Soy and the Thyroid: A Look at the Controversies

Edemame beans
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The issue of whether soy has a negative effect on the thyroid has been a continuing controversy. The potential effects of soy on the thyroid remains a divisive issue and shows no signs of being resolved in the near future.

The Sides of the Debate

On one side, we have health and nutrition magazines touting the benefits of soy as a cure-all for menopause, cancer prevention, heart disease, weight loss, and many other health concerns. And behind the many soy food products and supplements is a multi-billion-dollar industry that makes huge profits from soy. Soy has been a media darling for many years. And rounding out the pro-soy contingent are nutritionists and doctors who believe soy is a wonder food, even for thyroid patients. (Menopause "guru" Christiane Northrup, MD is, for example, a huge proponent of soy. Northrup even recommended that Oprah Winfrey incorporate a great deal of soy into her diet. Coincidentally—or not—both women are now hypothyroid.)

On the other side of the issue are the opponents of soy, who believe that soy is a toxin and endocrine disruptor and can be especially problematic for thyroid health, and thyroid patients. Various experts and organizations, including the Weston Price Foundation, are vocally opposed to soy.

In the center are experts who suggest that some soy—as long as it is in an unprocessed form, fermented forms, and not genetically modified (GMO)—may be safe for thyroid patients, as long as it's eaten only in moderation.

As a thyroid patient, how can you decide what to do? Here is a look at some of the issues to consider.

About Soy

Soy (or soybeans) are a type of legume that have been used for 5,000 years in Asia for food—i.e., tofu, tempeh, miso, and edamame beans—and medicinal purposes. Soybeans are considered a source of protein and are processed into many meat and dairy substitutes. The main producers of soy are the United States, Brazil, Argentina, China, and India.

Soy and many soy products contain isoflavones, which are phytoestrogens—plant-based estrogens. It is soy's weak estrogenic properties that are often touted as a health benefit of soy.

Soy is a highly profitable for some of the world's largest multinational agribusinesses. These include Cargill, Archer Daniels Midland, and Solae (a joint venture of DuPont and Bunge). (These companies are collectively sometimes referred to as "Big Soy.") In the past decade, the market for soy has exploded, and soy is now being incorporated into a variety of processed foods and included in various nutritional supplements.

Does Soy Have Health Benefits?

While soy is enjoying popularity, it's inconclusive whether soy has much in the way to offer, health-wise. A 2005 U.S. government-sponsored review of 200 different studies on soy found very limited evidence of health benefits from soy: primarily a small reduction in "bad" LDL cholesterol, and a small percentage of women who have a minor reduction in hot flashes when using soy during menopause. The Journal of the American Medical Association has reported that isoflavones do not improve cholesterol levels, cognitive function, or bone mineral density. The American Heart Association backtracked on its earlier support of soy and is now saying that there is no evidence that soy has specific benefits for heart health or for lowering cholesterol. Research on the use of soy and isoflavones for cancer prevention is also inconclusive. And there is no evidence that soy can "cause" weight loss, except its role in reducing calories, by replacing fattier, higher-calorie proteins with lower fat, lower-calorie soy.

In general, there is insufficient data to suggest that soy has a protective role against any medical conditions or diseases.

Soy and the Thyroid

Apart from the question as to whether soy even has demonstrable health benefits, there are long-standing concerns that soy may have negative effects on thyroid function and hormonal health. Soy falls into a category of foods known as goitrogens. Goitrogens are a category of foods that include certain vegetables, fruits and that promote the formation of goiter, an enlarged thyroid. Some goitrogens also have a definite antithyroid effect and appear to be able to slow thyroid function, and in some cases, trigger thyroid disease. These concerns have been studied for years but were raised specifically by Food and Drug Administration (FDA) researchers Daniel Doerge and Daniel Sheehan. Doerge and Sheehan were the FDA's key experts on soy. In 2000, Doerge and Sheehan wrote a letter of protest to their own employer, protesting the positive health claims for soy that the FDA was approving at the time. They wrote:

"...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."

