If you have thyroid disease, you may have been told to avoid soy products.
However, new research shows this outdated advice is unwarranted.
Soy’s reputation as an endocrine disruptor, it turns out, was based on animal and laboratory studies.
Human trials, on the other hand, show soy has little to no effect on thyroid function.
This article discusses soy and thyroid health.
It explains the previous misbeliefs and current understanding of soy’s impact on thyroid hormones.
Soy also contains bioactive compounds known as isoflavones, a phytoestrogen similar to the hormone estrogen.
Soy and soy isoflavones have been extensively studied and found to have many beneficial health properties.
A 2019 meta-analysis of 18 clinical trials found soy has no effect on overall thyroid function.
In other words, soy’s reputation as an endocrine disruptor is not supported by the current research.
Iodine, an essential dietary mineral, is a component of thyroid hormones.
Soy is believed to inhibit thyroid hormone production by interfering with the entry of iodine into thethyroid gland.
This can trigger a feedback mechanism that stimulates the pituitary gland to secrete more TSH.
TSH normally functions to spur thyroid hormone production.
When thyroid hormones are low, TSH levels continue to build to excessive levels.
This can overstimulate the thyroid gland and cause it to enlarge, forming agoiter.
However, iodine deficiency is rare in the United States due to universal salt iodization.
This effect is likely due to the isoflavones and not soy itself, a 2018 clinical trial found.
In this study, participants were given either soy protein plus isoflavones or soy protein alone.
The current research also found women are more susceptible to thyroid problems associated with soy products than men.
The reason for this different response between men and women, however, is unclear.
Soy and Thyroid Medication
Soy may prevent optimal absorption of thyroid replacement medications like levothyroxine.
This can cause your medication to provide inconsistent results.
In general, thyroid medication should be taken on an empty stomach to avoid irregular absorption.
Soy-containing foods and beverages should be avoided for four hours before and after taking your dose.
How Much Soy Is Healthy?
Soy is also sold as a dietary supplement in capsules and powder as soy protein or soy isoflavones.
Summary
Eating soy in moderation does not impact thyroid health, according to current research.
Excessive soy consumption, however, may alter thyroid hormone levels and interfere with the production of thyroid hormones.
However, high doses of soy isoflavones have been shown to raise TSH and T3 levels.
Women are particularly prone to this effect.
The FDA recommends adults consume 25 grams of soy protein daily.
Additional research suggests limiting soy isoflavone consumption to 100 mg or less a day.
U.S. National Library of Medicine: MedlinePlus.Soy.
Ichan School of Medicine at Mount Sinai.Soy.
American Thyroid Association.Low iodine diet.
