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 a health claim for the cholesterol-lowering properties of soy protein, concerns about the possible goitrogenic effects of soybean isoflavones persist. Concerns are based primarily on in vitro research, animal studies, and older reports of goiter in infants fed soy formula not fortified with iodine. In a randomized, double blind, placebo-controlled study, we investigated the effect on thyroid function of a daily supplement containing 90 mg (aglycone weight) of total isoflavones/day versus placebo in 38 postmenopausal 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, at baseline and 6 months, TSH (μ U/ml), T4 (nM), and T3 (nM) levels (mean ± SE) were 3.00 ± 0.44, 149.00 ± 5.04, and 1.53 ± 0.13, respectively, and 3.49 ± 0.52, 154.52 ± 2.0...