Thyroid Disease and Soy

Maybe you have an existing medical condition and you’re concerned about your soy consumption. Or perhaps, you’ve heard rumours about the effects of soy on your thyroid. Well, from the detailed research we’ve tracked down, you don’t have to worry about eating whole soyfoods each day — although some on medication for their thyroid have been recommended to avoid the soy at the same time as taking their medication. As always, consult with your doctor. Here’s what we found…

Here’s a Q and A from Dr, Dr. Joel Fuhrman’s website (author of “Eat to Live”):

Q: I’ve recently read that walnuts, soy products and high fibre foods are contraindicated to the efficacy of synthroid. I am hypothyroid, take .75/day, and all of those foods are a significant part of my diet. What, if any, foods effect the thyroid in this way?

A: The relationship between certain high fibre foods and binding of the drugs in the digestive tract inhibiting absorption of thyroid replacement therapy is slight. The PDR (Physician’s Drug Reference) recommends avoiding such foods at the exact time of taking the medication; however studies of those who take the medication with a meal compared to those who did not showed such a small difference that doctors don’t usually bother even mentioning it.

There was one case reported in the medical literature of a person who had to Increase the dose of her Synthroid because she took it at the same time as her high protein soy shake each morning. To sum up, it is not important, but if you can take your medication an hour before a meal or at bedtime and see if you can get by with a hair less, but I doubt it, especially because you are taking such a low dose already. JF

From Virginia Messina, MPH, RD & Mark Messina, PhD:

Soyfoods and the Thyroid

Many foods contain goitrogens, compounds that interfere with thyroid function (and in extreme cases can cause an enlarged thyroid, called a goiter). Along with soyfoods, millet, cruciferous vegetables and other foods contain goitrogens. Generally, these foods cause problems only in areas where iodine intake is low since this mineral is important for thyroid function. The effects of iodine deficiency can be made worse if the diet is high in goitrogens.

Although a concern about soy and thyroid function may be news to many vegans, it has actually been a focus of research for more than 70 years. Between 1951 and 1961, this research took on a special importance when about 10 cases of goiter were diagnosed in infants who had been fed infant formula made from soy flour. These old studies form some of the basis for arguments that soy is dangerous for infants. However, the situation for today’s soy formula-fed infant is very different. Since the 1960’s, soy-based infant formula has been made from soy protein isolate (which does not contain the goitrogens component; soy flour formulas did) and it is fortified with iodine. No cases of goiter have been diagnosed in infants fed this formula in the past 40 years.

Nor is there any evidence that consuming soy causes thyroid problems in healthy, well-nourished people who have adequate iodine in their diet. However, it is possible that eating a diet with generous amounts of soyfoods could be a problem for people whose iodine intake is marginal. And that might just include some vegans, since the main sources of iodine in western diets are fish and milk. But the appropriate response to this is not to limit healthful soyfoods; it’s to get enough iodine. Vegetables have varying amounts of iodine depending on where they are grown. In some parts of the world–specifically northern Europe–vegans may have low intakes of iodine. Foods that can supply iodine to vegan diets are sea vegetables, although contents vary quite a bit. Fortified foods are also a good source. Iodized salt is about the most reliable source. Vegans should be sure that, when they season foods with salt, it is iodized. If this isn’t a regular part of your diet, use an iodine supplement.

CONCLUSION: Soyfoods may contain goitrogenic compounds as do other foods. There is no evidence that regularly eating soyfoods causes thyroid problems in people who eat a balanced diet. Vegans should make an effort to include adequate sources of iodine in their diet.

(See their websites at Veg RD

Comments from readers like you on Marie Oser’s “Soytalk” board:

For those of you who are concerned about the possible negative impact of soy on the thyroid — I have been taking Synthroid for an under active thyroid for many years(years before I started consuming soy products). For the last couple of months I have been eating about 4 ounces of tempeh six nights a week with dinner, as well as miso several nights a week. For a couple of years before that, I was eating tofu on an almost nightly basis.

I just got the results from my blood test and my levels, according to my Dr., were exactly where they should be. In other words my soy consumption had no affect whatsoever on my thyroid levels. My Synthroid prescription level is the same as it was twenty years ago.

I have Hashimoto’s Disease and I eat soy products, but not in excessive amounts. I usually have about 3/4 c soy milk per day, and I eat tofu or soy beans maybe twice a week at most. Once or twice a week I might also have some kind of cake/dessert made with tofu. I asked my endocrinologist about consuming soy, and he seemed to think it was okay in the amounts that I consumed, particularly seeing that my blood work has been stable since I first saw him. He did warn me about the consumption of seaweed, which can cause thyroid problems as well, if eaten on a regular basis.

Personally, the only time I’ve ever had serious problems with my thyroid function, as well as other health problems, was when I first started developing thyroid disease (about 6 years ago), and when I went on a raw foods diet for about eight months (about 2 years ago). I am not concerned about the amount of soy that I eat, and I have read a lot of the anti-soy articles that have been written. I don’t think excess consumption of processed foods in general is a good idea for anyone, regardless of what kind of health you are in.

One idea, if you are concerned, would be to have blood work done *before* you start consuming soy products, start consuming soy in *small* amounts, have your blood work done in 4-6 weeks time, and see what kind of change has occurred. Please talk to your endo before doing this though; see what s/he has to say. If there is not much of a change (i.e you don’t need your T3 or T4 meds upped or lowered), then you are probably okay with *small* amounts.

Also, my endo *did* say to avoid consuming soy products within 2-3 hours after taking my meds, because soy products can interfere with the meds doing their job properly. Good luck!

Hello there!:) I have Grave’s Disease (which is actually like the opposite of Hashimoto’s). Grave’s is an overactive and Hash. is under active. Anyways, when I was first diagnosed, my endo told me to not eat any soy and that i should consume lots of protein (in other words: she said i should eat meat again and it would help me heal faster). Sadly I believed her and I think suddenly eating meat again may have made things even harder. I switched endos and he said that I could still be a vegetarian and that small amounts of soy should be no problem. He said that since they watch my levels so closely, if the soy were to effect it- they would know. I did the Radioactive Iodine Treatment, which made my thyroid under active and now finally my meds have stabilized my thyroid levels. He says that I can go ahead and eat soy just like any regular person now.


Now that you’ve learned the truth about soy and thyroid disease, why not check out quality research relating to soy and your healthy diet? We also have information on soy and thyroid cancer.