By the way, doctor: Do soy products cause thyroid problems?
Q. I read that soy can cause a low thyroid condition. What do you know about this?
A. 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.