iodine

The World’s Healthiest Foods

What can high-iodine foods do for you?

• Help ensure proper thyroid gland functioning

What events can indicate a need for more high-iodine foods?  

• Goiter (enlargement of the thyroid gland)
• Fatigue
• Weakness
• Depression
• Weight gain  

Concentrated food sources of iodine include sea vegetables, yogurt, cow’s milk, eggs, strawberries and mozzarella cheese. Fish and shellfish can also be concentrated sources of iodine.

For serving size for specific foods see the Nutrient Rating Chart.

Description Function Deficiency Symptoms Toxicity Symptoms Cooking, storage and processing Factors that affect function

Nutrient interaction Health conditions Food Sources Public Health Recommendations References

Description

What is Iodine?

If you backpack in the mountains, you may have used iodine tablets to purify your drinking water. Or, perhaps you’ve used an iodine-based disinfectant to clean a minor skin wound. But did you know that iodine is essential to life?

Iodine, a trace mineral, is required by the body for the synthesis of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). (T4 contains 4 iodine atoms. When one of the iodine atoms is stripped off of T4, it becomes T3, with 3 iodine atoms remaining.)

Under normal circumstances, your body contains approximately 20 to 30 mg of iodine, most of which is stored in your thyroid gland, located in the front of your neck, just under your voice box. Smaller amounts of iodine are also found in lactating mammary glands, the stomach lining, salivary glands, and in the blood.

How it Functions

What is the function of iodine?

As a component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), iodine is essential to human life. Without sufficient iodine, your body is unable to synthesize these hormones, and because the thyroid hormones regulate metabolism in every cell of the body and play a role in virtually all physiological functions, an iodine deficiency can have a devastating impact on your health and well-being.

Regulating thyroid hormones

The synthesis of thyroid hormones is tightly controlled. When the amount of thyroid hormone in your blood drops, the pituitary gland secretes a hormone called thyroid-stimulating hormone (TSH). As its name suggests, TSH then stimulates the thyroid gland to increase its uptake of iodine from the blood, so that more thyroxine (T4) can be synthesized. When necessary, thyroxine is then converted to the metabolically active triiodothyronine (T3), a process that involves removing one iodine atom from T4.

Several other physiological functions for iodine have been suggested. Iodine may help inactivate bacteria, hence its use as a skin disinfectant and in water purification. Iodine may also play a role in the prevention of fibrocystic breast disease, a condition characterized by painful swelling in the breasts, by modulating the effect of the hormone estrogen on breast tissue. Finally, researchers hypothesize that iodine deficiency impairs the function of the immune system and that adequate iodine is necessary to prevent miscarriages.

Deficiency Symptoms

What are deficiency symptoms for iodine?

In the early part of the 20th century, iodine deficiency was quite common in the United States and Canada. However, this problem has since been almost completely resolved by the use of iodized salt. In addition, iodine is now added to animal feed, which has increased the iodine content of commonly consumed foods, including cow’s milk.

Unfortunately, in countries where iodized salt is not commonly consumed, iodine deficiency remains a signficant problem. Dietary deficiency of this vital mineral results in decreased synthesis of thyroid hormone.

Goiter, or enlargement of the thyroid gland, is usually the earliest visible symptom of iodine deficiency. (Goiter can occur for many other reasons as well, but iodine deficiency is among the most common causes worldwide.) The enlargement of the thyroid results from overstimulation of the thyroid gland by thyroid stimulating hormone (TSH), as the body attempts to produce increased amounts of thyroid hormone.

Goiter is more common in certain geographical areas of the world where iodine is lacking in the diet and where selenium is lacking in the soil. (Selenium is directly involved with certain activities of the thyroid gland.)

Iodine deficiency may eventually lead to hypothyroidism, which causes a variety of symptoms including fatigue, weight gain, weakness and/or depression. Interestingly, iodine deficiency can also cause hyperthyroidism, a condition characterized by weight loss, rapid heart beat, and appetite fluctations.

Severe iodine deficiency during pregnancy or infancy causes cretinism, a condition characterized by hypothyroidism leading to failure of the thyroid gland and/or severe mental retardation, stunted physical growth, deafness, and spasticity. If discovered in its initial stages, cretinism can be corrected with iodine supplementation.

Toxicity Symptoms

What are toxicity symptoms for iodine?

Accidental overdose of iodine from medications or supplements in amounts exceeding one gram may cause burning in the mouth, throat and stomach and/or abdominal pain, nausea, vomiting, dirarrhea, weak pulse, and coma.

It is difficult to take in too much iodine from food sources alone. It is estimated that men and women consume at most 300 mcg and 210 mcg of iodine per day, respectively. In general, even high intakes of iodine from food are well-tolerated by most people.

