Are soy foods harmful to thyroid function?
There is no scientific evidence showing a correlation between soy consumption and thyroid disorders in humans. A number of clinical studies have also failed to note clinically significant changes in thyroid hormones in either men or women consuming soy protein or isoflavones 1-4. Recent studies in test tubes have shown that, under certain experimental conditions, the principle isoflavones in soy, genistein and daidzein, can inhibit the enzyme thyroid peroxidase (TPO)5. This enzyme is responsible for the conversion of thyroglobulin to thyroid hormone. It must be emphasised, however, that this occurs only with large amounts of soy isoflavones (more than an adult could typically consume as part of a normal diet), and/or when the diet is low in iodine. The inhibition of this enzyme can be completely abolished in the presence of iodine5. Further, soy isoflavones are not the only natural dietary constituents that inhibit TPO. A variety of other closely related flavonoids have been shown to b
