Soy and Thyroid Function

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Can soy suppress thyroid function?

I’ve heard that soy can suppress the thyroid. Is there an upper limit?

Eric Needs / Originally posted on Soy Hormones & Male Infertility


Excellent question! Soy does indeed have so-called “goitrogenic” compounds (as do broccoli-family vegetables and flax seeds), which can interfere with thyroid function in people with marginal iodine intake. The answer is not to avoid these super healthy foods but to just make sure you get enough iodine. See my videos Avoiding Iodine Deficiency and Pregnant Vegans at Risk for Iodine Deficiency.

For another reason, though, restricting one’s soy intake to 3-5 servings a day is probably a good idea. See How Much Soy Is Too Much? and Too Much Soy May Neutralize Benefits.

Image Credit: sleepyneko / Flickr


Michael Greger M.D., FACLM

Michael Greger, M.D. FACLM, is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous "meat defamation" trial.

14 responses to “Can soy suppress thyroid function?

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  1. Speaking of thyroid I don’t see information on your site about hypothyroidism . I am in my late 60’s and have always led a very active, healthy lifestyle and eat mainly a plant based low glycemic diet with very little if any sweets and moderate unrefined carbs. Have never had an overweight problem and was always skinny as a kid and could eat anything I wanted without gaining an ounce.
    I put on 20 lbs. during the past 5 years and no matter how much I diet or exercise the scale doesn’t move. Tests show that my thyroid is low and I was put on synthroid which after taking for several months hasn’t shown to be effective . I’ve kept a diet diary and even on a 800-1000 calories a day I still can’t lose weight. I get at least 30 minutes of exercise a day by walking and lifting weights. I have plenty of energy and am in otherwise good health -having never been hospitalized for anything serious other than having 3 children . Any suggestions ?

  2. Hi Dr Greger, I’m 30 years old, i’m vegan since a long time. Recently my doctor told me that I develop Hypothyroidism, I feel the symptoms for many years before it appears in my test. They don,t know for the moment if it’s an autoimmune problem or other… The doctor propose me to take syntroid, but i’m affraid of taking medication at my age… What do you propose or think about this situation.. I aready see acupuncturist and osteopath for years without long lasting result. Thanks a lot :)

  3. Browse the Internet on soy and thyroid function and one is bound to find a great deal of information on how soy foods negatively affect thyroid function. Soy has gotten a bad rap and misinformation abounds. It is true that compounds in soy, such as genistein and daidzein (isoflavones), have been shown to possess goitrogenic properties that may suppress thyroid function and/or inhibit the absorption of thyroid hormone medication. Most of the research wasperformed on animals and not humans. However, studies on Soy isoflavones and human thyroid health provide evidence that soy foods have a negligible impact on thyroid function, or no impact, as long as iodine in the diet is sufficient. You can find out more aboutiodine by viewing Dr. Greger’s video Too Much Iodine Can Be as Bad as Too Little .

  4. Browse the Internet on soy and thyroid function and one is bound to find a great deal of information on how soy foods negatively affect thyroid function. Soy has gotten a bad rap and misinformation abounds. It is true that compounds in soy, such as genistein and daidzein (isoflavones), have been shown to possess goitrogenic properties that may suppress thyroid function and/or inhibit the absorption of thyroid hormone medication. However, most of the research was performed on animals and not humans. Human studies on soy isoflavones and human health provide compelling evidence that soy foods have no or negligible impact on thyroid function as long as iodine in the diet is sufficient 1.

    You can find out more about iodine by viewing Dr. Greger’s video, Too Much Iodine Can Be as Bad as Too Little.


  5. hi, im 16 and have hypothyroidism and i heard taking the medication thyroxine i think its called can put you at a high risk of breast cancer? i was wondering if there is any way i could get off the medication and heal it naturally? thankyou

    1. Hi Georgia, sorry for this delayed response to your question but I just did a quick search of the literature and did not find any studies to substantiate the idea that thyroxine could put you at risk of breast cancer. I did find this study which was a meta-analysis (which means they compared the results of several studies on a single topic) and they looked at the association of breast cancer and thyroid disease in general. They found that breast cancer patients had higher levels of T3 and T4 (thyroid hormones). One might try to conclude that since thyroxine (or levo-thyroxine) can increase levels of T4 and T3 that it could increase your risk but that would be an incorrect conclusion since the study did not find a causative effect between hormone levels and breast cancer.

  6. I know this is an old post, but I just wanted to say, I have Hashimoto’s and my dose always seemed to go up and up after I had half of my thyroid removed. I tried paleo and gluten free variations as that’s what we’re often told to do. Since going vegan, my dose has had to be reduced yet again. A few months ago I was at 150mcg, now I’m going on 112mcg. For whatever reason, I seem to be absorbing things better – My iron has gotten better and I’ve always been anemic. My b12 and D are also better – supplements weren’t getting absorbed well before. And, I don’t avoid soy!

    1. That’s great to hear Kat! You’ll find that everything generally gets better when you have less inflammation in your body. Dr G has discussed the anti-inflammatory effects of a WFPB diet in many previous videos so it doesn’t surprise me that after going vegan your gut now seems to be working better.

  7. After doing a fruit based diet for a while. At first I felt extremely well and then I became increasingly fatigued. I started a WFPB diet, and to my surprise the symptoms have not improved. My blood work always comes back fine (iron stores only slightly low). I know this is asking a lot, but if you could give any tips or possible explanations for the fatigue. I’ve never felt this lethargic in my life, but know it is possible to thrive on a WFPB diet.

  8. Hi. My partner had an thyroidectomy long time ago and she is therefore taking T3 and T4 medication. We have seen an endocrinologue recently ( unfortunatly not open at all about veganism, but we do with what we have) and she told us about the danger of taking Soy because it could affect the effect of my partner medication. In fact my partner who had live without her thyroid since 13 years is also a soy consumer since years and it is true that she had to rise the dose of her medicine over the years. Is it link to soy?

    Doctors freak out in general when she told them she take 250 (I am not sure if this number is correct)

    Further more my partner is pregnant and when we show our endocrinologue the iodine bottle my partner have (lugol 5%) she was in the impossibility to give us a correct doze because she was not used to lugol.

    As I understood since she take T3 and T4 she don’t need iodine but the baby needs it. Further more it seams that she was not converting T3 since years but the placenta start to do it for her.

    If Iodine is the key to an healthy life for people with thyroid problems, how many drops my partner should take then? Does she need to stop Soy consumption? Or is that an other way to scare people?

    I will be grateful if there is anyone out there to answer my questions.

    Thank you


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