Soy and thyroid

Can soy cause hypothyroidism? - Some foods contain compounds called goitrogens that interfere with thyroid function. In extreme cases these goitrogens can cause goiter (an enlarged thyroid gland). Increasing iodine intake prevents this goitrogenic effect of soy. Some research also indicates that heating soy eliminates part of this goitrogenic effects. Around 1950 there were several cases of goiter in infants fed with soy flour based formulas, which were not fortified with iodine. These formulas do no longer exist and infant formulas are now based on soy protein isolates (not soy flour) and are also fortified with iodine. No cases of goiter have been reported in babies fed with these formulas.

Recent studies indicate that there is little risk to thyroid function in healthy people who consume soy. However, people who have too low intake of iodine could be at risk. Increasing iodine intake is easy: this can be done by using of iodized salt or by taking iodine supplements. According to the American Thyroid Association "soy does not cause hypothyroidism, but it does interfere with the body's ability to absorb thyroid replacement therapy". In practice, this means that thyroid medication and soy products must not be taken at the same time. The FDA rejected earlier claims that soy adversely affected the thyroid due to lack of evidence. In Asian countries where consumption of soy is 10 to 100 times higher than in the Western world there's no higher occurrence of goiter.

What if I have hypothyroisism? - Some doctors suggest that thyroid medications be taken at a different time of the day than soy protein to maximize absorption of the medication. Hypothyroidism is very common among women so you may want to have your thyroid levels checked every couple years or so, whether or not you eat soy. Hypothyroidism has not been a concern in human studies. Studies with athletes have not shown any negative effects of soy to thyroid health. The FDA has neither issued a consumer warning, nor stopped infant formula from being used, nor stopped soy foods from being sold. The FDA reviewed the medical literature for two years and found no proven evidence that soy would harm the thyroid. A lot of soy concerns come from one in vitro study, which uses pure isoflavones in high concentrations.