One of the most prevalent thyroid problems with a link to soy consumption is called hypothyroidism. It is a condition in which the gland, a butterfly-shaped organ in the neck, slows its activity and produces less thyroid hormone. The slowing down is gradual and can take months or even years before an individual notices any change.
Hypothyroidism's symptoms are rather subtle. They include fatigue, slight weight gain, constipation and often feeling cold. The condition also contributes to menstrual irregularities and even interferes with fertility.
The first link between soy consumption and hypothyroidism occurred when researchers began suspecting soy-based baby formulas were responsible for reported increases in the condition among infants.
The most recent such study was published in the February 2000 issue of the New Zealand Medical Journal. Its leading author, Mike Fitzpatrick, an environmental researcher in New Zealand and an expert on the subject, found the isoflavones in soy adversely affected the thyroid gland. Isoflavones have estrogen-like activity, which means they could behave like a hormone.
"As well as the effects on thyroid function, there is also strong evidence that soy may affect the uptake of essential minerals, immune system function, pancreatic function and sex-steroid hormones," Fitzpatrick told United Press International.
"On a very dark note, there is even evidence that certain cancers may be accelerated by the isoflavones that are found in soy products. This is particularly worrying given the widespread promotion of soy as a foodstuff that will help both prevent and fight cancer."
Although there has been some question whether soy products are dangerous for breast cancer patients, there is no concrete evidence linking soy consumption with the spread of various cancers.
A 1997 study published by the National Center for Toxicological Research in Jefferson, Ark., a division of the U.S. Food and Drug Administration, also found isoflavones in soybeans suppressed thyroid function and interfered with thyroid hormone balances. A researcher there, who spoke on condition of anonymity, told UPI ongoing animal studies investigating this perceived connection "make some clear conclusions" suggesting soy products influence proper thyroid function.
"The risks and benefits are different for different populations," the researcher said. "The idea is to educate."
For example, "we see the postmenopausal women population most at risk for thyroid problems and they're a group being heavily targeted for an intense marketing campaign for soy."
Thyroid conditions, particularly hypothyroidism, are more common among women than men, regardless of their dietary habits, according to the American Thyroid Association.
Dr. Hossein Gharib, president of the American Association of Clinical Endocrinologists and a professor of medicine at the Mayo Clinic in Rochester, Minn., told UPI the link between soy and thyroid problems is "an unproven belief. There's no proof that this is so."
Although there is some evidence suggesting soy-based infant formulas are linked to thyroid dysfunction, no such evidence exists for adults consuming moderate amounts of soy, Gharib said.
"There's a general belief there's some interference, but there's no documentation," he said.
Gharib said it is estimated about 10 percent of the U.S. population -- about 25 to 30 million people -- have hypothyroidism and the causes could be many, including genetic.
Keith Ayoob, a spokesman for the American Dietetic Association, said soy is widely consumed and therefore this is evident not everyone who eats soy foods has a thyroid problem.
"Billions of people eat this stuff, so if there was anything glaring in there, we'd probably know it on some level," Ayoob told UPI. "I'm not aware of any studies of any vegetarians who have increased problems with hypothyroidism."
All health experts agree more research is needed to determine what link, if any, exists between soy and thyroid problems.
Ayoob noted soy's health benefits outweigh any downsides. Soy protein has been conclusively linked to lower risks of heart disease, he explained, and soy is also a good source of dietary fiber and protein that is low in fat and cholesterol. Many grocery store products are supplemented with soy, such as soy milk, as soy's popularity grows in the United States.
Foods high in soy also include tofu, tempeh and soybeans, as well as many of the substitute meat products on grocery store shelves, such as soy burgers, soy hotdogs and soy sausages.
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'
Exploding whale video goes viral on Internet