Can You Eat Soy if You Have a Thyroid Condition?

If you have thyroid disease, you may have been told to avoid soy products. However, new research shows this outdated advice is unwarranted.

Soy's reputation as an endocrine disruptor, it turns out, was based on animal and laboratory studies. Human trials, on the other hand, show soy has little to no effect on thyroid function.

That said, soy may interfere with thyroid medications, and excessive consumption of soy products may be problematic for some individuals.

This article discusses soy and thyroid health. It explains the previous misbeliefs and current understanding of soy's impact on thyroid hormones.

Soy Protein and Isoflavones

Soy has been a dietary staple in Asia for centuries and a part of Western diets since the 1950s. Soybeans are a healthy and complete source of protein, packed with vitamins, minerals, fiber, and heart-healthy polyunsaturated fat.

Soy also contains bioactive compounds known as isoflavones, a phytoestrogen similar to the hormone estrogen.

Soy and soy isoflavones have been extensively studied and found to have many beneficial health properties. Soy may help to:

  • Ease menopausal hot flashes and night sweats
  • Lower LDL cholesterol levels
  • Lower blood pressure in people with hypertension
  • Preserve bone health after menopause
  • Reduce breast cancer risk

Soy and Thyroid Function

For years, it was thought that eating soy may interfere with thyroid function, possibly inducing hypothyroidism (low thyroid function).

In the 1930s, soy was first classified as a goitrogen—a category of foods and supplements that may alter thyroid hormone production and cause the thyroid gland to enlarge (goiter).

In the years following, laboratory and animal studies found soy isoflavones affect thyroid hormones in the following ways:

  • Prevent the action of thyroid hormones, causing hypothyroidism
  • Reduce the absorption of thyroid medications from the intestines
  • Stimulate thyroid growth, leading to goiter
  • Trigger autoimmune thyroid disease

However, human studies have not shown the same results. A 2019 meta-analysis of 18 clinical trials found soy has no effect on overall thyroid function. Soy supplementation was linked to a slight rise in thyroid-stimulating hormone (TSH) levels, but it does not appear to have clinical significance.

In addition, a 2022 review of 417 studies found soy isoflavones do not have a negative effect on thyroid function, thyroid hormone levels, or reproductive hormones. In other words, soy's reputation as an endocrine disruptor is not supported by the current research. 

Soy May Affect Iodine

Soy may alter thyroid function in people who are deficient in iodine, though. Thyroid hormones are produced in the thyroid gland. Iodine, an essential dietary mineral, is a component of thyroid hormones.

Soy is believed to inhibit thyroid hormone production by interfering with the entry of iodine into the thyroid gland. This can trigger a feedback mechanism that stimulates the pituitary gland to secrete more TSH.

TSH normally functions to spur thyroid hormone production. When thyroid hormones are low, TSH levels continue to build to excessive levels. This can overstimulate the thyroid gland and cause it to enlarge, forming a goiter.

However, iodine deficiency is rare in the United States due to universal salt iodization.

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7 Essential Facts About Goitrogens and Diet

Too Much Soy May Be a Problem

Excessive consumption of soy may alter the action of thyroid hormones, but the mechanism by which these changes occur is not clear.

A 2016 study published in Public Health Nutrition, for example, concluded that the chances of having high TSH were quadrupled in people who ate two daily servings of soy foods versus those who didn't eat any at all.

This effect is likely due to the isoflavones and not soy itself, a 2018 clinical trial found. In this study, participants were given either soy protein plus isoflavones or soy protein alone. After three months of daily supplementation, only the isoflavone group had elevated levels of thyroid hormones T3 and TSH.

The current research also found women are more susceptible to thyroid problems associated with soy products than men. The reason for this different response between men and women, however, is unclear.

Soy and Thyroid Medication

Soy may prevent optimal absorption of thyroid replacement medications like levothyroxine. This can cause your medication to provide inconsistent results.

In general, thyroid medication should be taken on an empty stomach to avoid irregular absorption. Soy-containing foods and beverages should be avoided for four hours before and after taking your dose.

Additionally, people taking radioactive iodine (RAI) therapy should avoid soy products for the duration of the treatment. According to the American Thyroid Association, high soy intake may interfere with radioactive iodine therapy and should be avoided.

How Much Soy Is Healthy?

The U.S. Food and Drug Administration (FDA) recommends adults consume 25 grams of soy protein a day as part of a heart-healthy diet.

Found in tofu, tempeh, miso, and edamame beans, soy is also used as a filler in processed meats and the manufacturing of meat and dairy substitutes. Soy milk, soy cheese, soy yogurt, soy nuts, and soy nut butter are widely available in grocery stores.

Soy is also sold as a dietary supplement in capsules and powder as soy protein or soy isoflavones. Research shows taking 50 milligrams (mg) to 100 mg of soy isoflavones a day is safe, but higher quantities have not been assessed.

Summary

Eating soy in moderation does not impact thyroid health, according to current research. Excessive soy consumption, however, may alter thyroid hormone levels and interfere with the production of thyroid hormones. 

Soy isoflavones, bioactive compounds with phytoestrogen properties, have beneficial effects on heart health and may relieve menopause symptoms. However, high doses of soy isoflavones have been shown to raise TSH and T3 levels. Women are particularly prone to this effect. 

If you are taking synthetic thyroid replacement hormones, avoid eating or drinking soy products for four hours before or after taking your medicine, as soy may hinder its absorption.

The FDA recommends adults consume 25 grams of soy protein daily. Additional research suggests limiting soy isoflavone consumption to 100 mg or less a day. 

14 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Mary Shomon
Mary Shomon is a writer and hormonal health and thyroid advocate. She is the author of "The Thyroid Diet Revolution."