How to Diet: Does Soy Affect Thyroid Function?

The rap: “Soy will destroy your thyroid.”
—foodrenegade.com

The real story: “Eating soy foods doesn’t harm the thyroid glands of most people,” says Hossein Gharib, president of the American Thyroid Association and an endocrinologist at the Mayo Clinic College of Medicine in Rochester, Minnesota.

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Doctors gauge thyroid function by looking at blood levels of thyroid hor¬mone (thyroxine, or T4) and thyroid stimulating hormone (TSH), which prods the gland to produce more T4 when needed.

In test tubes, soy isoflavones can interfere with the enzyme that helps the thyroid make T4. In most studies done so far in healthy people, however, soy foods or isoflavones have no impact on levels of T4 or TSH.

That was the case for 63 women who ate two to three servings a day of either soy foods (yogurt, burgers, and milk) or beef, chicken, and dairy for 10 weeks. And it was true for 206 middle-aged or older women who took 90 to 200 mg of soy isoflavones or a placebo every day for six months to two years.

But soy foods may make people with subclinical hypothyroidism more likely to become hypothyroid, says Gharib.

About 1 percent of the U.S. population has hypothyroidism (an underactive thyroid), which means they have low T4, high TSH, and symptoms like fatigue and increased sensitivity to cold. Another 3 to 5 percent have subclinical hypothyroidism. They have no symptoms and their T4 levels are normal, but their TSH is high (a sign that the body is working hard to keep T4 levels up).

In 2011, British researchers gave 60 middle-aged and older people with subclinical hypothyroidism a daily dose of 30 grams of soy protein. For eight weeks the powder contained 16 mg of soy isoflavones and for eight weeks it contained only 2 mg. Six of the 60—all women—progressed to hypothyroidism while taking the higher dose.

That’s just one small study that needs to be confirmed. In the meantime, “If a routine blood test shows that someone has an elevated TSH level of 5, 6, 7, or 8 and they want to continue eating a lot of soy, they should have their TSH levels tested every six to 12 months to make sure they aren’t progressing to full-blown hypothyroidism,” says Gharib.

Sources: J. Womens Health 20: 771, 2011; Arch. Intern. Med. 171: 1363, 2011; J. Med. Food 6: 309, 2003; Clin. Endocrinol. Metab. 96: 1442, 2011.

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