Question: What gland that controls growth and metabolism is iodine essential for?
About the article "Seafood", section "Health benefits".

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Research over the past few decades has shown that the nutrients and minerals in seafood can make improvements in brain development and reproduction and has highlighted the role for seafood in the functions of the human body.
Doctors have known of strong links between fish and healthy hearts ever since they noticed that fish-eating Inuit populations in the Arctic had low levels of heart disease.
One study has suggested that adding one portion of fish a week to your diet can cut your chances of suffering a heart attack by half.
Fish is thought to protect the heart because eating less saturated fat and more Omega-3 can help to lower the amount of cholesterol and triglycerides in the blood – two fats that, in excess, increase the risk of heart disease.
Omega-3 fats also have natural built-in anti-oxidants, which are thought to stop the thickening and damaging of artery walls.
Regularly eating fish oils is also thought to reduce the risk of arrhythmia – irregular electrical activity in the heart which increases the risk of sudden heart attacks.10-12% of the human brain is composed of lipids, including the Omega-3 fat DHA.
Recent studies suggest that older people can boost their brain power by eating more oily fish, what with regular consumers being able to remember better and think faster than those who don't consume at all.
Other research has also suggested that adding more DHA to the diet of children with attention-deficit hyperactivity disorder can reduce their behavioural problems and improve their reading skills, while there have also been links suggested between DHA and better concentration.
Separate studies have suggested that older people who eat fish at least once a week could also have a lower chance of developing dementia and Alzheimer's disease.
Including fish as a regular part of a balanced diet has been shown to help the symptoms of rheumatoid arthritis – a painful condition that causes joints to swell up, reducing strength and mobility.
Studies also show that sufferers feel less stiff and sore in the morning if they keep their fish oil intake topped up.
Recent research has also found a link between Omega-3 fats and a slowing down in the wearing of cartilage that leads to osteoarthritis, opening the door for more research into whether eating more fish could help prevent the disease.
Fish is high in minerals such as zinc, iodine and selenium, which keep the body running smoothly.
Iodine is essential for the thyroid gland, which controls growth and metabolism, while selenium is used to make enzymes that protect cell walls from cancer-causing free radicals, and helps prevent DNA damage caused by radiation and some chemicals.
Fish is also a source of vitamin A, which is needed for healthy skin and eyes, and vitamin D, which is needed to help the body absorb calcium to strengthen teeth and bones.
Iodine is very important for the proper functioning of the thyroid gland.
The thyroid gland in the neck secretes a hormone called thyroxine, of which iodine is an important element.
Lack of iodine stunts growth and causes the thyroid gland to become irregular - this disorder is called goitre.
People with goitre are advised to use iodised salt.
Iodine is an essential trace element; the thyroid hormones thyroxine and triiodothyronine contain iodine.
In areas where there is little iodine in the diet—typically remote inlandareas where no marine foods are eaten—iodine deficiency gives rise to goiter (so-called endemic goiter), as well as cretinism, which results in developmental delays and other health problems.
While reporting recent progress, "The Lancet" noted, "According to World Health Organization, in 2007, nearly 2 billion individuals had insufficient iodine intake, a third being of school age.
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Thus iodine deficiency, as the single greatest preventable cause of mental retardation, is an important public-health problem.
"In some such areas, this is now combated by the addition of small amounts of iodine to table salt in form of sodium iodide, sodium iodate, potassium iodide, and/or potassium iodate—this product is known as "iodized salt".
Iodine compounds have also been added to other foodstuffs, such as flour, water and milk in areas of deficiency.
Seafood is also a well known source of iodine.
Thus, iodine deficiency is more common in mountainous regions of the world where food is grown in iodine-poor soil. 
The thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are tyrosine-based hormones produced by the thyroid gland that are primarily responsible for regulation of metabolism.
Iodine is necessary for the production of T3 and T4.
A deficiency of iodine leads to decreased production of T3 and T4, enlarges the thyroid tissue and will cause the disease known as goitre.
The major form of thyroid hormone in the blood is thyroxine (T4), which has a longer half-life than T3.
The ratio of T4 to T3 released into the blood is roughly 20 to 1.
T4 is converted to the active T3 (three to four times more potent than T4) within cells by deiodinases (5'-iodinase).
These are further processed by decarboxylation and deiodination to produce iodothyronamine (T1a) and thyronamine (T0a).
All three isoforms of the deiodinases are selenium-containing enzymes, thus dietary selenium is essential for T3 production. 
Iodised salt (also spelled iodized salt) is table salt mixed with a minute amount of various salts of the element iodine.
The ingestion of iodide prevents iodine deficiency.
Worldwide, iodine deficiency affects about two billion people and is the leading preventable cause of intellectual and developmental disabilities.
Deficiency also causes thyroid gland problems, including "endemic goitre".
In many countries, iodine deficiency is a major public health problem that can be cheaply addressed by purposely adding small amounts of iodine to the sodium chloride salt.
Iodine is a micronutrient and dietary mineral that is naturally present in the food supply in some regions, especially near sea coasts, but is generally quite rare in the Earth's crust, since iodine is a so-called "heavy" element (with the highest atomic mass of any element needed by mammals for life), and abundance of chemical elements generally declines with greater atomic mass.
Where natural levels of iodine in the soil are low and the iodine is not taken up by vegetables, iodine added to salt provides the small but essential amount of iodide needed by humans.
An opened box of table salt with iodide will lose its iodine content in about four weeks through the process of oxidation and iodine sublimation. 
Excess iodine has symptoms similar to those of iodine deficiency.
Commonly encountered symptoms are abnormal growth of the thyroid gland and disorders in functioning and growth of the organism as a whole.
Iodides are similar in toxicity to bromides.
Excess iodine can be more cytotoxic in the presence of selenium deficiency.
Iodine supplementation in selenium-deficient populations is theoretically problematic, partly for this reason.