Isoflavone Supplements Do Not Affect Thyroid Function in Iodine-Replete Postmenopausal Women

Despite the safety review conducted by the U.S. Food and Drug Administration in the process of awarding ahealth claim for the cholesterol-lowering properties of soy protein, concerns about the possible goitrogenic effects of soybeanisoflavones persist. Concerns are based primarily on in vitroresearch, animal studies, and older reports of goiter in infantsfed soy formula not fortified with iodine. In a randomized, double blind, placebo-controlled study, we investigated the effecton thyroid function of a daily supplement containing 90 mg (aglycone weight) of total isoflavones/day versus placebo in 38postmenopausal women, 64 83 years old, not on hormone replacement therapy. Serum thyroid-stimulating hormone (TSH),thyroxine (T4), and triiodothyronine (T3) were measured at baseline and after 90 and 180 days. In the supplement group, atbaseline and 6 months, TSH (mU/ml), T4 (n M), and T3 (n M) levels (mean 6 SE) were 3.00 6 0.44, 149.00 6 5.04, and 1.53 60.13, respectively, and 3.49 6 0.52, 154.52 6 2.09, and 1.78 6 0.12, respectively. In the control group, levels at baseline andat 6 months were 3.35 6 0.51, 145.39 6 6.69, and 1.55 6 0.18, respectively, and 3.63 6 0.57, 153.77 6 6.64, and 1.75 60.10, respectively. Intragroup differences for all three measures were statistically indistinguishable at 6 months, and levelswere similar between the isoflavone supplement and placebo groups at each measurement. These results indicate that in thisgroup of healthy iodine-replete subjects, soy isoflavones do not adversely affect thyroid function.KEY WORDS . isoflavones . phytoestrogens. soy . thyroid function . thyroid hormones309 KCI Citation Count: 0