It has been frequently suggested that eating soy may interfere with thyroid function, possibly inducing hypothyroidism (low thyroid function). Soy falls into a category of foods known as goitrogens—foods and supplements that prevent proper thyroid hormone production and cause the thyroid gland to enlarge. Soy may also impact the thyroid by other mechanisms, such as preventing the action of thyroid hormones throughout the body and reducing absorption of thyroid medications from the intestines.
The evidence regarding the impact of soy on thyroid health is limited, and some studies do not show a link between soy and TSH or thyroid function. At the current time, there is no clear consensus regarding how much or how little soy is safe for thyroid function.
To further complicate matters, it has also been shown that soy can improve your risk of heart disease, and can help optimize your glucose, cholesterol, and fat levels if you have subclinical (not yet symptomatic) hypothyroidism. The importance of this is that thyroid disease increases the risk of heart disease, and reducing that risk at early stages can prevent the development of serious cardiac complications.
With all of this said, some thyroid experts recommend that you eat soy products sparingly if you are at risk of thyroid disease or if you already have thyroid disease.
Soy’s Effect on How the Thyroid Functions
Generally speaking, soy is a healthy source of protein. It is found in tofu, tempeh, miso, and edamame beans, and is also used as a filler in processed meats and in the manufacturing of meat and dairy substitutes. It does, however, have effects that can affect the thyroid.
Decreased Thyroid Hormone Production
Thyroid hormones are produced in the thyroid gland. Iodine, an essential dietary mineral, is a component of thyroid hormones. Goitrogens inhibit thyroid hormone production by interfering with the entry of iodine into the thyroid gland. The resulting low levels of thyroid hormone in the body trigger a feedback mechanism that stimulates the pituitary gland to secrete more thyroid stimulating hormone (TSH).
TSH normally functions to spur thyroid hormone production. When thyroid hormones remain low due to the effect of a goitrogen, TSH levels continue to build to excessive levels, overstimulating the thyroid gland and causing it to enlarge, forming a goiter.
Other goitrogens include cassava, broccoli, cauliflower, cabbage, arugula, bok choy, Brussels sprouts, kale, collard greens, horseradish, radish, and wasabi.
7 Essential Facts About Goitrogens and Diet
Inhibition of the Thyroid Hormone Actions
In addition to interfering with iodine entry into the thyroid gland, soy may also inhibit the action of thyroid hormones in the organs of the body. Some studies in humans have documented changes in thyroid function in response to soy, but the mechanism by which these changes occur is not clear. A 2016 study published in Public Health Nutrition, for example, concluded that the chances of having high TSH were quadrupled in people who ate two daily servings of soy foods versus those who didn’t eat any at all.
Most of the studies that have identified an association between soy and thyroid disease have shown that women are far more affected by this link than men are. The reasons for this different response between men and women is not clear.
Soy and Thyroid Disease Management
If you use thyroid replacement medication, it is important to know that soy can prevent optimal absorption of your thyroid medication, resulting in inconsistent medication effects.
If you take thyroid medications, you should also be aware that several other components of your diet, including calcium and iron, can also prevent adequate absorption of your medication.
In general, it is recommended that you take your thyroid medication on an empty stomach to avoid irregular absorption. If you eat soy-containing foods, be sure avoid eating them four hours before and after taking your dose.
Radioactive iodine (RAI) therapy is used for some types of thyroid disease, and the radioactive iodine must enter into your thyroid gland for this treatment to work. If you are receiving radioactive iodine, you need to steer clear of soy and other goitrogens and follow a special diet to ensure that your therapy will be effective.