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 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.


About kathyabascal

Herbalist with a background in neurobiology, biochemistry, and law. Teacher of the TQIDiet, how to quiet inflammation with food.
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11 Responses to Soy and the Thyroid

  1. Good information Kathy – thank you! I’ll be very interested in what you find out about soy’s phytoestrogens. I am a moderate to high user of soy and take Syntroid and have a history of breast cancer in my family at my age (!).

  2. Therese says:

    Hi Kathy,
    As a result of taking your class, I now prefer soy milk over milk in my coffee. And I love to find reasons to eat tofu so I’m wondering: what is “moderate use” of soy in the daily or weekly diet? Just curious how much is too much.

    • kathyabascal says:

      Hard to say. The one Japanese study on phytates in soy looked at people averaging 3 ounces a day. How much are you eating/drinking a day?

      • Therese says:

        It varies, of course. Over the course of a week I may have four soy lattes; “whitener” in seven cups of coffee; one tofu stirfry or other use with veggies; and four servings of soy snacks (soy nuts or soy crisps) with fruit. Not sure how that adds up.

      • kathyabascal says:

        Again, no hard data but in my opinion there is a big difference between a tofu stir fry and a soy latte. To me, soy lattes and soy crisps are not real food; soy nuts will vary depending on how they are processed but are certainly a step away from edamame. Personally, I’d give up the soy snacks and probably the lattes as well, especially if they are from the usual coffee bar which means non-organic, sweetened GM soy milk…

  3. Anastasia R. Ellis says:

    I bought some soy milk for my coffee, thinking it would be healthier. I read this on the label when I got home: “Soy milk, Cane Sugar, Palm Oil, Natural Flavor, Soy, Lecithin, Potassium Phosphate, Sodium Citrate, Tapioca Starch, Carrageenan”


    • kathyabascal says:

      Underscores the importance of reading labels to make sure you are buying what you thought you were (e.g. soy milk), doesn’t it. No reflexion on the pros and cons of “real” soy milk though.

  4. Courtney says:

    I really wish your book “The Abascal Way” was available via Kindle! I am having a difficult time locating your first book. I have been on your “diet” for over a week and don’t even find myself hungry any more. I eat to live and not live to eat any longer!

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