However, in certain circumstances, excessive consumption of iodine can actually inhibit the synthesis of thyroid hormones, thereby leading to the development of goiter (enlargement of the thyroid gland) and hypothyroidism. Excessive iodine intake may also cause hyperthyroidism, thyroid papillary cancer, and/or iodermia (a serious skin reaction).

In an attempt to prevent these symptoms of iodine toxicity, the Institute of Medicine established the following Tolerable Upper Intake Levels (TUL) for iodine:

• 1-3 years: 200 mcg
• 4-8 years: 300 mcg
• 9-13 years: 600 mcg
• 14-18 years: 900 mcg
• 19 years and older: 1,100 mcg
• Pregnant women 14-18 years: 900 mcg
• Pregnant women 19 years and older: 1,100 mcg
• Lactating women 14-18 years: 900 mcg
• Lactating women 19 years and older: 1,100 mcg

It is important to note that if you have an autoimmune thyroid disease (for example, Grave’s disease or Hashimoto’s disease) or if you have experienced an iodine deficiency at some point in your life, you may be more susceptible to the dangers of excessive iodine consumption, and may, therefore, need to monitor your intake of iodine more carefully.

Impact of Cooking, Storage and Processing

How do cooking, storage, or processing affect iodine?

Food processing practices often increase the amount of iodine in foods. For example, the addition of potassium iodide to table salt to produce “iodized” salt has dramatically increased the iodine intake of people in developed countries. In addition, iodine-based dough conditioners are commonly used in commercial bread-making, which increases the iodine content of the bread.

Factors that Affect Function

What factors might contribute to a deficiency of iodine?

Since absorption of iodine from the digestive tract is very thorough, deficiency of iodine typically occurs from too little intake of iodine-containing foods. However, there has been some controversy surrounding the impact of diet not on iodine absorption, but on iodine utilization by the thyroid gland. Since the thyroid requires iodine to make thyroid hormones, this utilization is essential. Two groups of substances found in food – isoflavones, most commonly found in soy foods, and thiocyanates, most commonly produced in the body from glucosinolates found in cruciferous vegetables like broccoli – have been shown to interfere with iodine utilization by the thyroid gland, but only under very specific circumstances. These circumstances involve simultaneous dietary deficiency of iodine or selenium (or both) and imbalanced overall dietary intake. We’re not aware of any evidence showing problems with iodine metabolism by the thyroid gland when either soy foods or cruciferous vegetables are eaten in moderate amounts in an overall balanced diet that also contains appropriate amounts of iodine and selenium. Since soy foods and cruciferous vegetables provide so many well-documented health benefits, we do not believe there is ordinarily any reason to eliminate these foods from the diet for iodine-related reasons. However, for individuals with a history of thyroid problems, poor dietary balance and deficient intake of iodine and/or selenium, we recommend consultation with a healthcare provider to decide about the role of these foods in health support.

Nutrient Interactions

How do other nutrients interact with iodine?

The conversion of thyroxine (T4) to triiodthyronine (T3) requires the removal of an iodine molecule from T4. This reaction requires the mineral selenium. The iodine molecule that is removed gets returned to the body’s pool of iodine and can be reused to make additional thyroid hormones.

If your body is deficient in selenium, the conversion of T4 to T3 is slowed, and less iodine is available for the thryoid to use in making new hormones.

Animal studies have shown that arsenic interferes with the uptake of iodine by the thyroid, leading to goiter. In addition, dietary deficiency of vitamin A, vitamin E, zinc and/or iron can exaggerate the effects of iodine deficiency.

Health Conditions

What health conditions require special emphasis on iodine?

Iodine may play a role in the prevention and/or treatment of the following health conditions:

• Cognitive impairment
• Cretinism
• Fibrocystic breast disease
• Goiter
• Hyperthyroidism
• Hypothyroidism
• Multiple miscarriages

Food Sources

What foods provide iodine?

The amount of iodine found in most natural foods is typically quite small and varies depending on environmental factors such as the soil concentration of iodine and the use of fertilizers. Some of the richest food sources of iodine are often processed foods that contain iodized salt, and breads that contain iodate dough conditioners.

Sea vegetables are an excellent source of iodine. Yogurt, cow’s milk, eggs, and strawberries are very good sources of idone. Good sources include mozzarella cheese.

Fish and shellfish require their own special category when it comes to iodine content, because the amount of iodine they contain is not always easy to predict. For example, the amount of iodine found in fish may not match up very predictably with the amount found in their home waters, or even with their diet. Four ounces of very low iodine fish might only provide about 70 micrograms of iodine, or less than half of the adult RDA. By contrast, four ounces of very high iodine fish might contain as much as 1,000 micrograms of iodine—an amount just below the Tolerable Upper Limit (UL, or safety level) of 1,100 micrograms.

