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Soy thyroid function

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Soy and Health

By Mark Messina, Ph.D. (Adapted from an article in The Soy Connection newsletter, Spring 2001)
There has been much discussion of late over the possible adverse effects of soy consumption on thyroid function in both infants and adults. In fact, researchers from the National Center for Toxicology (NCTR), which is part of the U.S. Food and Drug Administration, have even expressed some concerns. But concerns about effects of soy on thyroid function are not new.

The first report that soybean produced goiter in iodine-deficient rodents was published in 1933. Subsequent animal studies published in the 1930s and 1940s produced similar findings. However, iodine fortification of the diet was shown to largely eliminate this problem, although soy-fed animals required approximately twice as much iodine to prevent enlarged thyroids as animals fed soy-free diets, and in some cases, slight histological abnormalities of the thyroid gland still persisted.

In the late 1950s, 10-15 cases of goiter were identified in infants fed non-iodized soy flour-based infant formula. However, this type of formula has not been used since the 1960s. Today, soy formula is based on soy protein isolate and is fortified with iodine. No cases of goiter in infants, due to the consumption of soy protein isolate-based iodized formula as is used today, have been reported in the scientific literature. Unfortunately, no studies solely designed to study the effect of soy formula on thyroid function in infants has been conducted. Still, given the millions of infants fed soy formula over the past three to four decades, it is reasonable to assume that if a problem existed, more than likely it would have been reported upon by pediatricians in the medical literature.

There are, however, at least limited data suggesting that infants with congenital hypothyrodism who consume soy formula require about 25 percent more synthetic hormone than infants with congenital hypothryoidism on non-soy formulas. But this may not be a systemic effect, since fiber supplements also necessitate that patients increase their thyroid hormone medication. This suggests soy, like fiber, may interfere with either the absorption of thyroid hormone (in the case of medication), or may interfere with reabsorption by interrupting the enterohepatic circulation of thyroid hormone.

Furthermore, a recently conducted human trial showed no effects of isoflavone supplements on thyroid function. And several other human studies have also found little or no effect of soyfoods.

One study conducted in Japan did find that soy consumption was associated with adverse effects (increase TSH levels) on thyroid function in older women, including an increase incidence of goiter. Women in this three-month study consumed 30 g/ day of pickled soybeans stored in rice vinegar. However, this study suffers from many design flaws, and although these results should not be ignored, they directly conflict with the results from several, better-designed studies.

For example, recent work from the University of Minnesota indicates that the consumption of isoflavone-rich soy over a three-month period had little effect on thyroid hormone levels in either pre- or post-menopausal women. And a recent double-blind study involving 38 postmenopausal women over the age of 64 who were not on hormone therapy, found no differences in thyroid function, based on measures of thyroid stimulating hormone (TSH), total thyronine (T4), and triiodothyronine (T3), between women given daily either a placebo or a supplement that provided 90 mg of isoflavones (expressed as aglycone units).

Overall, there appears to be little reason to think that in healthy adults, either soy isoflavone supplements, or soyfoods, will exert adverse effects on thyroid function. Even the NCTR researchers acknowledge that soy is not likely to be a problem in iodine-replete individuals. In contrast, arguably, in people who are predisposed to goiter or who are consuming marginally iodine sufficient diets, soy could conceivably a risk factor for goiter. So, it is important to note that as many as 10 percent of post-menopausal women may have subclinical hypothyroidism. This group may be sensitive to the adverse effects of weak goitrogens.

The iodine status of the U.S. population is considered adequate, although there is a downward trend in iodine intake and subsets of the population may have marginal intakes.

In conclusion, there is no reason to restrict soy consumption over concerns about the impact on thyroid function. When consuming large amounts of soy, it is important to make sure iodine intake is adequate. But of course, all people, regardless of their dietary pattern, need to consume sufficient amounts of iodine. Any concerns about the effect of soy on thyroid levels can be definitively addressed by having thyroid hormone levels measured. Even this step is not unordinary, since the American Thyroid Association recommends that all people have their thyroid hormone levels checked every five years beginning at the age of 35.

Permission is granted to reprint this information, as long as credit is given to Soyfoods USA

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