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The Honolulu Advertiser
Posted on: Thursday, June 11, 2009

Soy products need not be avoided


By Amy Tousman

Q. Can eating soy products cause thyroid problems?

A. There is plenty of misinformation circulating on the Internet suggesting soy products can cause thyroid disease. Before throwing out your tofu, soybeans and soymilk, let's examine the facts.

There are two issues here: recommendations for folks who are healthy and recommendations for those who have hypothyroidism.

Hypothyroidism is when your thyroid gland cannot make enough thyroid hormone. Thyroid hormones control the way your body uses energy. The most common cause of hypothyroidism is an attack on the thyroid gland by the body's own immune system, rendering the thyroid gland non-functional. This type of hypothyroidism is thought to be genetic, rather than diet-related. Destruction of the thyroid gland through surgery or radiation treatment and iodine deficiency are other causes of hypothyroidism.

The connection between soy and the thyroid involves substances called isoflavones. Isoflavones have a similar chemical structure to thyroid hormone. Certain isoflavones in soy compete for the enzyme that is used to make thyroid hormone. This is where the rumor got started that soy is bad for the thyroid. However, this is not the whole story.

What the "avoid soy" Web sites fail to mention is that these effects are only seen when a person is iodine deficient. Iodine is also needed for thyroid hormone. Iodine deficiency is rare in the United States because iodine is added to salt and occurs naturally in dairy products, seafood and seaweed.

In folks with a healthy thyroid gland and adequate iodine intake, there's no need to limit soy. In fact, soy foods can promote good health by helping to lower your cholesterol, protecting bones against osteoporosis, relieving some symptoms of menopause and helping to prevent breast cancer.

For folks who have hypothyroidism and are being treated with synthetic thyroid hormones, some evidence suggests soy may interfere with the body's ability to absorb the medication. However, there's no evidence that adults with hypothyroidism need to avoid soy completely. If you eat soy foods often, consult your physician. He or she can use blood test results to determine if you need to increase your dose of medication.

If you have hypothyroidism, wait four hours after taking thyroid medication to consume soy products. This advice also applies to other products that may impair the body's ability to absorb thyroid medication, including high-fiber foods, iron and calcium supplements and antacids that contain aluminum or magnesium.