Despite Concerns, Soy Probably Safe for Thyroid

From Reuters Health Information

By Amy Norton

NEW YORK (Reuters Health) Apr 14 - Despite some concerns to the contrary, the soy-based dietary supplement genistein may not harm postmenopausal thyroid function, a new study finds.

Genistein is a type of soy isoflavone, a plant chemical that is structurally similar to estrogen. In a 2007 clinical trial, Italian researchers found that genistein supplements, along with calcium and vitamin D, appeared to help boost bone mass in postmenopausal women.

In this latest study, the researchers evaluated data from the same clinical trial -- this time looking at whether the genistein supplements had any effects on the women's thyroid function. The question stems from lab research showing that genistein and other isoflavones may decrease thyroid-hormone production.

The earlier research had suggested that isoflavones can interfere with iodine, explained Dr. Francesco Squadrito of the University of Messina, the senior researcher on the study.

However, he told Reuters Health by email, those studies used genistein doses that were 10 to 250 times higher than the doses used in his team's clinical trial -- 54 mg/day.

Dr. Squadrito and his colleagues found that among 77 study women followed for three years, those who used the genistein supplement showed no overall differences in thyroid function compared with women who were given a placebo.

The findings appeared online March 31 in the Journal of Clinical Endocrinology and Metabolism (http://jcem.endojournals.org/cgi/content/abstract/jc.2009-2779v1).

According to Dr. Squadrito, it is not surprising that studies would find thyroid effects of very high doses of genistein. However, he said, women are unlikely to consume such levels from soy-protein products or from soy foods.

As far as thyroid function is concerned, he said, "it is possible to conclude that genistein therapy is safe in postmenopausal women -- at least at the dose of 54 mg a day."

Soy contains several types of isoflavones, however, and more studies are needed to establish the safety of those compounds, according to Dr. Squadrito and his colleagues.

J Clin Endocrinol Metab 2010.