=936}} They increase the growth rate of young people,{{sfn|Guyton & Hall|2011|p=935-6}} and cells of the developing brain are a major target for the thyroid hormones T<sub>3</sub> and T<sub>4</sub>. Thyroid hormones play a particularly crucial role in brain maturation during fetal development and first few years of postnatal life{{sfn|Guyton & Hall|2011|p=936}}
* <!--Other-->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.{{sfn|Guyton & Hall|2011|p=937}} Sexual function, including libido and the maintenance of a normal [[menstrual cycle]], are influenced by thyroid hormones.{{sfn|Guyton & Hall|2011|p=937}}

After secretion, only a very small proportion of the thyroid hormones travel freely in the blood. Most are bound to [[thyroxine-binding globulin]] (about 70%), [[transthyretin]] (10%), and [[serum albumin|albumin]] (15%).{{sfn|Greenspan's|2011|p=169}} Only the 0.03% of T<sub>4</sub> and 0.3% of T<sub>3</sub> traveling freely have hormonal activity.<ref name=bowen>{{cite web| vauthors = Bowen R |title=Thyroid Hormone Receptors|url=http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/receptors.html|website=Colorado State University|access-date=22 February 2015|date=2000|archive-url=https://web.archive.org/web/20110927050157/http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/receptors.html|archive-date=27 September 2011|url-status=dead}}</ref> In addition, up to 85% of the T<sub>3</sub> in blood is produced following conversion from T<sub>4</sub> by [[iodothyronine deiodinase]]s in organs around the body.{{sfn|Davidson's|2010|p=736}}

Thyroid hormones act by crossing the [[cell membrane]] and binding to [[intracellular receptor|intracellular]] [[nuclear receptor|nuclear]] [[thyroid hormone receptor]]s [[thyroid hormone receptor#Isoforms|TR-α<sub>1</sub>, TR-α<sub>2</sub>, TR-β<sub>1</sub>, and TR-β<sub>2</sub>]], which bind with [[hormone response element]]s and [[transcription factor]]s to modulate [[DNA transcription]].<ref name=bowen />{{sfn|Greenspan's|2011|p=178}} In addition to these actions on DNA, the thyroid hormones also act within the cell membrane or within cytoplasm via reactions with [[enzyme]]s, including [[Plasma membrane Ca2+ ATPase|calcium ATPase]], [[adenylyl cyclase]], and [[glucose transporter]]s.{{sfn|Greenspan's|2011|p=179}}