There’s a perception among some people that certain types of greens increase your risk of thyroid cancer.

I know because a few people have asked me about it lately.  Of course the first impression upon hearing this sort of thing is to scratch your head.  It’s been beaten into our heads that the one thing you should do to avoid cancer is to eat more greens, not less.

What’s even weirder is that unlike some other health rumors, this one actually has some teeth.  If you read through PubMed or Google Scholar you can find some studies that talk about the positive relationship between the two.  That doesn’t make it true, but it’s better than the cayenne pepper/watermelon diet and stuff like that which is only talked about by homeopathic cranks.

So…..is there any reason to believe eating veggies increases your chances of having thyroid problems?

Luckily the answer is no.  And like most other diseases, greens seem to have a mildly protective effect.

The Skinny on Thyroid Cancer, Veggies and Iodine.

The biggest dietary component of thyroid functioning is easily iodine.  Extreme scarcity or abundance of the nutrient can cause your thyroid to do weird things, some of which can lead to thyroid cancer.

Cruciferous vegetables contain some compounds which are “goitrogenic”, which means they mildly suppress thyroid function.  Soy does the same thing.  In a few instances it’s been shown that people with extremely low levels of iodine intake accompanied by large amounts of cruciferous vegetables have an increased risk of thyroid cancer.

It’s my best guess that these studies are the basis of the cruciferous vegetables/thyroid cancer link.

The Best Evidence Says No

However, the majority of evidence suggests what common sense would lead you to believe:  eating your greens won’t increase your risk of cancer.

The best example is a  meta-analysis of the issue that was published in the journal Cancer Causes and Control.  It aggregated the results from the 11 most important studies done on the effect and found these results:

This combined analysis indicates that cruciferous vegetables are not positively related to thyroid cancer risk. Their effect does not seem to be substantially different from that of other vegetables, which appear to be protective on this cancer. 

The majority of other studies find about the same thing.

Why Cruficerous Vegetables Don’t Cause Thyroid Cancer

The presence of “goitrogenic” compounds in cruciferous vegetables is what accounts for their ability to effect thyroid function.  More specifically, they inhibit an enzyme called thyroid peroxidase which stimulates the production of thyroid hormone.

For the vast majority of people, this effect is simply not that big of a deal.  The presence or absence of these compounds in the diet pales in comparison to the presence of iodine and harmful radiation in causing thyroid cancer.  Only in extreme cases of iodine deficiency can the presence of cruciferous goitrogens seriously increase the risk of thyroid cancer.  If you live in America then you probably eat iodized salt at least once in a while, which makes the likelihood of this situation happening to you practically zero.  So you’re safe.  Don’t sweat it.

Cruciferous Vegetables Might Actually Protect Against It

Like I stated before, in most studies the presence of cruciferous vegetables decreases one’s chance of getting thyroid cancer, not increase.

The compounds that depress thyroid hormone function are called isothiocyanates.  In general these compounds are considered to be very healthy for you, and tend to reduce various types of cancers such as breast and   prostate cancers.  In the absence of iodine deficiency they seem to have the same effect on thyroid cancer cells as well.

I don’t know of human clinical studies (a large emission, I digress), but cancerous thyroid cells that are treated with the isothiocyanates 3,3?-diindolylmethane and indole-3-carbinol cause it to retreat.

This makes sense if you consider that these compounds in general cause thyroid function to slow down.  Again, it’s possible in the presence of unusual thyroid conditions this might not be the case.  But in any event they don’t seem to be the cause of any problems, just the multiplier.

References

Higdon, Jane, et. al. “Cruciferousvegetables and human cancer risk: epidemiologic evidence and mechanistic basis”

http://www.sciencedirect.com/science/article/pii/S1043661807000321

Horn-Ross, Pamela, et. al. “Phytoestrogens and Thyroid Cancer Risk The San Francisco Bay Area Thyroid Cancer Study”

http://cebp.aacrjournals.org/content/11/1/43.short

Bosetti, Cristina, et. al. “A pooled analysis of case–control studies of thyroid cancer. VII. Cruciferous and other vegetables (International)”

http://www.springerlink.com/content/j63k422204767104/

Tadi, Kiranmayi, et. al. “3,3?-Diindolylmethane, a cruciferous vegetable derived synthetic anti-proliferative compound in thyroid disease”

http://www.sciencedirect.com/science/article/pii/S0006291X05021194

Marine, David, et. al. “Studies on Simple Goiter Produced by Cabbage and Other Vegetables”

http://ebm.rsmjournals.com/content/26/9/822.short

Dal Maso, Luigini, et. al. “Risk factors for thyroid cancer: an epidemiological review focused on nutritional factors.”

http://www.springerlink.com/content/1u6171713356j03t/

Memon, A, et. al. “Benign thyroid disease and dietary factors in thyroid cancer: a case–control study in Kuwait”

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2375394/

Truong, Therese, et. al. “Role of dietary iodine and cruciferous vegetables in thyroid cancer: A countrywide case-control study in New Caledonia”

http://hal-riip.archives-ouvertes.fr/docs/00/47/13/39/PDF/dietary_factors_in_thyroid_cancer_20100308_CCC_R1.pdf