Low thyroid function is a common problem in women after menopause, affecting as many as 10 percent of older women. Decreased levels of thyroid hormones can cause weight gain, fatigue and intolerance to cold. In people who are already hypothyroid, compounds in certain foods might make the problem worse.
Results from animal studies suggest that soyfoods could be among those foods. Soybeans contain isoflavones, compounds that may be responsible for this relationship. But the same effects have not been observed in humans despite intensive investigation of this issue. One reason may have to do with the length of the clinical studies. Critics suggest that the human studies have been too short to identify thyroid problems in response to isoflavones and that only the results from prolonged consumption can prove these soybean components to be safe.
Researchers at the University of Messina in Italy recently addressed that criticism. They studied 389 women living an area where risk for thyroid problems is somewhat higher than usual. Half of the women were given 54 milligrams per day of genistein, which is the main isoflavone found in soybeans. The other half received a placebo. The 54-milligram dose was equal to the amount of isoflavones in about four servings of soyfoods like tofu and soymilk.
All of the women were followed for two years and about half of them for an additional year. At the end of the third year, the researchers assessed thyroid function based on a number of tests, including those that are considered to be extremely sensitive to changes in thyroid function. They found no evidence that genistein had any effect on thyroid function.
In fact, there was one interesting benefit: Levels of LDL-cholesterol dropped by about 10 percent in the women taking the genistein supplements, suggesting a potential heart-healthy benefit. Soyfoods were awarded a health claim for coronary heart disease in 1999 but this claim was based on the cholesterol-lowering effects of soy protein, not isoflavones. Based on these new 3-year results, soyfoods can safely be consumed even by women at elevated risk for thyroid problems. And if they are at risk for heart disease, soyfoods will likely provide some potentially important benefits.