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10.1: Iodine

Last updated

Mar 5, 2022
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- 10: Macronutrient Metabolism Micronutrients
- 10.2: Manganese

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Why is iodine first in this chapter? Partly because it is a mineral (but so is manganese), but there is also a connection between iodine and selenium (last antioxidant in the previous chapter). Iodine's only, yet critical, function is that it is required for thyroid hormone synthesis. The figure below shows that the thyroid gland is a butterfly-shaped organ found in the neck. The parathyroid glands are also found within the thyroid gland.

Figure $\PageIndex{1}$ : Location of thyroid and parathyroid glands1

Query $\PageIndex{1}$

Iodine is found in foods primarily as iodide ( $\ce{I-}$ ), some bread dough has iodate ( $\ce{IO3-}$ ) added to help with gluten cross-linking2. This used to be more commonly used in the past than it is now. Like selenium, iodide concentrations of the soil vary greatly, causing food concentrations to greatly fluctuate. Sea water is high in iodine, thus foods of marine origin, such as seaweed and seafood, are good dietary sources of iodine. Dairy products also tend to be good sources of iodide because it is added to cattle feed. Cattle receive iodine-containing medications, and iodide-containing sanitizing solutions are used in dairy facilities3 ,4.

For most Americans, we consume ample iodine through the consumption of iodized salt. Consumption of 1/2 teaspoon of iodized salt meets the RDA for iodine. There is a global logo for iodized salt. However, I must admit that I do not recall ever seeing it myself.

The other link below is to a page that contains a scorecard map that depicts access to iodized salt worldwide. It also contains a Youtube video that displays the reduction in iodine deficiency over the last 2 decades.

Web Link

Global Iodized Salt Icon

Global Iodine Scorecard

Query $\PageIndex{2}$

Salt is iodized with either potassium iodide ( $\ce{KI}$ ) or potassium iodate ( $\ce{KIO3}$ ). The positives of each are:

Potassium iodide

Less expensive
Higher iodine content (76% vs. 59% for $\ce{KIO3}$ )
More soluble

Potassium Iodate

More stable

The U.S. uses potassium iodide, but the form, and amount, used varies from country-to-country. Most Americans’ salt intake comes from processed foods, many of which are made with non-iodized salt. Iodine is well absorbed (~90%). Some dietary compounds interfere with thyroid hormone production or utilization. These compounds are known as goitrogens. However, it is not believed that goitrogens are of clinical importance unless there is a coexisting iodine deficiency5.

Some examples of foods that contain goitrogens are3 ,4,6:

Cassava

The cassava plant is cultivated as an annual crop in tropical and subtropical regions for its edible starchy tuberous root. It is often called yuca in Spanish America and in the United States.

Millet

Millets are important crops in the semiarid tropics of Asia and Africa (especially in India, Mali, Nigeria, and Niger).

Cruciferous Vegetables (broccoli, cabbage, Brussels sprouts)

Cruciferous vegetables are vegetables of the family Brassicaceae with many species being raised for food production including

Onions
Garlic
Soybeans
Peanuts

Query $\PageIndex{3}$

Thyroid Hormone

The thyroid accumulates most absorbed iodine, keeping it for use to synthesize thyroid hormone. The following video shows the thyroid and describes its function.

Video $\PageIndex{1}$ : In this animation thyroid gland, its structure and function along with the hormones produced are described. https://www.youtube.com/watch?v=xKQa-MbZUPY

As mentioned in the video, the two primary forms of thyroid hormone are triiodothyronine (T3) and thyroxine (T4).

Figure $\PageIndex{4}$ : (left) The structure of triiodothyronine (T3)11 and (right) The structure of thyroxine (T4)12

T4 is the primary circulating form, and is really a prohormone that is converted to the active T3 form.

Query $\PageIndex{4}$

The enzymes that metabolize thyroid hormones are known as deiodinases. There are three deiodinases (Type I , Type II, Type III) that are selenoenzymes whose location and function are summarized in the table below.

Table $\PageIndex{1}$ : Location and function of the three deiodinases13
Enzyme	Tissues	Function
Deiodinase Type I (DI1)	Liver, kidney, thyroid gland	Plasma T3 production
Deiodinase Type II (DI2)	Brain, pituitary, brown adipose	Local T3 production
Deiodinase Type III (DI3)	Brain, placenta	T3 degradation

Thyroid hormone regulates the basal metabolic rate and is important for growth and development. Thyroid hormone is particularly important for brain development, but hypothyroidism (low thyroid hormone) also leads to decreased muscle mass and skeletal development13.

Query $\PageIndex{5}$

Iodine Deficiency & Toxicity

There are two iodine deficiency disorders (IDD): goiter and cretinism (also known as congenital hypothyroidism and congenital iodine deficiency syndrome). Goiter is a painless deficiency condition that results from the enlargement of the thyroid to help increase its ability to take up iodine. A couple of pictures of goiter are shown below.

Figure $\PageIndex{5}$ : Pictures of women with goiters14 ,15

A more serious consequence of iodine deficiency occurs during pregnancy to the fetus. Iodine deficiency during this time can lead to cognitive impairment and stunted growth and is known as cretinism. This condition is characterized by severe hypothyroidism, speech loss, and paralysis13 ,16. Proposed mechanisms have been attributed to the chronic consumption of poorly detoxified cassava, selenium deficiency, and the immune system causing thyroid dysfunction17. The following links show some examples of individuals with cretinism.

Web Link

Cretinism

The World Health Organization calls iodine deficiency "the world's most prevalent, yet easily preventable, cause of brain damage18." By saying it is easily preventable, they are referring to the ability of salt iodization to prevent brain development problems. The following New York Times article talks about how salt iodization may be the cheapest way to raise the world's IQ.

Web Link

In Raising the World’s I.Q., the Secret’s in the Salt

Iodine toxicity is rare, but like iodine deficiency, it can result in thyroid enlargement, and hypothyroidism or hyperthyroidism. Acute toxicity results in gastrointestinal irritation, abdominal pain, nausea, vomiting, and diarrhea2.

Query $\PageIndex{6}$

References

en.Wikipedia.org/wiki/File:Il...arathyroid.jpg
Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.
Whitney E, Rolfes SR. (2008) Understanding nutrition. Belmont, CA: Thomson Wadsworth.
Anonymous. (2001) Dietary reference intakes for vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. Washington, D.C.: National Academies Press.
en.Wikipedia.org/wiki/File:Casava.jpg
en.Wikipedia.org/wiki/File:Ma...a_dsc07325.jpg
en.Wikipedia.org/wiki/File:PeeledCassava.jpg
en.Wikipedia.org/wiki/Millet...on,_7-3-02.jpg
en.Wikipedia.org/wiki/Staple...harvesting.jpg
en.Wikipedia.org/wiki/File:Tr...othyronine.svg
en.Wikipedia.org/wiki/File:Thyroxine.svg
Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
en.Wikipedia.org/wiki/File:Ko...tor_struma.jpg
en.Wikipedia.org/wiki/File:Goitre.jpg
Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
Eastman CJ, Zimmermann MB. (2018). The Iodine Deficiency Disorders. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK285556/
http://www.who.int/nutrition/topics/idd/en/

Cassava

Millet

Cruciferous Vegetables (broccoli, cabbage, Brussels sprouts)

Thyroid Hormone

Iodine Deficiency & Toxicity

References

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