Recent reports have suggested that soy may adversely affect the thyroid gland, increasing goiter formation and thyroid cancer risk (5). Isoflavones in soy inhibit the enzyme thyroid peroxidase (TPO) and thus negatively affect thyroid hormone synthesis in vitro (7). However, studies in rats have found that some additional, unidentified components in soy protein may be needed besides the isoflavones to exert anti-thyroid actions in vivo. Soy protein has been found to induce goitrogenesis in rats, but only in those that are iodine-deficient (10). Therefore, susceptibility to soy-induced goiters may not be substantial unless some other goitrogenic risk factor, such as iodine deficiency, is also present. Studies have also found that iodine-deficient diets containing 30% soy protein can induce thyroid carcinoma in rats (13). In contrast, however, retrospective human studies such as the Bay Area Thyroid Cancer Study found soy to reduce thyroid cancer incidence (14,15). Hence, more research is needed, particularly in the form of prospective human clinical trials, to clarify the relationship between soy and the thyroid. At present, it would be wise for consumers to abstain from excessive soy protein consumption and consume only moderate levels of soy before more research is done.