Soy and the Thyroid

There are widely differing opinions on the wisdom of eating soy. Some are vehemently opposed and talk of soy as if it were the food of the devil. Others are carried off in the opposite direction, recommending soy foods (often in chemicalized form) and soy supplements many times a day. Most of us just want to know if eating a reasonable amount of soy foods is good or bad for us. I am in the process of reviewing research on soy with an eye toward answering those questions

One of the most common claims against soy is that it causes hypothyroidism, an underactive thyroid state. Thyroid governs our tendency to gain or lose weight. The last thing we want added to our woes is an underactive thyroid, making it difficult to lose weight, so this is a big concern. In addition, there are reports that soy will lessen the effectiveness of any thyroid medication you are taking to overcome hypothyroidism. So let’s begin our soy exploration by looking at soy and the thyroid.

You would think that there would be a ton of good information on the topic. Millions of people are eating—or not eating—soy foods daily. How hard can it be to gather some people from each group and measure their thyroid levels? Apparently, very hard. I did locate one study from a few years back that reviewed 14 human studies that in some way measured the effect of soy on thyroid. None of those 14 studies were perfectly designed.

First, few were primarily designed to measure the effect of soy on thyroid function.

Second, none tested the effect of tofu and other commonly eaten soy foods. Instead, all except one used a form of soy protein isolate.

Soy protein isolates are a form of highly refined soy protein. (See http://en.wikipedia.org/wiki/Soy_protein for more information.) Likely they are used in research to limit variability but isolated plant constituents are not the same as real, whole foods and we should not assume that their effects are the same. Moreover, as the study pointed out, different soy protein isolates (SPI) contain differing amounts of important compounds, such as soy isoflavones. So the fact that a study uses SPI does not mean its SPI is identical (or particularly similar to) the SPI used in another study. <sigh>

Nonetheless, the research is interesting because our notion that soy will cause hypothyroidism grew out of animal studies looking at soy protein isolates. Conclusion: All but one study showed that, in humans with iodine levels within the norm, soy did not have a negative effect on thyroid. In other words, if you have adequate amounts of iodine in your diet, soy will not cause your thyroid to malfunction. Most of us eat processed and/or fast foods and are getting plenty of iodized salt in our diet. (In fact, a Danish study recently found that we may be getting so much iodine that it is making us hypothyroid—but that is a study for another blog.) If you are following a traditional Asian diet that includes seaweed and fish, you also are getting enough iodine. On the other hand, if you are eating a low salt, whole foods diet and only seasoning with sea salt, you may want to double-check your iodine intake: A combination of low iodine and soy flavones can cause thyroid issues.

The study also looked at the effect of soy on thyroid medications such as Levothyroxine. It seems that soy may affect the absorption of the medication. For adults, following the usual instructions (take the medication at least ½ hour before breakfast) should suffice to prevent a reduction in the absorption of the drug. But just to be on the safe side, the reviewers also recommend thyroid tests any time a person’s diet is changed to regularly include a lot more or a lot less soy food. (They do not think this is necessary if you once in a while eat more or less soy).

All but one of the reviewed studies looked at the effect of soy protein isolates. That one study looked at 37 people eating about an ounce of roasted soybeans pickled and stored in rice vinegar. Although most thyroid hormone levels were unaffected, many very quickly developed goiters (15-71%), digestive issues (35-50% constipation or diarrhea), and malaise/sleepiness (42-53%). There was no control group and the test food was poorly described. The researchers could not explain the data showing goiters quickly developing in response to no more than 30 mg soy isoflavones in a Japanese population that typically eats at least 30-50 mg/day but has a very low incidence of goiter. They classified the study as an anomaly and I tend to agree.

One of the reviewers, Dr. Messina is the president of the Soy Nutrition Institute. Despite this potential for bias, I think the summary of the research in this review seems reliable. While I intend to look further into research on real soy foods and thyroid (as well as other soy issues), I am convinced that eating moderate amounts of soy foods does not cause thyroid problems in people with adequate iodine levels

Photo: Ninja Soy Bean Creative Commons some rights reserved by Marcoa Zerene

NOTE: You are welcome to use my blog’s original images and content for non-commercial purposes if you attribute the work to me (Kathy Abascal) and link back to the blog. This work is licensed under a Creative Commons Attribution-NonCommercial 3.0 United States License.

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