Editing Thyroid (section)

==Other animals==
[[File:The thyroid gland in health and disease (1917) (14780977681).jpg|thumb|Goat affected by a [[goitre]]]]
The thyroid gland is found in all [[vertebrate]]s. In fish, it is usually located below the gills and is not always divided into distinct lobes. However, in some [[teleost]]s, patches of thyroid tissue are found elsewhere in the body, associated with the kidneys, spleen, heart, or eyes.<ref name=VB>{{Cite book| last1 = Romer | first1 = Alfred Sherwood | last2 = Parsons | first2 = Thomas S. | name-list-style = vanc | year=1977 |title=The Vertebrate Body |publisher=Holt-Saunders International |location= Philadelphia, PA|pages= 555–556|isbn= 978-0-03-910284-5}}</ref>

In [[tetrapod]]s, 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 not joined. However, there is only ever a single thyroid gland in most [[mammal]]s, and the shape found in humans is common to many other species.<ref name=VB/>

In larval [[lamprey]]s, the thyroid originates as an [[exocrine gland]], secreting its hormones into the gut, and associated with the larva's filter-feeding apparatus. In the adult lamprey, the gland separates from the gut, and becomes endocrine, but this path of development may reflect the evolutionary origin of the thyroid. For instance, the closest living relatives of vertebrates, the [[tunicate]]s and [[amphioxi]] (lancelets), have a structure very similar to that of larval lampreys (the [[endostyle]]), and this also secretes iodine-containing compounds, though not thyroxine.<ref name=VB/>

Thyroxine is critical to [[metabolic regulation]], and growth throughout the vertebrate clade. Iodine and T<sub>4</sub> trigger the [[metamorphosis|change]] from a plant-eating water-dwelling [[tadpole]] into a meat-eating land-dwelling [[frog]], with better neurological, visuospatial, smell and cognitive abilities for hunting, as seen in other predatory animals. A similar phenomenon happens in the [[neotenic]] amphibian [[salamanders]], which, without introducing iodine, do not transform into land-dwelling adults, and live and reproduce in the larval form of aquatic [[axolotl]]. Among [[amphibian]]s, administering a thyroid-blocking agent such as [[propylthiouracil]] (PTU) can prevent tadpoles from metamorphosing into frogs; in contrast, administering thyroxine will trigger metamorphosis. In amphibian metamorphosis, thyroxine and iodine also exert a well-studied experimental model of [[apoptosis]] on the cells of gills, tail, and fins of tadpoles. Iodine, via iodolipids, has favored the evolution of terrestrial animal species and has likely played a crucial role in the [[evolution]] of the human brain.<ref>{{Cite journal| last = Venturi | first = Sebastiano | name-list-style = vanc |title=Evolutionary Significance of Iodine|journal=Current Chemical Biology|volume=5 |pages=155–162|year=2011|issn=1872-3136|doi=10.2174/187231311796765012|issue=3}}</ref><ref>{{Cite journal| last = Venturi | first = Sebastiano | name-list-style = vanc |title=Iodine, PUFAs and Iodolipids in Health and Disease: An Evolutionary Perspective|journal=Human Evolution|volume= 29 |issue= 1–3|pages=185–205|year=2014|issn=0393-9375}}</ref>