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10.1: Iodine

  • Page ID
    21024
  • 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.

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    Figure \(\PageIndex{1}\): Location of thyroid and parathyroid glands1
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    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.

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    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.

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    Figure \(\PageIndex{2}\): (left) Cassava plants are typically grown in tropical and subtropical environments.6 (center) The cassava roots are what are typically eaten, but first they must be peeled. These are unprocessed7 (right) Peeled cassava roots8

    Millet

    Millet is an important crops in the semiarid tropics of Asia and Africa (especially in India, Mali, Nigeria, and Niger).

    clipboard_eb39d0f6cda8412ecc841dcf4a2488bc8.png
    clipboard_e3cf3b8d47afb2665b6f38de61cb5957a.png
    Figure \(\PageIndex{3}\): (left) Millet growing in a field9 (right) Millets10

    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
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    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).

    clipboard_ea50491d09e627b3b8be14619d496ad27.png
    clipboard_e5a96a63b00e8aacbb2d959dd0fbb6021.png
    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.

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    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.

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    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.

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    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 article talks about how salt iodization has lead to IQ gain in the United States.

    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.

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    References

    1. en.Wikipedia.org/wiki/File:Il...arathyroid.jpg
    2. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
    3. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.
    4. Whitney E, Rolfes SR. (2008) Understanding nutrition. Belmont, CA: Thomson Wadsworth.
    5. 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.
    6. en.Wikipedia.org/wiki/File:Casava.jpg
    7. en.Wikipedia.org/wiki/File:Ma...a_dsc07325.jpg
    8. en.Wikipedia.org/wiki/File:PeeledCassava.jpg
    9. en.Wikipedia.org/wiki/Millet...on,_7-3-02.jpg
    10. en.Wikipedia.org/wiki/Staple...harvesting.jpg
    11. en.Wikipedia.org/wiki/File:Tr...othyronine.svg
    12. en.Wikipedia.org/wiki/File:Thyroxine.svg
    13. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
    14. en.Wikipedia.org/wiki/File:Ko...tor_struma.jpg
    15. en.Wikipedia.org/wiki/File:Goitre.jpg
    16. Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
    17. 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: www.ncbi.nlm.nih.gov/books/NBK285556/
    18. http://www.who.int/nutrition/topics/idd/en/

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