Along with the increasing presence of soy foods in grocery stores and on restaurant menus has come increasing controversy over soybeans and thyroid health. We're not surprised to find strong conflicting opinions in this area because scientific research on thyroid and soy is both complicated and inconclusive. Here's what we know - and what we don't know - about this important issue.
You may sometimes hear soy foods being described as foods that contains "goitrogens," or as foods that are "goitrogenic." For helpful information in this area—including our WHFoods Recommendations—please see our article What is meant by the term "goitrogen" and what is the connection between goitrogens, food, and health?.
Research on soybeans and thyroid health has focused on one specific category of nutrient found in soybeans: isoflavones. Isoflavones are phytonutrients that belong to the much larger phytonutrient family called flavonoids. (Most foods contain flavonoids, and many vegetables are especially rich in this family of phytonutrients.) The best studied isoflavones in soybeans are genistein, daidzein, malonylgenistin, and malonyldaidzin. It is very clear that at a molecular and biochemical level, isoflavones in soy have the ability to change thyroid cell events in at least two ways that might be interpreted as posing a risk to the thyroid's health. The first type of change involves interference with activity of an enzyme called thyroid peroxidase (TPO). TPO is an enzyme that helps attach iodine to an amino acid called tyrosine. This iodine-tyrosine combination forms the basis for production of thyroid hormones. How isoflavones are able to interfere with TPO activity is not fully understood. But it's clear that they have the ability to do so. (In fact, it's clear that most flavonoids - and not only isoflavones - have the ability to interfere with TPO.) The second type of change involves uptake of iodine into the thyroid cells by a protein referred to as the sodium-iodide symporter (and abbreviated as "NIS," where the "N" comes from "Na," which is the scientific abbreviation for the element sodium). Like TPO activity, uptake of iodine into the thyroid cells by NIS can be interfered with by isoflavones. Once again, it's not 100% clear how isoflavones are able to interfere with NIS activity, but it's clear that they can.
Considered by themselves, these two biochemical events might justify strong concerns about the impact of isoflavones on thyroid function. But in this case, we have more information, including a good number of studies that show a lack of thyroid symptoms in adults (for example, no increase in the weight of the thyroid gland) even when there is evidence that TPO and NIS activity has been interfered with by soy isoflavones. In other words, even though scientists are certain that soy isoflavones can trigger these biochemical changes, the health consequences in adults do not appear to be automatic or guaranteed, and may be contingent on the involvement of many additional factors.
Many researchers agree on the list of additional factors that might combine with soy isoflavone intake to put the thyroid at added risk. From a nutritional standpoint, the mostly likely factor is iodine deficiency. High isoflavone intake, in combination with iodine deficiency, has been associated with decreased thyroid function in animals and in humans. "High intake" in these studies has usually involved intake of soy isoflavones in supplemental form, and/or intake of processed soy products. While iodine deficiency can itself be associated with decreased thyroid function, we have not seen studies on animals or humans in which the addition of an average daily serving of any ordinary soy food (for example, tofu, tempeh, soybeans, natto, soy miso, or soy sauce) resulted in greater thyroid dysfunction than was already present due to iodine deficiency. (Nor have we seen studies on animals or humans in which an average daily serving of any ordinary soy food - for example, tofu, tempeh, soybeans, natto, soy miso, or soy sauce - resulted in thyroid dysfunction all by itself.)
Along with iodine deficiency, we believe that there is good research to support a list of 3 additional factors that can combine with high intake of soy isoflavones to put the thyroid at risk. These 3 factors are: (1) intestinal problems, including poor absorption or colitis; (2) metabolic problems in the liver; and (3) the production of antibodies by the immune system to thyroid enzymes, receptors, or hormone precursors. As mentioned earlier, when everyday serving sizes of whole soy foods are combined with any of the health problems listed above, they do not appear to increase thyroid risk. However, we also think it makes sense to err on the side of extra caution here, and recommend that persons with a known health history of these problems seek the advice of their healthcare provider when trying to decide about the role of soy foods in their everyday meal plan.
Soy-Based Infant Formulas (SBIF) and Thyroid: A Special Case
The reason that we qualified several of our above statements as applying to adults (versus children or infants) was based upon our interpretation of the research on soy-based infant formulas (SBIF), and also upon some conclusions that seem logical to us when we think about the "big picture" involving infants and diet.
Several studies on infant feeding show greater rates of autoimmune thyroid problems in infants who are exclusively fed soy formula. Some infants may have had thyroid problems (or have been predisposed to thyroid problems) even before consuming soy formula. Others may have had problems with digestive function, liver function, or immune function that combined with soy formula intake to compromise the health of their thyroid. But even if these other factors were involved, the association between thyroid problems and exclusive intake of soy formula by infants still seems like a good reason to take a special look at this area of diet and health.
From a logical standpoint, we're equally concerned about the soy protein isolate (SPI) form of soy found in most infant soy formulas. SPI is a highly processed form of soybean, and it is very likely to expose an infant's digestive tract to unexpected proteins and protein components. Since the infant's immune system is functioning at a very early stage of development, this exposure to unexpected proteins and protein fragments would logically be expected to increase risk of adverse reactions, including allergic reactions.
At present, the American Academy of Pediatrics has established no special precautions for the use of soy-based infant formula (SBIF), except in the case of preterm infants or infants with digestive tract problems (specifically enteropathy or enterocolitis). Additionally, the Canadian Paediatric Association recommends the monitoring of thyroid hormone levels when using SBIF with infants who are born with hypothyroidism. If you are considering exclusive soy-based feeding for an infant, we believe a consult with your pediatrician or healthcare provider is worthwhile, even if the none of the above problems have been identified.
Based on research studies, culinary traditions, and the logic of whole, natural foods intake, we believe that the vast majority of adults can enjoy the tastes, textures, and nutritional benefits of soybeans or whole soybean-based foods (including tofu, tempeh, natto, soy miso, and soy sauce) without increased risk of thyroid problems. We make a definite distinction between these whole food-related forms of soy and soy isolates, extracts, or related supplements, and we believe that the enormous advantages of whole foods (versus processed components or dietary supplements) shines through in this health area. We do believe that there is sufficient evidence to raise concerns about soy intake for individuals with iodine deficiency, or problems with intestinal function, immune function, or liver function. If you think that you might fall into one of the categories above, we recommend that you consult with your healthcare provider when determining the best role for soy foods in your meal plan. We also recommend that anyone who is considering exclusive soy formula feeding of an infant consult with their pediatrician or healthcare provider when making this decision.
Brent GA. Environmental Exposures and Autoimmune Thyroid Disease. Thyroid 2010 July; 20(7): 755—761. doi: 10.1089/thy.2010.1636
de Souza dos Santos MC, Goncalves CFL, Vaisman M et al. Impact of flavonoids on thyroid function. Food and Chemical Toxicology, Volume 49, Issue 10, October 2011, Pages 2495-2502.
Doerge DR and Chang HC. Inactivation of thyroid peroxidase by soy isoflavones, in vitro and in vivo. Journal of Chromatography B, Volume 777, Issues 1—2, 25 September 2002, Pages 269-279.
Messina M and Redmond G. Effects of Soy Protein and Soybean Isoflavones on Thyroid Function in Healthy Adults and Hypothyroid Patients: A Review of the Relevant Literature. Thyroid. March 2006, 16(3): 249-258.