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New Findings on the Soy/Thyroid Connection
Soy Expert Daniel Doerge Revisits the Issue

from Mary Shomon

June 2002 -- Dr. Daniel Doerge of the Division of Biochemical Toxicology at the National Center for Toxicological Research, who is one of the nation’s top soy researchers, has published an article in the June 2002 issue of Environmental Health Perspectives, looking at the effect of soy on the thyroid.

In a February 18, 1999 official letter of protest to the FDA, Doerge and Daniel Sheehan, who at that time were the Food and Drug Administration's (FDA) two key experts on soy, protested the health claims approved by the FDA on soy products, saying:

"there is abundant evidence that some of the isoflavones found in soy, including genistein and equol, a metabolize of daidzen, demonstrate toxicity in estrogen sensitive tissues and in the thyroid. This is true for a number of species, including humans. Additionally, isoflavones are inhibitors of the thyroid peroxidase which makes T3 and T4. Inhibition can be expected to generate thyroid abnormalities, including goiter and autoimmune thyroiditis. There exists a significant body of animal data that demonstrates goitrogenic and even carcinogenic effects of soy products. Moreover, there are significant reports of goitrogenic effects from soy consumption in human infants and adults."
In this new research, Doerge has looked at the goitrogenic and estrogenic effects of soy in greater depth. According to Doerge, soy is known to produce estrogenic isoflavones. Genistein, which is the major soy isoflavone, also has a an estrogenic effect in women. Research has already shown that soy consumption is linked to increased risk of goiter. When iodine is deficient, the antithyroid effects of soy are intensified. Soy’s ability to affect the thyroid, therefore, depends on the relationship between iodine status and thyroid function. In animal studies, rats given a genistein-fortified diets showed an increase in thyroid antibodies, while other measures of thyroid function apparently remained normal.

These findings have led Dr. Doerge to conclude that additional factors appear necessary for soy to cause overt thyroid toxicity. These factors include:
  • iodine deficiency
  • consumption of other soy components
  • other goitrogens in the diet
  • other physiological problems in synthesizing thyroid hormones.
According to Dr. Doerge, more needs to be known. Says the research findings: “Although safety testing of natural products, including soy products, is not required, the possibility that widely consumed soy products may cause harm in the human population via either or both estrogenic and goitrogenic activities is of concern. Rigorous, high-quality experimental and human research into soy toxicity is the best way to address these concerns.”

Source: Doerge, DR. “Goitrogenic and estrogenic activity of soy isoflavones,” Environ Health Perspect 2002 Jun;110 Suppl 3:349-53

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