Could Kale Cause Thyroid Problems?

Thursday Feb 19 | BY |
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Baked Kale Chips

Is it possible that too much kale could be bad for you?

American novelist Edna Ferber was quoted as saying, “Perhaps too much of anything is as bad as too little.”

A lot of people were shocked to hear similar wisdom applied to…


Is it possible this highly revered vegetable could be falling off its pedestal?

From Superstar to Superbad?

U.S. farmers grew 57 percent more kale in 2012 than in 2007, with California, Georgia, and New Jersey leading the nation in production. Whole Foods Market sells more of the prized veggie than any other green leafy type.

People “feel virtuous after eating it,” says chef and author Alison Fishman Task. Full of nutrients and capable of imparting numerous health benefits, kale experienced a meteoric rise over the last few years, surpassing spinach as America’s favorite superfood.

It’s become so popular that in the summer of 2014, one of the world’s leading kale seed suppliers struggled to meet demand. (They quickly recovered.)

But then came the famous New York Times article entitled, “Kale? Juicing? Trouble Ahead.”

In case you didn’t see it, it’s author Jennifer Berman’s story of being diagnosed with hypothyroidism (underactive thyroid), and subsequently giving up her beloved kale smoothies—because she read that they could increase risk of thyroid problems.

Could it be true? Could all those kale juices actually be bad for you?

Could We Be Eating Too Much?

Look up “foods to avoid with hypothyroidism” and you’ll see kale high up on the list, along with its cruciferous cousins, including broccoli, Brussels sprouts, cauliflower, bok choy, and cabbage.

These foods, we’re told, can interfere with thyroid function.

Where did that come from?

Scientists have actually been aware of this possibility for decades. In 1982, for example, G. Roger Fenwick and colleagues published an animal study showing that very high intakes of cruciferous vegetables could cause hypothyroidism (low levels of thyroid hormone).

In 2010, doctors from the New York University School of Medicine had a letter published in the New England Journal of Medicine that described an incident of an 88-year-old woman developing severe hypothyroidism after eating about 1.0 to 1.5 kg of raw bok choy daily (2.2 to 3.3 pounds) for several months. (She thought it would help control her diabetes.)

Since an estimated 12 percent of the U.S. population will develop a thyroid condition during their lifetime, these studies are concerning.

Is it possible that we’re all eating too much kale?

How Kale May Affect the Thyroid

Kale and other cruciferous veggies contain compounds called “glucosinolates” that may interfere with the production of thyroid hormone. These are sulfur-containing chemicals responsible for the pungent aroma and bitter flavor, and that may have detoxifying and anti-cancer effects.

When these compounds are broken down in the body, they release other compounds, two of which can potentially affect thyroid health:

  1. Goitrin, which can directly interfere with the synthesis of the thyroid hormone.
  2. Thiocyanate ions, which compete with iodine for uptake by the thyroid gland.

As for number two, you may remember that the thyroid needs iodine to produce thyroid hormones, and without enough, it simply can’t do its job. Therefore, theoretically, too many cruciferous veggies could result in the thyroid not getting enough iodine—and consequently not producing enough hormones.

Thyroid hormones are critical to metabolism. When your body doesn’t produce enough, you feel tired, sluggish, and cold, and you may gain weight. Without treatment, hypothyroidism can lead to an enlarged thyroid (goiter), heart problems, depression, nerve damage, infertility, and birth defects.

The thinking goes that too much kale and other cruciferous vegetables could potentially out-muscle the iodine in your blood, blocking it from getting to the thyroid and robbing the thyroid of what it needs to produce its product.

Only if two conditions are present, however, can these compounds actually have any detrimental effect:

  1. There are enough glucosinolates in the bloodstream to overpower the iodine supply.
  2. There isn’t enough iodine in the blood to compete.

From what we know so far, it seems both conditions have to be present. According to the Linus Pauling Institute, increased exposure to thiocyanate ions from cruciferous vegetables or from smoking (where higher exposure more typically comes from) didn’t seem to increase the risk of hypothyroidism unless there was also an iodine deficiency present.

