Nutrition

The Truth About Soy + Hypothyroidism

December 12, 2017  |  By: Whitney E. RD

Welcome to the third and final post in our series on SOY!

Oh, poor, misunderstood soy, will it ever get its rightful respect?

As we saw in the last two posts, there is a mountain of research supporting soy’s benefit for cancer prevention and treatment and no good research suggesting that it is harmful to male and female fertility, PCOS, or acne.

However, our next topic is a little dicier. Today we’re talking about soy and thyroid health, and specifically, whether soy causes hypothyroidism. 

Soy and Thyroid Health

This is one area that could certainly use more research.

In mice, soy has shown to deactivate an enzyme involved in the production of thyroid hormones. Surprisingly though, this doesn’t lead to a decrease in thyroid hormone production or result in hypothyroidism.

Other studies in mice that have shown that a decrease in thyroid hormones with soy intake only occurs when the mice also have an iodine deficiency.

Similar results were shown in old studies of infants receiving soy formula.

These studies initially sparked concern when they showed an increase in the incidence of goiter, which is an enlargement of the thyroid gland.

However, more recent studies show that this only occurs when the infants are also iodine deficient. With supplementation of iodine, the condition is reversed.

SOY and Hypothyroidism

In humans, the evidence on soy intake and thyroid health is mixed and appears to vary based on person’s current thyroid health status and iodine intake.

One study of middle-aged females with subclinical hypothyroidism showed that supplementation of a mixture of soy protein and high-dose isoflavones increased the risk of progression to overt hypothyroidism compared to low-dose isoflavone supplementation.

Six of the sixty patients in the study progressed to overt hypothyroidism over the eight-week trial. However, at the same time, these patients saw decreases in blood pressure and CRP (a marker of inflammation) as well as improvements in insulin resistance.

On average, studies show that 5.6% of patients with subclinical hypothyroidism will progress to overt hypothyroid anyway and the researchers in this study suggested that perhaps the high dose supplementation may have accelerated an underlying autoimmune condition.

Other studies in women with normal thyroid function have not shown detrimental effects on hormone levels.

One randomized control trial of postmenopausal women with normal thyroid function found that supplementation with soy protein actually increased levels of thyroid hormones.

Another 3-year randomized controlled trial of isoflavone supplementation in postmenopausal women also showed no effect on thyroid hormone levels or production of thyroid autoantibodies, which are produced in autoimmune thyroid disease.

Meanwhile, many epidemiological studies have failed to show an increase in hypothyroidism in vegans, a population with a high soy intake. One study showed that a vegan diet was actually associated with a reduced risk of hypothyroidism, although the result was insignificant.

At the very least, this suggests that this eating pattern is not harmful to people with normal thyroid health. 

Lastly, soy inhibits absorption of the drug used to treat hypothyroidism, levothyroxine. However, coffee, fiber, and many other foods also inhibit absorption of this drug, which is why we recommend patients take it on an empty stomach.

In summary:

1. Soy has not shown to be harmful to people with normal thyroid functioning as long as they also are consuming adequate amounts of the essential micronutrient iodine.
2. Soy is also likely not harmful to patients with hypothyroid as long as it is not consumed at the same time as their medication.
3. People with subclinical hypothyroidism may want to watch their soy intake, as some research shows it could trigger the progression to full-blown hypothyroidism. However, this may only occur for some people and it may occur either way.

And that concludes our investigation of soy and various health claims. We now know that soy has shown to be safe and in most cases beneficial for a variety of health concerns including breast and prostate cancer, male and female fertility, PCOS, acne, and thyroid conditions.

But there’s still one big thing people are scared of when it comes to soy, and that’s GMOs.

Soy + GMOs

Approximately 90% of the soy grown in the US is genetically modified.

While research has not yet found evidence that GMOs are harmful to human health, there are many reasons you may not want to consume genetically-modified products, including their potential detrimental effects on the environment.

So if you’re concerned, I’ve got an amazing solution for you.

Are you ready for it?

Don’t eat GMO soy. It’s that simple.

Until the research can catch up to the innovation, stick with organic soy to reap all of the amazing benefits we discussed without any of the potential unknown effects of GMOs.

Lastly, let’s talk types.

Whole Soy vs. Processed Soy

Not all soy products are created equally.

As we discussed before, some research shows no effect on health with the consumption of isolated soy isoflavones.

Some of the best evidence we have comes from populations who consume large amounts of soy, and these populations are eating whole or minimally processed soy products as opposed to the soy found in dietary supplements, oils, or protein substitutes here in the US.

Studies show that 80-90% of the beneficial isoflavones found in soy can be lost in the processing or refining of products like isolated soy protein.

Your best bet is to consume things like soybeans, aka edamame, soy milk, tofu, tempeh, and miso.

Fermented products are specifically beneficial for anyone with digestive issues.

Whole soy contains prebiotic fibers known as oligosaccharides which are digested by our gut bacteria in the colon. While this is a natural, benign process, it can cause bloating and gas for people with GI conditions like IBS.

Consuming fermented soy products however, like tofu and tempeh, can decrease this effect.

THE BOTTOM LINE

So now, let’s sum up everything we learned in this series on soy:

1. Soy is not only NOT a cause of cancer, but it may actually help treat and prevent cancer.
2. Soy doesn’t cause man boobs. It also doesn’t affect fertility, and in some cases it may help infertile couples.
3. There is absolutely no research suggesting soy causes acne.
4. Soy is not harmful to people with normal thyroid health.
5. Whole soy foods are likely better for your health than processed foods and supplements.
6. If you’re afraid of GMOs in soy, buy organic soy.

I hope you guys enjoyed this series and learned some valuable information that will help you make an educated decision about the foods you eat!

I hope you also feel armed with the evidence to shut down the crazy claims about soy the next time you hear them from some pseudoscience spouting wellness practitioner.

If you missed them, be sure to check out the first two articles in this series:

• The Truth About Soy + Cancer
• The Truth About Soy+ Fertility, PCOS, Acne + “Man Boobs”

And if you haven’t already, please SUBSCRIBE to my channel to get more episodes of “The Sitch” delivered straight to your inbox the moment they’re released.

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Weigh in: Did you enjoy this series on the health effects of soy? Did you learn anything new? What nutrition topic should I tackle next?!