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What Is the Impact of Soy Products on Thyroid?

Moderate consumption of soy products may not pose a problem, but some people find they need a higher dose of thyroid hormone.

When people turn away from meat and cow’s milk, often they replace these foods with soy products. Indeed, soy is central to the diets of many North American vegetarians. But do soy foods carry unanticipated side effects? One reader discovered some.

Does Soy Milk Interfere with Thyroid Hormones?

Q. I really like the flavor of soy milk, but I’ve found that drinking it interferes with my thyroid treatment. I’ve been taking the same dose of Armour thyroid for more than a decade. After I started drinking a couple of glasses of soy milk every day, I noticed symptoms of hypothyroidism. I would feel cold and not be able to warm up.

Once I stopped the soy milk, my hypothyroid symptoms went away. Now I stay away from all soy. I think I could tolerate small amounts here and there, but not a lot every day. I make sure to take my thyroid medicine separately from any food intake.

When I mentioned this to my doctor, she gave me the side eye. How well known is this interaction?

Soy Products May Affect Thyroid Function:

A. It sounds as if you have done your own N of 1 experiment to determine how soy products affect your thyroid function. There are relatively few randomized controlled trials addressing a potential interaction.

Back in 2002, scientists found that certain natural compounds in soy called isoflavones can inactivate an enzyme crucial for thyroid hormone regulation (Journal of Chromatography B: Analytical Technologies in the Biomedical and Life Sciences, Sep. 25, 2002). This led some researchers to suspect that soy isoflavones might interfere with thyroid function (Environmental Health Perspectives, June 2002).

A few case studies have confirmed that some individuals react as you do (Endocrinology, Diabetes & Metabolism Case Reports, July 19, 2021).  A meta-analysis revealed some contradictory findings (Scientific Reports, March 8, 2019). Although TSH rose (an indication that thyroid function is subpar), the investigators thought the increase did not have clinical significance.

Other Foods That May Affect Thyroid:

Soy is not the only food that might affect thyroid hormones. You are smart to take your medicine well away from meal time, since coffee, tea and supplements containing iron or calcium can reduce absorption. You can learn more about this from our eGuide to Thyroid Hormones.

What Is the Impact of Soy Products on Thyroid?

Q. I started eating lots of soy products when I was a young woman. I developed low-level hypothyroidism around age 40.

My nurse practitioner put me on levothyroxine and I have been taking that. I still consume soy products just about every day even though I’ve read that it is bad for my thyroid.

Dairy products give me sinus problems, so I only use butter and a small amount of cheese. I drink soy milk every day. I’ve tried to cut back on soy but I have eaten it for so long that it is hard to change!

My TSH levels are tested every year and are normal. Do I really need to worry about soy and thyroid?

How Do Soy Foods Affect Thyroid?

A. You might need to find some ways to consume fewer soy products, but since your TSH is normal and you feel good, it is not an immediate problem. Soy foods might reduce the absorption of your thyroid pill, but presumably the dose can be adjusted (Messina & Redmond, Thyroid, March 2006).

You will want to make sure you are getting adequate iodine; you can do this simply by using iodized salt in your cooking and at the table.

The idea that soy products might interfere with levothyroxine absorption is not the only concern. Some research suggests that eating a diet containing nitrate, thiocyanate or soy isoflavones might trigger an autoimmune reaction to the thyroid hormone T3 (Colucci et al, Archives of Environmental Contamination and Toxicology, Aug. 2015).

Cruciferous vegetables such as broccoli, Brussels sprouts, cauliflower, cabbage and turnips are sources of thiocyanate. Processed meats such as bacon or salami are rich in nitrate. Vegetables such as beets, spinach and collard greens also contain nitrates. Obviously, all these vegetables belong in a healthful diet, just as soy products do. The key is to consume traditionally prepared soy foods in moderation (Zaheer & Humayoun Akhtar, Critical Reviews in Food Science and Nutrition, Apr. 13, 2017).

Learn More:

You may want to listen to our interview with thyroid patient advocate par excellence Mary Shomon and thyroid expert Antonio Bianco, MD. It was Show 1015: Thyroid Mysteries, Controversies and the Latest Research. There is also more information in our Guide to Thyroid Hormones.

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About the Author
Terry Graedon, PhD, is a medical anthropologist and co-host of The People’s Pharmacy radio show, co-author of The People’s Pharmacy syndicated newspaper columns and numerous books, and co-founder of The People’s Pharmacy website. Terry taught in the Duke University School of Nursing and was an adjunct assistant professor in the Department of Anthropology. She is a Fellow of the Society of Applied Anthropology. Terry is one of the country's leading authorities on the science behind folk remedies..
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  • Doerge DR & Chang HC, "Inactivation of thyroid peroxidase by soy isoflavones, in vitro and in vivo." Journal of Chromatography B: Analytical Technologies in the Biomedical and Life Sciences, Sep. 25, 2002. DOI: 10.1016/s1570-0232(02)00214-3
  • Doerge DR & Sheehan DM, "Goitrogenic and estrogenic activity of soy isoflavones." Environmental Health Perspectives, June 2002.
  • Kumarathunga PADM et al, "Over-the-counter protein supplement resulting in impaired thyroxine absorption in a hypothyroid patient." Endocrinology, Diabetes & Metabolism Case Reports, July 19, 2021. DOI: 10.1530/EDM-21-0070
  • Otun J et al, "Systematic review and meta-analysis on the effect of soy on thyroid function." Scientific Reports, March 8, 2019. DOI: 10.1038/s41598-019-40647-x
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