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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