Isoflavone Research Initiative

Another type of potential risk regularly discussed is the safety at the thyroid gland. Again the concerns can be traced back to in vitro-studies and animal experiments, from which it was extrapolated that isoflavones might aggravate the symptoms of patients with thyroidal gland insufficiency, especially when these women are iodine deficient. Again, this risk has not been confirmed through clinical observations. A recent review of the available clinical data concluded that even adults with thyroidal gland insufficiency do not need to avoid soy. Consequently, the authors call for a sufficient iodine intake by subjects at risk of developing thyroidal disease (Messina and Redmond 2006).

Since the publication of this review more clinical data has become available, supporting the conclusions of Messina & Redmond (2006). Effects of soy isoflavones on the thyroidal gland have been explicitly tested in several studies (Cheng et al. 2007; Imhof et al. 2008; Knight et al. 2001; Teas et al. 2007; Watanabe et al. 2000), none of which yielded indications of a negative impact of isoflavones on thyroidal hormone levels. A multiethnic case control study in women aged between 20 and 74 years living in the San Francisco area even showed an inverse correlation between the intake of soy isoflavones and the risk of thyroidal cancer (Haselkorn et al. 2003).

Warnings against soy, red clover, soy/red clover extracts or isoflavones are entirely based on extrapolations, and are highly questionable. Should isoflavones in fact aggravate thyroidal functions in subjects with iodine deficiency (which is not supported by the clinical data), the logical consequence would be the recommendation of an adequate intake of iodine, not a restriction of isoflavones. The latter would not contribute to the solution of the health problems of iodine deficient subjects. With a sufficient iodine intake, however, no negative impact of isoflavones on the thyroidal gland is to be expected (Doerge and Sheehan 2002; Messina and Redmond 2006).

References

Cheng, G., Wilczek, B., Warner, M., Gustafsson, J. A., and Landgren, B. M. (2007). Isoflavone treatment for acute menopausal symptoms. Menopause 14 (3 Pt 1): 468-473.

Doerge, D. R. and Sheehan, D. M. (2002). Goitrogenic and estrogenic activity of soy isoflavones. Environ. Health Perspect. 110 Suppl 3: 349-353.

Haselkorn, T., Stewart, S. L., and Horn-Ross, P. L. (2003). Why are thyroid cancer rates so high in southeast asian women living in the United States? The bay area thyroid cancer study. Cancer Epidemiol. Biomarkers Prev. 12 (2): 144-150.

Imhof, M., Gocan, A., Imhof, M., and Schmidt, M. (2008). Improvement of menopausal symptoms by soy isoflavones: A randomized, double-blind study. Planta Med 74 (9): 928

Knight, D. C., Howes, J. B., Eden, J. A., and Howes, L. G. (2001). Effects on menopausal symptoms and acceptability of isoflavone-containing soy powder dietary supplementation. Climacteric. 4 (1): 13-18.

Messina, M. and Redmond, G. (2006). Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid 16 (3): 249-258.

Teas, J., Braverman, L. E., Kurzer, M. S., Pino, S., Hurley, T. G., and Hebert, J. R. (2007). Seaweed and soy: companion foods in Asian cuisine and their effects on thyroid function in American women. J. Med Food 10 (1): 90-100.

Watanabe, S., Terashima, K., Sato, Y., Arai, S., and Eboshida, A. (2000). Effects of isoflavone supplement on healthy women. Biofactors 12 (1-4): 233-241.