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Can soy products cause problems with thyroid?
Q. I read that soy can cause a low thyroid condition. What do you know about this?
Although many people consider soy to be part of a healthy diet, there have been persistent concerns about its effects on the thyroid gland. Animal studies dating back as far as 1959 have suggested that certain chemical components of soy -- the isoflavones -- may be linked to thyroid disorders, especially goiter and low thyroid. The two main soy isoflavones, genistein and daidzein, inhibit thyroid peroxidase, an enzyme necessary for making thyroid hormone. But luckily iodine blocks this antithyroid effect.
In 2003, a randomized clinical trial compared a daily dose of 90 mg of isoflavones with a placebo in 38 healthy postmenopausal women with normal iodine intake. After six months, there were no differences in thyroid function between the treatment and placebo groups. So soy shouldn't cause thyroid problems as long as you get enough iodine in your diet. Fortunately, that isn't hard to do. The recommended dietary allowance (RDA) of iodine is 150 micrograms (mcg) per day. Two good sources of iodine include plants grown in iodine-rich soil and dairy products from animals that feed on such plants. Iodine-rich soil is found mainly on land that was once under salt water. But you needn't worry if you don't live near such an area, because you are almost certainly eating food from all over the country and all over the world.
Saltwater seafood, including deep-sea fish, shellfish, and even seaweed are also good sources of iodine. In the United States, iodized salt is another major source. A half-teaspoon provides 134 mcg of iodine.
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Dr. Celeste Robb-Nicholson, Harvard Women's Health Watch
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