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Significance of iodine
In areas of the world where iodine is lacking in the diet,
the thyroid gland can become considerably enlarged, a condition called endemic goiter.
Pregnant women on a diet that is severely deficient of iodine can give birth to infants with
thyroid hormone deficiency (congenital hypothyroidism),
manifesting in problems of physical growth and development as well as brain development
(a condition referred to as endemic cretinism).
In many developed countries, newborns are routinely tested for congenital hypothyroidism
as part of newborn screening.
Children with congenital hypothyroidism are treated supplementally with levothyroxine,
which facilitates normal growth and development.
Thyroxine is critical to the regulation of metabolism and growth throughout the animal kingdom. Among amphibians, for example, 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.
Iodine (and T4) trigger the amphibian metamorphosis that transforms the vegetarian aquatic tadpole into a carnivorous terrestrial adult frog, with better neurological, visuospatial,
olfactory and cognitive abilities for hunting, as seen in other predatory animals.
A similar phenomenon happens in the neotenic amphibian salamanders, which,
without introducing iodine, don't transform into terrestrial adults,
and live and reproduce in the larval form of aquatic axolotl.[30]
Because the thyroid concentrates iodine, it also concentrates the various radioactive isotopes
of iodine produced by nuclear fission.
In the event of large accidental releases of such material into the environment,
the uptake of radioactive iodine isotopes by the thyroid can, in theory,
be blocked by saturating the uptake mechanism with a large surplus of non-radioactive iodine, taken in the form of potassium iodide tablets.
One consequence of the Chernobyl disaster was an increase in thyroid cancers in children
in the years following the accident.[31]
The use of iodination salt is an efficient way to add iodine to the diet.
It has eliminated endemic cretinism in most developed countries,
and some governments have made the iodination of flour, cooking oil, and salt mandatory. Potassium iodide and sodium iodide are typically used forms of supplemental iodine.
As with most substances, either too much or too little can cause problems.
Recent studies on some populations are showing that excess iodine intake could cause an increased prevalence of autoimmune thyroid disease, resulting in permanent hypothyroidism.[32]
History
Historical references to what we now know as the thyroid gland arise early in medical history.
In 1600 BC the Chinese were using burnt sponge and seaweed for the treatment of goitres (enlarged thyroid glands).
Celsus first described a bronchoceole (a tumour of the neck) in 15 AD.
Around this time Pliny referred to epidemics of goitre in the Alps and also mentioned the use
of burnt seaweed in their treatment,
in the same way as the Chinese had done 1600 years earlier.
In 150 AD Galen, an instrumental figure in the transition from ancient to modern medicine, referred to 'spongia usta' (burnt sponge) for the treatment of goitre.
He also suggested (incorrectly, as it turns out) that the role of the thyroid was to lubricate
the larynx.
There are several findings that evidence a great interest for thyroid disorders just in
the Medieval Medical School of Salerno (12th century).
Rogerius Salernitanus, the Salernitan surgeon and author of "Post mundi fabricam" (around 1180) was considered at that time the surgical text par excellence all over Europe.
In the chapter "De bocio" of his magnum opus, he describes several pharmacological and surgical cures, some of which nowadays are reappraised as scientifically effective.[33]
It was not until 1475 that Wang Hei anatomically described the thyroid gland and recommended that the treatment of goitre should be dried thyroid.
Paracelsus, some fifty years later, attributed goitre to mineral impurities in the water.
In modern times, the thyroid was first identified in 1656 by the anatomist Thomas Wharton
(whose name is also eponymised in Wharton's duct of the submandibular gland).[34]
In 1656 Thomas Wharton named the gland the thyroid, meaning shield,
as its shape resembled the shields commonly used in Ancient Greece.
In 1909, Theodor Kocher from Switzerland won the Nobel Prize in Medicine
"for his work on the physiology, pathology and surgery of the thyroid gland".[35]
Other animals
The thyroid gland is found in all vertebrates.
In fish, it is usually located below the gills and is not always divided into distinct lobes.
However, in some teleosts, patches of thyroid tissue are found elsewhere in the body,
associated with the kidneys, spleen, heart, or eyes.[36]
In tetra-pods, 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 together.
However, there is only ever a single thyroid gland in most mammals,
and the shape found in humans is common to many other species.[36]
In larval lampreys, 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 tunicates and Amphioxus,
have a structure very similar to that of larval lampreys (the endostyle),
and this also secretes iodine-containing compounds (albeit not thyroxine).[36]
Additional images
Position of the Thyroid in Males and Females
Section of the neck at about the level of the sixth cervical vertebra.
Diagram showing common arrangement of thyroid veins.
Sagittal section of nose mouth, pharynx, and larynx.
Muscles of the pharynx, viewed from behind,
together with the associated vessels and nerves.
Thyroid
Thyroid gland
Thyroid gland
Thyroid cartilage lamina
Muscles, nerves and arteries of neck.
Deep dissection. Anterior view.
Larinx, pharinx and tongue.
Deep dissection.Posterior view.
Larinx, pharinx and tongue.
Deep dissection.Posterior view.
Thyroid gland
Surgery to remove thyroids
See also