Certain foods such as millet, cassava, and cruciferous vegetables contain compounds thought capable of interfering with the body's ability to produce thyroid hormones.
Soybeans, too, contain isoflavones which in vitro (test tube) studies have shown to interfere with thyroid hormone-synthesizing enzymes.
However, the polyphenolic compounds (different classes of phytochemicals) found in fruits and vegetables are even more potent than isoflavones when it comes to potentially interfering with thyroid function. And of course, no one would recommend consuming fewer fruits and vegetables.
Also, what goes on in vitro doesn't necessarily occur in vivo (in organisms). Recently, in fact, several human studies have looked at the effect of soyfoods on thyroid function and have found no adverse reactions. (One of these studies was conducted for a full year.)
If soy does have some slightly negative effects on thyroid function, it would likely be a problem only in populations who have inadequate or very marginal intakes of the mineral iodide, which is needed for hormone synthesis. Therefore, maintaining an adequate intake of iodide intake is important, not forgoing soy.
There is an important exception, however, to the safety of soy. Several investigators have reported that infants with congenital hypothyroidism require larger amounts of synthetic thyroid hormone if fed soy formula compared to cow's milk formula. This likely has to do with the inhibitory effect of soy formula on thyroxine (thyroid hormone) absorption and possibly reabsorption.
One approach to lessen this effect would be to give the hormone separately from feeding times, but this might be difficult and would likely not totally eliminate the problem.
A healthy adult with adequate iodide intake, however, can keep enjoying soy without reservation. It offers so much in terms of nutrients and health benefits—some of which are only now being discovered.
Mark Messina, Ph.D., is a nutritionist and author. His work at the National Cancer Institute and National Institutes of Health helped identify research needs in the area of diet and cancer prevention.