Iodine Intake and Effect on Thyroid Problems

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The New England Journal of Medicine published research from China that looks at the relationship between iodine intake and thyroid disease. Reporting in the June of 2006 issue, the researchers concluded that "more than adequate or excessive iodine intake may lead to hypothyroidism and autoimmune thyroiditis."

These findings were accompanied by an editorial by Dr. Robert Utiger, who said that "the small risks of chronic iodine excess are outweighed by the substantial hazards of iodine deficiency," adding to the controversy over iodine, in particular, the iodization of salt, and iodine supplementation,

Iodine Deficiency

On one side of the controversy is the serious issue of iodine deficiency. Iodine is obtained through food, water, iodized salt, and supplementation. The thyroid uses iodine to produce ​thyroid hormone, making iodine an essential, necessary nutrient.

Iodine is particularly critical for pregnant women and fetuses, as well as young children. According to the World Health Organization (WHO), an estimated 2 billion people, including 285 million school-age children, are iodine deficient. And among them, iodine deficiency disorders (IDD) affect some 740 million -- with almost 50 million of them suffer from some form of brain damage resulting from the iodine deficiency.

According to the International Council for the Control of Iodine Deficiency Disorders (INCCIDD):

Iodine deficiency is the single most common cause of preventable mental retardation and brain damage in the world. It also decreases child survival, causes goiters, and impairs growth and development. Iodine deficiency disorders in pregnant women causes miscarriages, stillbirths, and other complications. Children with IDD can grow up stunted, apathetic, mentally retarded, and incapable of normal movements, speech, or hearing.

There are many areas of the world where iodine deficiency is a severe health crisis. The INCCIDD has a map online that shows iodine nutrition worldwide. This map reveals that while most areas of the Western Hemisphere tend to be sufficient in iodine, Europe, Russia, Asia, Australia and Africa, have areas at varying risk of deficiency.

Iodine Excess

On the other side of the controversy is the recognition that excessive iodine can trigger autoimmune thyroid disease and hypothyroidism.

According to animal studies, high iodine intake can initiate and worsen infiltration of the thyroid by lymphocytes. Lymphocytes are white blood cells that accumulate due to chronic injury or irritation. In addition, large amounts of iodine block the thyroid's ability to make hormone.

In this New England Journal study, researchers, led by Dr. Weiping Teng, of China Medical University in Shenyang, looked at the thyroid effects of giving supplemental to three separate groups: people who were mildly iodine-deficient, those with adequate iodine intake, and those with excessive iodine intake. They found that giving iodine to people who had adequate or excessive iodine intake could lead to hypothyroidism autoimmune thyroiditis.

In the study, researchers found that the primary risk factors for continuing subclinical hypothyroidism included:

  • TSH above 6
  • High levels of antithyroid antibody (thyroid peroxidase antibody or thyroglobulin antibody)
  • A shift in iodine intake from mildly deficient to more than adequate

They also found that the key risk factors for new subclinical hypothyroidism in people who started with normal thyroid function included:

  • TSH level greater than 2
  • High antithyroid antibody levels
  • A shift in iodine intake from mildly deficient to more than adequate, or excessive iodine intake

The researchers concluded:

...although iodine supplementation should be implemented to prevent and treat iodine-deficiency disorders, supplementation should be maintained at a safe level. Levels that are more than adequate (median urinary iodine excretion, 200 to 299 µg per liter) or excessive (median urinary iodine excretion, >300 µg per liter) do not appear to be safe, especially for susceptible populations with either potential autoimmune thyroid diseases or iodine deficiency. Supplementation programs should be tailored to the particular region. No iodine supplementation should be provided for regions in which iodine intake is sufficient, whereas salt in regions in which iodine intake is deficient should be supplemented with iodine according to the degree of iodine deficiency.