Due to the great variability in fish iodine content, and the relative lack of good information for consumers to base their nutritional decisions in this area, we would not recommend reliance on fish alone to provide all of your dietary iodine needs. However, you can count on getting iodine from most fish, and on any one particular day, we would recommend thinking about a 4-6 ounce fish meal serving as providing at least 50% of your iodine needs. Conversely, if you are trying to greatly restrict your iodine intake, you might want to eat fish on a less frequent basis to lower your risk of iodine intake above the RDA level.

Nutrient Rating Chart

Introduction to Nutrient Rating System Chart

In order to better help you identify foods that feature a high concentration of nutrients for the calories they contain, we created a Food Rating System. This system allows us to highlight the foods that are especially rich in particular nutrients. The following chart shows the World’s Healthiest Foods that are either an excellent, very good, or good source of iodine. Next to each food name, you’ll find the serving size we used to calculate the food’s nutrient composition, the calories contained in the serving, the amount of iodine contained in one serving size of the food, the percent Daily Value (DV%) that this amount represents, the nutrient density that we calculated for this food and nutrient, and the rating we established in our rating system. For most of our nutrient ratings, we adopted the government standards for food labeling that are found in the U.S. Food and Drug Administration’s “Reference Values for Nutrition Labeling.” Read more background information and details of our rating system.

Public Health Recommendations

What are current public health recommendations for iodine?

In 2000, the Institute of Medicine at the National Academy of Sciences developed new Dietary Reference Intakes for iodine. Adequate Intakes were established for children up to one year old, and Recommended Dietary Allowances were determined for all people over one year old. These recommendations appear below:

• 0-6 months: 110 mcg
• 7-12 months: 130 mcg
• 1-8 years: 90 mcg
• Boys 9-13 years: 120 mcg
• Girls 9-13 years: 120 mcg
• Boys 14-18 years: 150 mcg
• Girls 14-18 years: 150 mcg
• Men 19 years and older: 150 mcg
• Women 19 years and older: 150 mcg
• Pregnant women 14 years and older: 220 mcg
• Lactating women 14 years and older: 290 mcg

In an attempt to prevent these symptoms of iodine toxicity, the Institute of Medicine established the following Tolerable Upper Intake Levels (UL) for iodine:

• 1-3 years: 900 mcg
• 4-8 years: 300 mcg
• 9-13 years: 600 mcg
• 14-18 years: 900 mcg
• 19 years and older: 1,100 mcg
• Pregnant women 14-18 years: 900 mcg
• Pregnant women 19 years and older: 1,100 mcg
• Lactating women 14-18 years: 900 mcg
• Lactating women 19 years and older: 1,100 mcg

For more details on this, see the Toxicity Symptoms section above.

References

• Delange F. The role of iodine in brain development. Proc Nutr Soc 2000 Feb;59(1):75-9. 2000. PMID:15560.
• Dunn JT, Dunn AD. Update on intrathyroidal iodine metabolism. Thyroid 2001 May;11(5):407-14. 2001. PMID:15530.
• Feldt-Rasmussen U. Iodine and cancer. Thyroid 2001 May;11(5):483-6. 2001. PMID:15510.
• Groff JL, Gropper SS, Hunt SM. Advanced Nutrition and Human Metabolism. West Publishing Company, New York, 1995. 1995.
• Institute of Medicine. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academy Press: Washington DC, 2001. 2001.
• Lininger SW, et al. A-Z guide to drug-herb-vitamin interactions. Prima Health, Rocklin, CA, 2000. 2000.
• Mahan K, Escott-Stump S. Krause’s Food, Nutrition, and Diet Therapy. WB Saunders Company; Philadelphia, 1996. 1996.
• Rasmussen LB, Ovesen L, Bulow I et al. Relations between various measures of iodine intake and thyroid volume, thyroid nodularity, and serum thyroglobulin. Am J Clin Nutr 2002 Nov;76(5):1069-76. 2002.
• Roti E, Uberti ED. Iodine excess and hyperthyroidism. Thyroid 2001 May;11(5):493-500. 2001. PMID:15500.
• Ruwhof C, Drexhage HA. Iodine and thyroid autoimmune disease in animal models. Thyroid 2001 May;11(5):427-36. 2001. PMID:15520.
• Spitzweg C, Heufelder AE, Morris JC. Thyroid iodine transport. Thyroid 2000 Apr;10(4):321-30. 2000. PMID:15570.
• Venturi S, Donati FM, Venturi A, et al. Role of iodine in evolution and carcinogenesis of thyroid, breast and stomach. Adv Clin Path 2000 Jan;4(1):11-7. 2000. PMID:15550.

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