One of the most discussed issues of dietary soy consumption and human health over the years has been thyroid function. Animal and cell culture studies reported that soy isoflavones inhibit the activity of enzymes involved in the production of thyroid hormones [1 – 3]. However, animal and lab cell culture studies are often very different than human consumption. Despite this enzyme inhibition, consumption does not appear to actually alter thyroid function, since the hormones and the gland were unaffected in rats [2, 3].
Studies on human subjects confirmed the findings reported in the animal studies regarding the lack of any effect of dietary soy on thyroid function [4, 5]. Despite this, the concern over soy consumption and thyroid function has persisted. This appears to be due to the fact that many of the human clinical trials have been of short duration, and several internet groups selling anti-soy books have failed or refused to report the newer human studies.
A new human clinical trial addressed this issue by examining thyroid function in postmenopausal women taking a soy isoflavone supplement (54 milligrams of genistein) daily for 3 years . For this study, blood levels of thyroid hormones and auto-antibodies were assessed in 40 women taking genistein daily and 37 women taking a placebo capsule daily. Additionally, changes in thyroid hormone receptors and thyroid hormone enzymes were measured in 71 postmenopausal women taking genistein daily and 67 postmenopausal women taking a placebo daily. The results of this new soy research report showed that daily consumption of genistein:
- Had no effect on thyroid hormones, which remained within the normal range for the full duration of the study,
- Did not alter the function of enzymes involved in thyroid hormone production,
- Did not cause any changes in thyroid hormone auto-antibodies, and
- Had no effect on the expression of thyroid hormone receptors.
These results continue to confirm earlier studies that soy does not appear to alter thyroid function in healthy postmenopausal women. This is an excellent study in regards to its 3-year duration; however, this study focused only on genistein, the main isoflavone present in soy protein. Additional long-term studies looking at soy protein with all the isoflavones present or looking at the other isoflavones individually will be beneficial in continuing to clear up the issue of soy and thyroid function. Individuals taking thyroid medications need to separate soy consumption and medication intake by 3 – 4 hours, so as not to interfere with absorption of the medication [7 – 9], which is also true for other foods like walnuts, calcium, and dietary fiber.
 Divi RL, Chang HC, Doerge DR. Anti-thyroid isoflavones from soybean: isolation, characterization, and mechanisms of action. Biochem Pharmacol 1997; 54:1087-96.
 Chang HC, Doerge DR. Dietary genistein inactivates rat thyroid peroxidase in vivo without an apparent hypothyroid effect. Toxicol Appl Pharmacol 2000; 168:244-52.
 Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect 2002; 110 (Suppl 3):349-353.
 Bruce B, Messina M, Spiller GA. Isoflavone supplements do not affect thyroid function in iodine-replete postmenopausal women. J Med Food 2003; 6:309-316.
 Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature.
 Bitto A, Polito F, Atteritano M, Altavilla D, Mazzaferro S, Marini H, Adamo EB, D’Anna R, Granese R, Corrado F, Russo S, Minutoli L, Squadrito F. Genistein aglycone does not affect thyroid function: results from a three-year, randomized, double-blind, placebo-controlled trial. Journal of Clinical Endocrinology & Metabolism 2010; 95:Published online ahead of print.
 Jabbar MA, Larrea J, Shaw RA. Abnormal thyroid function tests in infants with congenital hypothyroidism: the influence of soy-based formula. J Am Coll Nutr 1997; 16:280-2.
 Bell DS, Ovalle F: Use of soy protein supplement and resultant need for increased dose of levothyroxine. Endocr Pract 2001; 7:193–194.
 Pinchera A, MacGillivray H, Crawford JD, Freeman AG. Thyroid refractiveness in an athyreotic cretin fed soybean formula. N Engl J Med 1965; 273:83-87.