Many foods—including soy, broccoli, and millet—contain goitrogens, which are compounds that interfere with thyroid function. Most people can eat these foods regularly without problems. Problems do occur in people who have low intakes of the mineral iodine, which is needed for a healthy thyroid.
In the 1950s approximately 10 cases of goiter—a symptom of thyroid problems—were identified in infants consuming soy infant formula. These 50-year-old studies have been used to fuel arguments that soy is dangerous for infants—but in fact, they have no relevance to infants who are currently fed soy infant formula. Today’s formula is fortified with iodine and is processed differently than older formulas. As a result, there has not been one case of goiter reported in the medical literature in the more than 20 million babies fed soy infant formula over the past 40 years.
Soy doesn’t adversely affect thyroid function in healthy adults either. As long as people meet the recommendations for iodine intake, consuming large amounts of soy has not been shown to have adverse effects. Whether soy is harmful to those who have deficient iodine intakes hasn’t been determined. But if this turns out to be the case, the answer will be to consume adequate iodine, not give up soy.
This can be a relevant issue for vegans, however, since those who don’t consume milk can sometimes have low iodine intake. It really depends on where you live (or where your food comes from) since the amount in fruits and vegetables varies according to where they are grown.
The most consistent and reliable sources of iodine are milk (often held in tanks that are cleaned with an iodine solution) and seafood. Sea vegetables are one good source of iodine in vegan diets, although the amount varies greatly and some types may contribute excessive amounts of iodine. The most reliable source of this nutrient is iodized salt—or an iodine supplement. The recommended intake for iodine is 150 micrograms per day for adults, an amount that is included in many multivitamin tablets.