After publication of their letter, Doerge and Sheehan refined their concerns, and in the journal Environmental Health Perspectives, suggested that for soy to cause toxicity, there need to be several factors, including iodine deficiency, defects of hormone synthesis, or additional goitrogens in the diet. They also stated that: "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."

Other studies raise concerns about soy's effect on hormones, for example:

  • A 2011 study published in the Journal of Clinical Endocrinology and Metabolism found that in people who have mild or subclinical hypothyroidism, "there is a 3-fold increased risk of developing overt hypothyroidism with dietary supplementation of 16 mg soy phytoestrogens."  
  • 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.

One of America's most well-known holistic doctors, Andrew Weil, MD, while usually a proponent of soy, has some thyroid-related concerns about soy. He has said at his "Ask Dr. Weil" website:

"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'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 the 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. 

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, and eat is as a condiment and not as a primary protein, similar to the Asian diet. 

There are estimates suggesting that Asians consume some 10 to 30 milligrams of isoflavones from soy in a day at most. That soy is typically in traditional food form that is not processed or genetically modified. In the U.S., however, some people take in 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.

Mary Anthony, a pro-soy researcher at Wake Forest University School of Medicine in Winston-Salem, NC, told the Los Angeles Times: "There's a tendency in our culture to think if a little is good, then a lot's better. I personally am very concerned about isoflavone pills and soy protein supplemented with extra isoflavones. Isoflavones, after all, seem to act like hormones or drugs in our body—even if for regulatory purposes they are classified as nutritional supplements."

The issue of genetically modified soy is also controversial, as the corporations that are farming soy claim that genetically modified organisms (GMO) in foods, including soy, are safe. At the same time, some nations in Europe are banning or severely restricting use of GMO foods, due to concerns about the potential effects GMO foods may have on health, including causing allergic responses, contributing to antibiotic resistance, producing new toxins, concentrating toxic metals, enhancing the growth of toxic fungi, and molecular or DNA damage. In the U.S., various experts and organizations, including consumer watchdog Public Citizen, holistic physician Dr. Joseph Mercola, and environmental group Greenpeace, among others, have serious concerns about GMO foods, including soy. Activist and author Jeffrey K. Smith's bestselling book "Seeds of Deception" chronicles many of the scientific concerns about GMO foods and pushback from industry.

Is Soy Actually Safe for the Thyroid?

On the other side of the controversy are those who wholeheartedly support soy. Proponents of soy point to a study, frequently touted as evidence of soy's safety for the thyroid, which was published in 2006 in the journal Thyroid. The researchers looked at 14 trials involving soy, and in 13 out of 14 trials, either no effects or modest changes were noted in thyroid function as a result of soy consumption. The researchers claim that the findings provide little evidence that "in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function."

The researchers also stated that: 

There remains a theoretical concern based on in vitro and animal data that in individuals with compromised thyroid function and/or whose iodine intake is marginal soy foods may increase risk of developing clinical hypothyroidism. Therefore, it is important for soy food consumers to make sure their intake of iodine is adequate." They also claim that "some evidence suggests that soy foods, by inhibiting absorption, may increase the dose of thyroid hormone required by hypothyroid patients."

This study is suggesting that soy is safe unless you have a thyroid condition or iodine deficiency. It also suggests that soy foods can inhibit absorption of thyroid medication.

The study also goes on to say that despite these factors, soy foods are in fact safe, and all that is needed is to ensure sufficient iodine in the diet along with regular retesting and dosage changes of thyroid medication to make up for any effect the soy has on the thyroid medication.

The study doesn't address the fact that it's estimated that as much as one-fourth of the U.S. population is now iodine deficient and that the number is on the rise. At the same time, many millions of Americans also have undiagnosed autoimmune thyroid disease. At minimum, if you accept the premise of this study, that means that more than millions of Americans with iodine deficiency may be at risk of thyroid problems from soy consumption. 