An Important Note: The researchers were defining overt hypothyroidism as a TSH above 4.8, with elevated Free T4 levels. Subclinical hypothyroidism was defined as a TSH above 4.8, with normal range Free T4 levels. The American Association of Clinical Endocrinologists, along with the National Academy of Clinical Biochemistry, recommended in late 2002/early 2003 that the TSH normal range is narrowed substantially to .3 to 3.0. So the point at which someone would be considered overtly hypothyroid might be different, based on these new guidelines.

Do You Need Iodine?

Many decades ago, iodization of salt was voluntarily instituted in the U.S. and other industrialized countries as a means to counteract iodine deficiency. In these areas with iodized salt, iodine deficiency disorders were all but eliminated, and most Americans do have sufficient iodine.

During the past two decades, however, reductions in salt intake for health reasons, reduced use of iodized salt in processed foods, and the fact that iodization is not mandatory in the U.S. (even then, some 70% of table salt is iodized) have resulted in a cutback in iodine intake even in countries like the U.S. So after a period where iodine deficiency in the U.S. had been all but eliminated, it is now on a slow by steady upward rise.

The greatest concern is in pregnant women. In fact, the rate of pregnant women with iodine deficiency has increased in the U.S. over the past 20 years Utiger, from just 1 percent in the 1970s to 7 percent in 2002. These women and their babies face the greatest risks from insufficient iodine in their diet.

Some experts recommend that iodine supplementation be standard during pre-conception and pregnancy. The recommended dietary allowance for iodine is 200 mcg/day during pregnancy and 75 mcg/day while breastfeeding.

For the rest of us, the answer for optimum thyroid health is, therefore, to get enough -- but not too much -- iodine. You might be deficient in iodine if you have, for health reasons, cut iodized salt out of your diet, or switched to non-iodized sea salt.

So, do you need supplemental iodine? How can you tell for sure if you are getting enough iodine? It's almost impossible to gauge on your own. You can do a very rough estimate, however, based on the following questions:

  • Do you use iodized salt?
  • How much salt do you eat daily?
  • Do you take a vitamin or supplement with iodine? (How much iodine is in the supplement?)
  • Do you eat, meat, dairy products or seafood regularly?

Some alternative, holistic and herbal practitioners are almost knee-jerk in their insistence that anyone with a thyroid problem requires iodine supplementation (either liquid iodine or an herb that contains iodine, such as kelp or bladderwrack). This can aggravate symptoms and worse thyroid problems in some people.

But, unless you are planning to get pregnant, are currently pregnant or you're breastfeeding, you'll want to be very careful about taking iodine unless you and your practitioner have some very strong evidence that you are deficient. If your practitioner recommends iodine supplementation as a thyroid treatment, you may wish to ask for a more specific test that can measure iodine levels -- the "urinary excretion" test. This test which evaluates the iodine excreted in the urine, and gives an indirect but fairly accurate assessment of iodine levels, and can document deficiency.

Also, watch out for the so-called "thyroid support" vitamin and supplement formulas, including the heavily marketed and promoted Alvidar. Most, like Alvidar, include substantial amounts of iodine, and if you are not iodine-deficient, they can end up having the unintended and opposite effect of actually making your symptoms worse and aggravating your thyroid condition.


Teng, Weiping M.D., et. al. "Effect of Iodine Intake on Thyroid Diseases in China" New England Journal of Medicine, Volume 354:2783-2793, June 29, 2006, Number 26 Abstract

Utiger, Robert D. M.D. "Iodine Nutrition - More Is Better," New England Journal of Medicine, Volume 354:2819-2821, June 29, 2006, Number 26

Higdon, Jane Ph.D. et. al. "Iodine," Micronutrient Information Center, Linus Pauling Insitute, Oregon State University, 2003 Article

International Council for the Control of Iodine Deficiency Disorders

Shomon, Mary J. The Thyroid Guide to Fertility, Pregnancy and Breastfeeding Success, Thyroid-Info, 2006

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