Soy and Isoflavone Estrogenic Activity

349 The potential health benefits of soy, and the soy isoflavones in particular, are widely publicized. Although soy and soy isoflavones exhibit both risks and benefits (1,2), in this article we focus on the thyroid toxicity of genistein. Genistein is the major isoflavone synthesized by the soybean; genistein possesses both estrogenic and goitrogenic activities. The claimed benefits of genistein are being examined in numerous experimental, epidemiologic, and clinical studies investigating breast and prostate cancer chemoprevention, relief of postmenopausal symptoms, and prevention or slowing of osteoporosis. Here, we first present a brief summary of the estrogenic activity and toxicity of genistein and then explore the potential for thyroid toxicity of these chemicals, both from a historical perspective and from data reported from recent investigations on the mechanisms of potential toxicity. We also integrate research results on isoflavone thyroid effects in a manner useful for predicting and identifying potential risks from soy consumption in various human populations.