Is soy bad for thyroid function

August 14, 2012

Depression, Estrogen, Food, Thyroidism, Women

A high dosage of soy products in a person with iodine deficiency and/or exposure to environmental goitrogens has the ability to negatively affect thyroid function.

by Dr. Sima Aidun — 

Long-standing debates exist as to whether or not soy is safe for thyroid function. Soy products are currently heavily marketed to women in the menopausal age group to help with symptoms such as hot flashes and night sweats. The potential anti-thyroid effect of soy might be more relevant with these women because they represent the age group in which overt hypothyroid (up to 40 percent) and subclinical hypothyroid (up to 10 percent) are most likely to occur.

Brief introduction about soybeans

The soybean is a species of legume native to East Asia. Soybeans are composed of 40 percent protein and 20 percent oil. The remainder consists of 35 percent carbohydrate and about 5 percent ash. Soybeans contain significant amounts of all the essential amino acids that must be provided to the body due to its inability to synthesize them.

Soybeans also contain isoflavones, which are organic chemicals with a weak estrogenic effect (phytoestrogenic). The two main isoflavones present in soybeans are genistein and daidzein. Genistein, the major soy isoflavone, also has a weak estrogenic effect in women, as well as ability to effect thyroid function — an activity independent of estrogenicity.

Effect of soy isoflavones on thyroid function

Soy isoflavones inhibit the activity of TPO (thyroid peroxidase), one of the main enzymes inside the thyroid gland responsible for making thyroid hormones. Studies show that the effect of soy on TPO activity is dose dependant and happens in a much higher dosage than is usually consumed, and additional risk factors for thyroid dysfunction are necessary before soy consumption can induce an anti-thyroid effect.

Additional risk factors such as iodine deficiency greatly increase soy anti-thyroid effects, whereas iodine supplementation is protective. Thus, soy effects on the thyroid involve the critical relationship between iodine status and thyroid function. A link between iodine deficiency and soy-induced hypothyroidism in humans comes from the studies of infants on soy formula in whom goiter was reversed upon supplementation with iodine.

Exposure to environmental elements present in water, food and air is another factor that could influence the development of thyroid function abnormalities in presence of soy intake. Goitrogens can cause enlargement of the thyroid gland, possibly affecting its function.

A high dosage of soy products in a person with iodine deficiency and/or exposure to environmental goitrogens has the ability to negatively affect thyroid function. Most people neither consume soy products in high dosage nor are iodine deficient, and therefore may enjoy a moderate amount of (unprocessed) soy products. Follow these guidelines:

  • Limit daily soy consumption to optimally 30 mg and a maximum of 50 mg of isoflavone.
  • Consume a minimum of 150 micrograms of iodine daily.
  • Take thyroid medication hours before or after consuming soy products.

Under-consumption of iodine deprives the thyroid gland of manufacturing active thyroid hormones. Iodinated salt, along with other sources such as seaweed, for persons on a sodium-restricted diet are good choices to obtain daily iodine intake.

Please note: patients with autoimmune thyroiditis (Hashimoto’s) should avoid taking more than a minimum recommended dosage since it will exacerbate the autoimmune response.


Dr. Sima Aidun is a naturopathic physician in Scottsdale, Ariz., who focuses on female health issues from puberty to menopause, as well as bio-identical hormone replacement therapy. 480-451-1602, or

Reprinted from AzNetNews, Volume 29, Number 3, June 2010.

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