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The Iodine Controversy

Too Much vs. Not Enough, and What It Does To Your Thyroid?


Updated June 02, 2014

Pregnant woman holding stomach
Sam Edwards/OJO Images/Getty Images
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.

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