Soy and Thyroid Health: What You Need to Know

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Soy, once a popular go-to food, has now become controversial, both in terms of its overall health benefits (or not), as well as its link to your thyroid function.

In fact, for almost two decades, there's been a continuing debate on whether soy can negatively affect the thyroid, and this debate continues. So, in the meantime, here are some dos and don'ts for soy consumption, keeping your thyroid health in mind.

About Soy

Soy (or soybeans) are a type of legume, high in protein, that contain isoflavones, which are plant-based estrogens. In recent years, soy has become popular and can now be found not only in its traditional food forms such as miso, tempeh, tofu, and edamame, but also processed into burgers, protein bars, protein powders, shakes, soy milk, and nutritional supplements.

Soy's Pros

  • There is limited evidence that soy can help some women alleviate menopausal symptoms such as hot flashes.
  • Substituting soy for protein sources higher in saturated fat may result in a small reduction in your "bad" LDL cholesterol.
  • While the ways in which soy or soy isoflavones could interfere with thyroid function have been reported (albeit inconsistently), most human studies have found no major effect of soy on thyroid function in adults.

Soy's Cons

  • There is scant and insufficient evidence to support soy's claims of having specific benefits for heart health or weight loss.
  • Soy may hinder your ability to absorb thyroid medication properly.
  • Much of the soy available in the United States is genetically modified "GMO" soy, and there is some concern about the health effects of genetically modified soy and other foods.

Tips on Eating Soy for People with Thyroid Disease

Until we have definitive, rigorous, high-quality studies on soy toxicity and the effects of soy on thyroid function, you can't assume that soy is universally safe for your thyroid health. But if you want to include soy in your diet, here are some guidelines.

  • Avoid soy if you are preparing to undergo a radioactive iodine uptake scan: According to the American Thyroid Association, soy products like soy sauce, soy milk, and tofu should be avoided (as part of a low iodine diet), since high soy consumption may interfere with the radioactive iodine uptake. 
  • Consider avoiding soy. If you are a thyroid patient with optimized thyroid treatment, and you're still suffering from hypothyroidism symptoms, you may consider eliminating soy from your diet to see if that helps relieve symptoms. You should talk to your doctor about this first, though. 
  • Eat the right soy. If you are eating soy foods, you may want to avoid GMO soy foods until the debate over their safety has been definitively resolved. In addition, select fermented and food forms of soy, for example, tofu, tempeh, and miso. Avoid processed soy products such as soy powders, protein shakes, and other manufactured forms of soy.
  • Don't overconsume soy. It's probably safe to include some soy in your diet, but a daily diet of soy milk, edamame, tofu, tempeh, miso, soy burgers, soy bars, soy ice cream, and soy protein shakes is going overboard. While there are really no official recommendations for soy consumption, avoiding excess intake is likely best.  
  • Keep soy apart from your thyroid medication. The one thing experts are sure about is that soy can interfere with levothyroxine absorption in the gut. This is why it's important to only consume soy foods within three to four hours of taking your thyroid hormone replacement medication.
  • Soy allergy. Keep in mind that soy protein is a potential allergy-triggering food for some people—and likewise, if you are allergic to soy, you should avoid eating it. 

A Word From Verywell

The bottom line is that soy is a controversial nutrient with regards to many aspects of your health. Until experts can tease out all the inconsistencies, it's best to be safe and consume soy (if desired) in moderation.

Lastly, with regards to your thyroid health, the main thing to remember is to not consume soy with your thyroid replacement medication, as it can interfere with the drug's absorption in your gut. 

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Article Sources
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  • Garber et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012 Nov-Dec;18(6):988-1028.
  • Messina M. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients. 2016 Dec;8(12):754.
  • National Center for Complementary and Integrative Health. (2016). Soy.