Soy and goiter
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Soy and Thyroid
Claim : Soy contains natural chemicals known as goitrogens that interfere with thyroid function. These can cause an enlargement of the thyroid gland (a "goiter") and symptoms of hypothyroidism, such as lethargy, dullness, coldness, and depression.
It is true that soy contains goitrogens, as do many other foods such as cruciferous vegetables (cabbage, cauliflower, broccoli, and brussel sprouts), sweet potatoes, lima beans, and millet. However, these foods have been found only to cause problems when iodine intake is low , because goitrogens do their damage by interfering with the thyroid gland's ability to utilize iodine. Between 1951 and 1961, several cases of goiter were diagnosed in infants who had been fed infant formula made from soy flour. These cases are frequently cited by the anti-soy lobbyists to prove soy damages thyroid function (especially in infants). But not a single case of goiter in infants has been caused by soy formula since the 1960s. At that time the soy formula base was changed from soy flour to soy protein isolates, which are low in goitrogens, and manufacturers began fortifying soy formula with iodine.
Soy does not cause thyroid problems in healthy, well-nourished people who are not deficient in iodine. However, people who do not have a reliable source of iodine could increase their risk of thyroid problems if they eat a lot of soy and/or other foods rich in goitrogens. Iodized salt, dairy products, and fish are the main dietary sources of iodine, and most multivitamin/mineral supplements provide the recommended daily allowance. So the answer is not to avoid soy or cruciferous vegetables, but to get enough iodine.
Conclusion : There is no evidence that eating soy foods regularly causes thyroid problems in healthy people who include sufficient iodine in the diet.