Truth be told, though, we don’t have a lot of studies on this yet, particularly anything that tests the consumption of a lot of raw cruciferous veggies and any potential effect on the thyroid. There was one small test done in 1986 that showed consumption of 150 grams of Brussels sprouts (about 5.3 ounces) daily for four weeks did not affect thyroid function, even though the sprouts contained high concentrations of glucosinolates.

These were cooked Brussels sprouts, however, which can make a difference. Anytime you cook cruciferous veggies, the so-called “goitrogenic” properties are dramatically reduced.

Does this mean health enthusiasts juicing raw kale every day should be concerned?

Some Groups May be At Risk for Kale Overdose

We don’t have enough information to know yet just how much kale is too much. An interview with Dr. Jeffrey Garber, chief of endocrinology at Harvard Vanguard Medical Associates and lead author of the latest clinical practice guideline on hypothyroidism in adults, may serve to put most minds at ease.

According to Dr. Garber, though it’s theoretically possible that eating too much kale (or other cruciferous veggies) could lead to thyroid problems, the risk is “miniscule” for two reasons:

  1. Most Americans get plenty of iodine.
  2. Most Americans would never consume enough of the veggies to cause problems.

There are exceptions, however, and some readers of this blog may be included in them:

  1. Vegetarians and vegans
  2. Pregnant women
  3. Those deficient in iodine
  4. Those who already suffer hypothyroidism or other thyroid disorder
  5. Those who consume tons of cruciferous veggies every single day and also fit in one of the above three groups

It seems most Americans don’t have to worry about an iodine deficiency, because it’s added to table salt, breads, and dairy, and it’s present in animal foods (including seafood). There has been some evidence, though, that we aren’t getting as much as we used to.

A 2003 study, for instance, measured iodine levels in schoolchildren, and found that levels were low in almost half of the students. A later 2011 study showed that iodine nutrition seemed adequate in most Americans since 2000, except certain populations, including pregnant women and non-Hispanic black participants.

Vegetarians and vegans are at particular risk for deficiency because they consume fewer (or no) animal foods, and iodine levels in plant foods are quite a bit lower by comparison. A 2003 study, for example, found that one-fourth of vegetarians and 80 percent of vegans suffered from iodine deficiency, compared to only 9 percent of those on mixed diets.

An earlier 2011 study also found that vegans were often low on iodine. Scientists measured iodine levels in vegetarians and vegans, and found that though vegetarians had an average level of 147 micrograms (recommended levels are between 100 and 199 for adults, and between 150 and 249 for pregnant women), vegans had an average of only 79 micrograms.

Iodine deficiency has also been found to be prevalent in pregnant women. A May 2014, researchers reported that about a third of pregnant women in the U.S. are iodine-deficient. The researchers added that as consumption of processed foods has increased, so has iodine deficiency, because the salt in processed foods is not iodized.

An earlier 2013 study also found that more than two-thirds of pregnant women were short on iodine, and that these shortages could affect childhood development. Both studies suggested supplements for both pregnant and breastfeeding women.

Of course, those who are already experiencing reduced thyroid function may be more at risk for the effects of reduced iodine, though regular consumption of cruciferous veggies (in normal amounts) has not been found so far to cause any worsening of the disorder.

Kale is Super Good For You

There are some other changes in the food industry that may be affecting our iodine levels, including a reduction in daily salt intake (robbing us of one source of iodine), and the reduced use of iodine in the processing of flour. (Today’s manufacturers often use bromide instead, which can block the activity of iodine.)

It may be wise to have your iodine levels checked, no matter who you are. Otherwise, eating greens in their normal amounts is likely to give you a lot more health benefits than health concerns. We’re all fairly familiar with kale’s shining reputation as being a great source of protein, fiber, vitamins, minerals, and omega-3 fatty acids, and its potential as a cancer fighter, cholesterol reducer, and eye health protector.

Still, as with most things, too much of a good thing may indeed be as bad as too little. Here, we’re talking about buckets of raw kale in a daily smoothie for weeks on end—something most of us aren’t going to do. But just in case you’ve become obsessed with the stuff, it may be good to shake it up now and then.

How about some spinach, instead?

