Soy and the Thyroid

There are long-standing concerns that soy may have negative effects on thyroid function. Soy falls into a category of foods known as goitrogens — vegetables, grains and foods that promote formation of goiter — an enlarged thyroid. Some goitrogens also have a definite antithyroid effect, and appear to be able to slow thyroid function, and in some cases, trigger thyroid disease. As usual, things aren’t entirely clear with respect to soy.

The link between isoflavones and decreased thyroid function is, in fact, one of the few areas in which soy intake has called into question as problematic. Isoflavones, like genistein appear to reduce thyroid hormone output by blocking activity of an enzyme called thyroid peroxidase. This enzyme is responsible for adding iodine onto the thyroid hormones.

A quick look at the literature will reveal studies on both sides of the argument.  On the other hand, a quick google search reveal hundreds of websites claiming that soy is a toxin to the thyroid gland.

A recent study published in 2006 in the journal Thyroid seems to clear up some of the confusion (1). The researchers looked at 14 trials involving soy, and in 13 out of 14 trials, either no effects or modest changes were noted in thyroid function as a result of soy consumption. The researchers state that there is  little evidence that “in euthyroid, iodine-replete individuals, soy foods, or isoflavones adversely affect thyroid function.” The researchers also stated that “there remains a theoretical concern based on in vitro and animal data that in individuals with compromised thyroid function and/or whose iodine intake is marginal soy foods may increase risk of developing clinical hypothyroidism. Therefore, it is important for soy food consumers to make sure their intake of iodine is adequate.” They also claim that “some evidence suggests that soy foods, by inhibiting absorption, may increase the dose of thyroid hormone required by hypothyroid patients.”

This study is suggesting that soy is safe — unless you have a thyroid condition, or iodine deficiency. It also suggests that soy foods can inhibit absorption of thyroid medication. The study also goes on to say that despite these factors, soy foods are in fact safe, and all that is needed is to ensure sufficient iodine in the diet along with regular retesting and dosage changes of thyroid medication to make up for any effect the soy has on the thyroid medication.

So, where does that leave us? Here are my recommendations:

Excess consumption of soy can affect thyroid function, if you have a thyroid condition, or are not getting enough iodine.  For the average patient (without thyroid concerns) you’re unlikely to get too many isoflavones as a result of adding soy foods to your diet — but you probably will take in too much if you take soy supplements in pill form.

I can say that in 7 years of practice, I’ve only seen a handful of patients who have clearly had their thyroid function altered by eating soy foods.  And, in most of those cases, a vegan diet with very high intakes of soy foods was to blame.  I have never seen thyroid function be affected my low to moderate intake of soy foods.  But, if you have a thyroid condition, especially if you have ongoing symptoms, a soy-free trial may be in order.

If you are a thyroid patient being treated with medications, and you’re still suffering from hypothyroidism symptoms, consider eliminating soy from your diet to see if that helps relieve symptoms.

If you are eating soy foods, you may want to avoid genetically-modified soy, until the debate over their safety has been resolved.

If you are going to eat soy, select fermented and food forms of soy, for example tofu, tempeh, and miso. Avoid processed soy products — including soy powders, protein shakes, and other processed forms of soy.

1.  Messina, M., Redmond, G.  Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006 Mar;16(3):249-58.