It's also troubling to note that the author of this study, along with other studies claiming soy is not a danger to the thyroid, is Mark Messina, PhD. Messina, though not a medical doctor, also goes by the name "Dr. Soy." Messina had been in charge of grant funding at the National Institutes of Health (NIH), where he oversaw a $3 million grant for soy studies. Soon after he left NIH, he was hired to serve on the scientific advisory boards of both the United Soybean Board and international soy agribusiness Archer Daniels Midland. He still serves on both scientific advisory boards as a paid advisor. In addition to his work on these advisory boards, Messina has served as paid consultant and speaker for the United Soybean Board, and edited its soy-related newsletter. Messina has also published a number of books promoting soy. Numerous sources have documented the close relationship between Messina and the various corporate players in the soy industry.

So, is the study accurate? Honestly, it's impossible to say at this point. There is a clear ethical and financial conflict of interest in commissioning research on soy's safety from someone who is a longstanding representative of, and who is lucratively employed by, the soy industry itself.

Hopefully, more studies will be done by researchers who do not have any ties to the industry, or who do not have a vested interest in presenting a rosy picture of soy vis a vis thyroid concerns.

Who Should Thyroid Patients Believe? What Should Thyroid Patients Do?

Until we have the sort of definitive, rigorous, high-quality experimental and human studies into soy toxicity that soy experts Daniel Doerge and Daniel Sheehan called for, it's not advisable to assume that soy is universally safe for thyroid patients. It's also clear that soy does have the potential to cause thyroid problems in a segment of the population that is susceptible, due to iodine deficiency or other conditions.

If you feel it's necessary to include soy in your diet, here are some guidelines.

  • Be sure that you are not iodine deficient. This is tricky, however, because the only way to really determine if you are deficient in iodine is to have a urinary iodine clearance test. (The process of how to assess iodine deficiency is described well in Dr. David Brownstein's book, Iodine: Why You Need It, Why You Can't Live Without It). Be careful not to just decide to supplement with iodine without clearly establishing that you are deficient, however, because just as iodine deficiency can trigger a thyroid problem, excess iodine can also aggravate and worsen thyroid problems.
  • If you have elevated thyroid antibodies or autoimmune thyroid disease that is not being treated, be aware that soy can be a trigger for developing hypothyroidism.
  • If you are a thyroid patient with optimized thyroid treatment, and you're still suffering from hypothyroidism symptoms, consider eliminating soy from your diet to see if that helps relieve symptoms.
  • If you are eating soy foods, you may want to avoid GMO soy foods until the debate over their safety has been resolved.
  • If you are going to eat soy, select fermented and food forms of soy, for example, tofu, tempeh, and miso. Avoid processed soy products, such as soy powders, protein shakes, and other processed forms of soy.
  • Limit soy and isoflavone consumption to less than 30 mg per day, at most. Ideally, limit soy foods to several servings per week.
  • Do not eat soy foods within three to four hours of taking your thyroid hormone replacement medication, to avoid any interference with your thyroid medication.
  • Do not use soy or isoflavone supplements.
  • Be careful about the overall quantity of goitrogenic foods that you consume raw, especially if they are in addition to soy foods, which are known goitrogens.

Keep in mind that soy is one of the most common allergy-triggering foods. Even if soy is not affecting your thyroid specifically, it can trigger allergy symptoms, including acne, swelling, a stuffy nose, diarrhea, stomach pains, heart palpitations, skin rashes, itching, hives, swelling in the throat, fatigue, and episodes of low blood pressure.

Also, remember that if you do not have a thyroid gland (due to congenital hypothyroidism or surgery) or you have a totally non-functioning gland (due to radioactive iodine ablation treatment), you don't need to be concerned about the effects of soy on your thyroid gland. Soy can, however, still interfere with absorption of your thyroid hormone replacement medication, so be sure to take your medication at least three hours apart from soy foods.

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