People managing a chronic condition like hypothyroidism want to avoid simple activities that might interfere with the best outcome. They may have heard that you have to take your thyroid medication early in the morning, an hour before coffee or breakfast. But what about other dietary considerations? Does soy interfere with thyroid hormones?
Does Soy Interfere with Thyroid Treatment?
Q. I have hypothyroidism and have read from some sources that soy products can make symptoms worse, while others report that soy is beneficial for thyroid disorders. Should I avoid soy or consume it?
A. Although the idea of soy disrupting thyroid function has been kicking around for decades, the science is surprisingly thin and quite confusing. An analysis of data from the National Health and Nutrition Examination Survey (NHANES) found that people consuming more flavonoids, including those from soy, had lower levels of thyroid hormone (PLoS One, May 21, 2024). That finding is slightly worrisome but inconclusive.
A six-month randomized trial in China found that soy and the soy isoflavone daidzein are safe, without any impact on thyroid function (Phytotherapy Research, Oct. 2021). Some studies in rats demonstrate that soy isoflavones can inhibit thyroid peroxidase in certain situations such as iodine depletion (Molecules, March 10, 2023). Most people using iodized salt in the US would not have inadequate iodine intake. Consequently, you might not want to make soy a mainstay of your daily diet, but including it occasionally is probably just fine.
Can You Take Thyroid Medicine in the Evening?
Q. I have hypothyroidism. I read that soy products can make symptoms worse. I’ve seen other reports that soy is beneficial for women with thyroid disorders.
I take my thyroid medicine first thing 30 minutes before I eat or drink anything. Then I have soy milk and fiber with my cereal.
Can I take thyroid medicine several hours after dinner (with no soy or fiber) just before bedtime? Should I avoid soy? I hope you can clear up my confusion.
A. You are not the only one who is confused. Researchers found that compounds in soy can inactivate an enzyme important for thyroid function (Environmental Health Perspectives, June 2002, Suppl. 3). Ever since then, scientists have been arguing about the practical importance of this discovery. People low in iodine seem especially susceptible, but we don’t know how common that is in the US. People consuming cow’s milk rather than soy milk may have an easier time getting adequate iodine (Public Health Nutrition, Sep. 2023).
Learn More:
Our eGuide to Thyroid Hormones covers several controversial issues, including the use of Armour desiccated thyroid, the balance between T3 and T4 hormones and a list of foods that can affect thyroid function. You might also want to listen to one of our podcasts on treating hypothyroidism, such as Show 1330: Rethinking Hypothyroidism.
It’s smart to avoid fiber when you take your pill, as that might reduce its absorption. In fact, taking it in the evening at least a few hours after you finish eating gives the best results (Clinical Endocrinology, Jan. 2007).
