10.1 Iodine

Jill Reid, VCU

10.1 Iodine

Iodine

Why is iodine first in this chapter? Not only is it the only non-B vitamin in this chapter, but there is also a connection between selenium (discussed in the previous chapter) and iodine. Iodine‘s only, yet critical, function is that it is required for thyroid hormone synthesis. Figure 10.11

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 10.11 Location of thyroid and parathyroid glands1

Iodine is found in foods primarily in its ionized form, known as iodide (I-). Like selenium, soil concentrations of iodide vary greatly, thus 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 feeds. Cattle also receive iodine-containing medications, and iodide-containing sanitizing solutions are used in dairy facilities, both of which also contribute to the iodine levels in dairy products,4.

Iodine is well absorbed (~90%), and most Americans consume ample iodine through the consumption of iodized salt. Consumption of 1/2 teaspoon of iodized salt meets the RDA for iodine.

The link below is a video that illustrates the reduction in iodine deficiency over the last 2 decades.

Required Web Link:Global Iodine Scorecard

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

Potassium iodide

+ Less expensive

+ Higher iodine content (76% vs. 59% for KIO₃)

+ More soluble

Potassium Iodate

+ More stable

The U.S. uses potassium iodide for supplementation, 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. Some dietary compounds interfere with thyroid hormone production or utilization. These compounds are known as goitrogens due to the increased likelihood of goiter (discussed in Section 10.12) formation5.

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

Cassava

Figure 10.12 Cassava plants are typically grown in tropical and subtropical environments6

Figure 10.13 The cassava roots are what are typically eaten, but first they must be peeled.

Unprocessed cassava is on the left, and peeled cassava root is on the right7, 8

Millet

Figure 10.14 Millet growing in a field9

Figure 10.15 Millets10

Cruciferous Vegetables (broccoli, cabbage, Brussels sprouts)
Onions
Garlic
Soybeans
Peanuts

For more information on iodine, see the Required Web Link below.

Required Web Link:Iodine Fact Sheet for Health Professionals

Subsections:

10.11 Thyroid Hormone
10.12 Iodine Deficiency & Toxicity

References & Links

http://en.wikipedia.org/wiki/File:Illu_thyroid_parathyroid.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.

http://en.wikipedia.org/wiki/File:Casava.jpg
http://en.wikipedia.org/wiki/File:Manihot_esculenta_dsc07325.jpg
http://en.wikipedia.org/wiki/File:PeeledCassava.jpg
https://en.wikipedia.org/wiki/Millet#/media/File:Grain_millet,_early_grain_fill,_Tifton,_7-3- 02.jpg 10.https://en.wikipedia.org/wiki/Staple_food#/media/File:Pearl_millet_after_combine_harvest ing.jpg

Links

Global Iodine Scorecard – http://www.ign.org/scorecard.htm

Iodine Fact Sheet for Health Professionals – https://ods.od.nih.gov/factsheets/Iodine- HealthProfessional/

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.

Required Web LinkVideo: Thyroid (0:37)

As mentioned in the video, the two primary forms of thyroid hormone are triiodothyronine (T₃) and thyroxine (T₄). T₄ is the primary circulating form, and is really a prohormone that is converted to the active T₃ form.

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 10.11 Location and function of the three deiodinases1

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 development1.

References & Links

1. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.

Video

Thyroid – http://www.youtube.com/watch?v=7V0HB4cKIMw

Iodine Deficiency & Toxicity

There are two iodine deficiency disorders (IDD): goiter and cretinism. 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 goiters are shown below.

Figure 10.121 Pictures of women with goiters1,2

A more serious consequence of iodine deficiency occurs during pregnancy to the fetus. Iodine deficiency during this time can lead to the mental and physical retardation known as cretinism. This condition is characterized by severe hypothyroidism, stunted growth, speech loss, and paralysis3,4. The following links show some examples of individuals with cretinism.

Required Web LinkCretinism

The World Health Organization calls iodine deficiency “the world’s most prevalent, yet easily preventable, cause of brain damage5.” By saying it is easily preventable, they are referring to the ability of salt iodization to prevent brain development problems. The New York Times article in the following Required Web Link talks about how salt iodization may be the cheapest way to raise the world’s IQ.

Required Web LinkIn 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 diarrhea6.

References & Links

http://en.wikipedia.org/wiki/File:Kone_med_stor_struma.jpg
http://en.wikipedia.org/wiki/File:Goitre.jpg
Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.
http://www.who.int/nutrition/topics/idd/en/
Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.