- One study found that children with autoimmune thyroid disease are more likely to have been fed soy-based infant formula.
- A 1991 Japanese study found that soy consumption can suppress thyroid function and cause goiters in healthy people, especially elderly subjects.
- Czech researchers in 2006 reported on a study that looked at thyroid hormones and thyroid autoantibodies, along with blood levels of two isoflavones -- daidzein and genistein. The study looked at children without overt thyroid disease, who were not iodine deficient. They found a "significant positive association of genistein with thyroglobulin autoantibodies and a negative correlation with thyroid volume." They concluded that "even small differences in soy phytoestrogen intake may influence thyroid function, which could be important when iodine intake is insufficient."
- In 2004, researchers found that infants fed soy formula had a prolonged increase in their thyroid stimulating hormone (TSH) levels, compared to infants fed non-soy formula.
- European researchers found in one study that even a week of consuming unprocessed boiled natural soybeans caused modest changes to thyroid levels.
- A 1997 study published in the journal Biochemical Pharmacology wrote: "it was observed that an...extract of soybeans contains compounds that inhibit thyroid peroxidase- (TPO) catalyzed reactions essential to thyroid hormone synthesis."
Excess consumption of soy can affect thyroid function, if you have a thyroid disorder to begin with or if you're not getting enough iodine in your diet...you're unlikely to get too many isoflavones as a result of adding soy foods to your diet -- but you probably will take in too much if you take soy supplements in pill form. At this point, I can only recommend that you avoid soy supplements entirely.In my book Living Well With Hypothyroidism, Dr. Mike Fitzpatrick, an internationally known expert on soy, was profiled. Dr. Fitzpatrick is an environmental scientist and phytoestrogen researcher who has extensively researched the issue of soy formulas, and the impact of soy consumption on thyroid function. I wrote:
Dr. Fitzpatrick is so concerned that he is calling for soy formula manufacturers to remove the isoflavones -- the agents that are most active against the thyroid -- from their products. .. There are also concerns for adult consumption of soy products. One UK study involving premenopausal women gave 60 grams of soy protein per day for one month. This was found to disrupt the menstrual cycle, with the effects of the isoflavones continuing for a full three months after stopping the soy in the diet. Another study found that intake of soy over a long period causes enlargement of the thyroid and suppresses thyroid function. Isoflavones are also known to modify fertility and change sex hormone status, and to have serious health effects -- including infertility, thyroid disease or liver disease -- on a number of mammals… Dr. Fitzpatrick believes that people with hypothyroidism should seriously consider avoiding soy products, and predicts the current promotion of soy as a health food will result in an increase in thyroid disorders."While the U.S. has stayed out of the fray over soy, other countries have taken action to limit the possible dangers of soy. The French Center for Cancer Research put out a warning saying that soy products -- in any amount -- should not be eaten by children under 3 years of age or women who have breast cancer or are at risk of the disease. The Israeli Health Ministry has also issued a public warning on soy, suggesting that soy consumption be limited in young children and avoided if possible in infants. In Germany, the Federal Institute for Risk Assessment is doing a study of isoflavone supplements, and has reported that there's a lack of evidence to confirm the safety of such supplements, and some evidence to suggest that there may be health risks. (New Zealand's Soy Online Service is an excellent resource with information on studies and findings relating to soy's health effects, as well as recent developments in legislation to control or limit soy around the world.)
Is Overconsumption of Soy the Main Concern?Some experts suggest that soy itself is not inherently a problem, but it's primarily overconsumption -- and secondarily, the issue of genetic modification -- that are the concerns. They argue that soy that is not genetically modified, and consumed in food forms -- like tofu, tempeh and miso -- can be safely incorporated into the diet when used in moderation -- like the Asians do -- as a condiment, and not as a primary protein source.
There are estimates suggesting that Asians consume some 10 to 30 milligrams of isoflavones from soy a day at most -- and it's soy in traditional food form that is not processed or genetically modified. In the U.S., however, some people are getting as much as 80 to 100 milligrams of soy isoflavones a day, by consuming soy milk, soy nuts, soy protein shakes, soy candy bars, soy cereal, and foods enriched with soy, as well as soy supplements. Some soy and isoflavone supplements have as much as 300 milligrams of isoflavones. Isoflavones are also increasingly being added as a so-called "healthy" component of foods and other supplements.
Kaayla Daniels, Ph.D., author of The Whole Soy Story, suggests that the thyroid-toxic effects of soy are most often seen at levels above 30 mg of soy per day.