Does Soy Have a Negative Effect on Your Thyroid Function?

Does Soy Have a Negative Effect on Your Thyroid Function?

The benefits of soy for its protective effects against heart disease have been promoted for several years, making it a popular supplement among the post-menopausal population.  More recently there have been negative  relationships drawn between breast cancer and some soy components which mimic the female hormone, estrogen.  Now further attention has brought soy supplementation into the spotlight for possible negative effects the isoflavones present in soy products can have on the function of the thyroid gland.

Probably best known for its role in maintaining metabolism, the thyroid gland does have many essential functions.  Most people are aware that an underactive thyroid can cause weight gain, or at least difficulty with attempts at weight loss.  Some other lesser-known symptoms of hypothyroidism may include feeling unusually cold more often, suffering fatigue and sluggishness, and having irregular menstrual cycles.

So what's the relationship between soy and thyroid function?  Some of the soy components, known as isoflavones, indirectly inhibit synthesis of the thyroid hormone.  If your body makes less of this hormone, you will suffer the symptoms of hypothyroidism.  Keep in mind, of course, that many times these symptoms can be due to other factors--including a poor diet and inadequate sleep.

After reviewing several studies on the effects of soy on thyroid function it appears those most at risk are people who consume soy supplements or very large amounts of soyfood.  On average  these studies suggest limiting intake to 30-60 grams per day of the isoflavones.  Examples of soyfoods containing 30 grams of isoflavones follow, and are not always typical serving sizes: 12 ounces of soy cheese, 6 ounces of soy hot dogs, 30 ounces of soy breakfast 'sausages', or 12 ounces of soymilk each contain approximately 30 grams of isoflavones.  The most likely scenario for an effect on the thyroid would be someone eating tofu on a daily basis, or someone taking supplements containing the isolated forms of the isoflavones, genistein and daidzein in amounts that exceed 60 grams.

Should you stop eating foods with soy in them?  Even the popular  healthfood guru, Dr. Andrew Weil, recommends that soy supplements should be avoided (to prevent effects on thyroid function) but concedes it is unlikely consumption of foods containing soy would have such a dramatic effect on one's thyroid.

Soy has its benefits when used in moderation and is a good substitute for meat products in people who follow vegetarian diets.  But always remember there can be a drawback to consuming too much of a good thing. Use common sense and  avoid extremes of any supplements to help assure your best state of health.  Talk with your doctor if you take soy supplements, especially if you are having symptoms of hypothyroidism or are already being treated for the condition.