Were you concerned about kale and thyroid problems? Have you cut back because of those concerns?

* * *

Morgan Korn, “Behind the Rise of the Almighty Kale,” Yahoo, August 17, 2014,

G. Roger Fenwick, et al., “Glucosinolates and their breakdown products in food and food plants,” Crit Rev Food Sci Nutr., 1983; 18(2):123-201,

Michale Chu, Terry F. Seltzer, “Myxedema Coma Induced by Ingestion of Raw Bok Choy,” N Engl J Med. 2010; 362:1945-1946,

Kristin Della Volpe, “News Update: Can Kale Cause Hypothyroidism?” EndocrineWeb, July 7, 2014,

“Cruciferous Vegetables and Cancer Prevention,” National Cancer Institute, June 7, 2012,

McMillan M, et al., “Preliminary observations on the effect of dietary Brussels sprouts on thyroid function,” Hum Toxicol., January 1986; 5(1):15-9,

Rachel Zimmerman, “Thyroid Doc: Kale Risks ‘Theoretical’ But In Reality, Very Low to Miniscule,” wbur’s CommonHealth Reform and Reality, January 17, 2014,

Krajcovicova-Kudlackova M., et al., “Iodine deficiency in vegetarians and vegans,” Ann Nutr Metab., 2003; 47(5):183-5,

Kathleen Doheny, “Iodine Deficiency Common in Pregnancy, Pediatricians Warn,” HealthDay, May 26, 2014,

“Iodine Deficiency in Pregnant Women May Harm Babies’ IQs,” MedicalNewsToday, May 23, 2013,

Genevra Pittman, “Vegans may be at risk for low iodine: study,” Reuters, June 4, 2011,

Gur E, et al., “Prevalence and risk factors of iodine deficiency among schoolchildren,” J Trop Pediatr., June 2003; 49(3):168-71,

Caldwell KL, et al., “Iodine status of the U.S. population, National Health and Nutrition Examination Survey, 2005-2006, and 2007-2008,” Thyroid, April 2011; 21(4):419-27,

Colleen M. Story

Colleen M. Story

Colleen M. Story, a northwest-based writer, editor, and ghostwriter, has been creating non-fiction materials for individuals, corporations, and commercial magazines for over 17 years. She specializes in the health and wellness field, where she writes and ghostwrites books, e-books, blogs, magazine articles, and more.

Colleen is the founder of Writing and Wellness. Her fantasy novel, “Rise of the Sidenah,” was released with Jupiter Gardens Press in September 2015. Her literary novel, “Loreena’s Gift,” is forthcoming in spring 2016 from Dzanc Books. She lives in Idaho.


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  1. debra says:

    What about the idea of “countering as you go”? I put a liquid iodine on my inner thigh (a suggestion of Dr. Klinghardt from an interview) whenever I’m juicing a goiterogen or using a fair amount of the iodine-inhibiting foods in salads.
    By applying the iodine topically, I’m letting my own body decide how much it wants to absorb.

  2. Julie says:

    Great to see you getting the word out about kale and other cruciferous veggies.
    I knew these foods were goitrogenic from my own research a couple years or so ago. For me, when I was visiting LA and shopping at Erewhon market, I saw a bottled raw juice there by Elissa Goodman that was for thyroid. I looked her up online and read through some blog entries on her website. Then I started looking online for info on goitrogenic foods to see what I should be avoiding.
    I think a point that’s missing in this article is that you can still have these foods if you cook them; I think even light steaming is sufficient. Could you check this and confirm?
    Maybe for readers who do want to do raw juicing and blending, they could supplement a day ahead with iodine, before the day they plan on using a raw cruciferous veg. Or they can help themselves with seaweed in their diet.
    Also perhaps adaptogenic foods and herbs can help as well, such as holy basil, which Kevin is a fan of, or maca or other similar foods and herbs.

  3. Olin says:

    Key to this discussion is that current standards for iodine sufficient may be inadequate. David Brownstein. M.D. finds about 95% of his patients iodine deficient! I suggest that most of the US population is deficient and would benefit from daily iodine supplementation as iodine is not only required for the thyroid, but the breast tissue, ovaries, thyroid, immune system and all cells of the body. Iodine sufficiency is necessary for efficient apoptosis and in deed may be a limiting factor in cancer development.
    I would suggest that iodine sufficiency will allow one to consume cruciferous vegetables liberally both raw and cooked. An excellent resource on the importance of iodine by Gabriel Cousens. M.D. is available at:

  4. While I appreciate this article, I DO think those that eat raw kale and crucerf. veggies need to be careful. While it is possible I had an iodine shortage prior to my starting on green smoothies 2 years ago, this practice definitely made it worse. Once I cut the raw kale and used romaine, spinach, or such I did better. Also I didn’t have the same effects with the cooked kale. I understand that cooking it alleviates the iodine/thyroid issues.
    At the time I started to do the smoothies, I was in training as a Holistic Health Coach. I also found out how EMF radiation has had a big effect on my thyroid. I use protectors on my person for this. The thyroid can be a tricky thing to balance for many of us.
    Hope this helps with input.

  5. jodi says:

    i’m taking thyroid medication for an underactive thyroid. yes, i used to juice my kale and eat mega kale chips… but when i found out about the goitrigen or whatever connection between the thyroid and kale i basically stopped eating kale. =( i switched back to spinach. it would be nice to eat kale again.. but sometimes i also wonder- like when i feel really tired after eating a lot of cabbage or broccoli. i don’t want to cut out aaaaaaaall of my anti cancer veggies… thanks for the article.

  6. Minia says:

    Great article. I have hypothyroidism and take NatureThroid most days. I love kale and use it in smoothies but not every day. I add a little kelp powder to smoothies so I get some of my iodine from that whether I juice with kale, collards, or not.

  7. Great article on the thyroid! Keep them coming! Izabella Wentz, a pharmacist, is very knowledgeable on the iodine topic. In her book, Root Cause of Hashimoto’s, she covers iodine studies.

  8. Kuru says:

    I always add plenty of salt and miso to my smoothies. Nothing makes me feel better than kale. Love it cooked as well, again with lots of salt. Yum.

  9. Viviane Proulx says:

    I experience gas bloating when I eat smoothies with raw kale. I want to test adding fermented foods to it, to see if that helps.
    If you ”feel really tired after eating a lot of cabbage or broccoli”, it could be because you have digestion problems with those raw cruciferous vegetables. Cooking them lightly helps digestion but does affect nutrient density.
    Great article which summarizes very well this famous question.

  10. Candice says:

    Recently, I left customer feedback at Wholefoods in regard to this vary issue. I am
    slightly hypothyroid and have been able to bring my numbers up by supplementing
    with nascent iodine drops (3 in the morning) in water. I have stopped drinking
    raw kale juice and have requested that Wholefoods begin offering juices with
    spinach in lieu of kale. I still eat shaved brussel sprouts, cauliflower and broccoli raw
    in salads on occasion, but for the most part, am eating these veggies lightly steamed.
    I suppose the jury is still somewhat out on this subject. Keep up the good
    work sharing these types of articles…..

  11. Riley says:

    Wait uh, Spinach is cruciferous too, no?
    Also, you mentioned that “animal foods, and iodine levels in plant foods are quite a bit lower by comparison,” but that’s an overly general and therefore misleading statement. Sea vegetables are also plant foods, and frequently contain much higher levels of iodine than a lot of animal foods.

  12. Great article on thyroid.I have let me know how can I join any hypothyroidism patients association.regds

  13. ara says:

    Since the problem only occurs if you eat three pounds of kale and are iodine deficient I don’t think there is any need to worry. Supplement iodine-( maybe in your multi) and eat less than three pounds of kale. That’s a huge amount of kale. I have a smoothie with kale and collards every day and don’t come near that much. Kale has too many health benefit to abandon for fear of a problem that will not occur if one is iodine sufficient and does not eat ridiculous amounts. By the way substituting spinach does not provide the benefits of cruciferous vegetables and spinach is high in oxalic acid.

  14. Aaliyah says:

    Great article and helpful information. Thank you for sharing with us. Thank you so much.

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