Do Raw Kale and other Cruciferous (Brassica) Leafy Greens Cause Thyroid Problems?

“I have been reading that certain raw green leafies are bad for your thyroid. I am really concerned about that. I have been eating green smoothies since the latter part of December 2008. Although I have not had problems with my thyroid, I was a bit alarmed with what I read. Please shed some light on that.” – Shawna

Every so often, there’s an alarming tale spun on the Internet about somebody, somewhere who was diagnosed with hypothyroidism. The cause? That super-healthy leafy green that everybody else was telling you was good for you. The cautionary tale concludes with a dire warning: “Kale killed my thyroid”.

Well, not quite.

Cruciferous vegetables such as kale, bok choy, collards, turnip and broccoli do contain goitrogenic glucosinolates, which produce thiocyanates that compete with iodine for absorption by the thyroid gland. Sounds alarming, right?

However, the overwhelming body of research shows (as is the general medical/scientific consensus) that consuming foods that contain glucosinolates does not increase the risk of hypothyroidism in healthy individuals unless accompanied by iodine deficiency.

An Endocrinologist Speaks On The Issue

I recently read an online interview with Dr. Jeffrey Garber, who is the chief of endocrinology at Harvard Vanguard Medical Associates, and author of clinical guidelines for hypothyroidism in adults. When asked specifically about whether or not raw kale in green smoothies can pose a negative effect on a healthy thyroid, his answer was this:

“Basically the goitrogens are challenges to the thyroid. But in the absence of iodine deficiency, substantial or prolonged ingestion of dietary goitrogens and lastly the absence of an underlying thyroid disorder, the risk in this country [United States] of having problems in this area are very, very low, almost minuscule. Again, that’s because the vast majority of people have adequate iodine levels to counteract the effect of goitrogens.”

When pressed even further about whether or not consuming raw kale in a smoothie every day would cause thyroid disorder in someone who was iodine deficient, his answer was that, “It could theoretically happen, but it would be unusual”.

My Experience Consuming Raw Kale For 5+ Years

I have consumed three to four cups of raw kale, bok choy or other cruciferous vegetable nearly every single day in my green smoothies since early 2008. I recently had routine blood work done, and I checked my thyroid just to see. The lab results indicated that my thyroid function is 100% normal and the numbers are right in the middle of the healthy range (not too high, not too low).

My husband also checked his, just out of curiosity since he’s been guzzling kale smoothies with me the whole time. His thyroid is also performing perfectly five years and counting.

Basically, if you have a healthy thyroid and adequate dietary iodine intake, you do not need to worry about eating leafy greens from the cruciferous (brassica) families like kale or bok choy.

What About Stories From People Who Became Hypothyroid After Juicing or Blending Kale?

It is important to remember that there are numerous causes of hypothyroidism, so kale can’t be blamed on all of them. I’d wager that the majority of hypothyroid cases in the United States are diagnosed in people who do not eat anywhere near the amount of kale that I do.

People tend to look for reasons for why things happen in their lives. Those who are into natural health tend to blame all of their ills on food or environmental toxins. If somebody juices kale and is diagnosed with a thyroid disorder two years later, they might blame the goitrogens in kale for “destroying their thyroid”, when in fact, kale might not had anything to do with it. They might have become hypothyroid regardless of their kale consumption (or avoidance).

News media and other bloggers absolutely love to swoop in on stories like these and spin them in the most alarming way possible. This generates lots of clicks, but it doesn’t educate people on the causes of thyroid disorders, or the safety of consuming cruciferous vegetables.

Support Your Thyroid And Stop Worrying About Kale

Regardless of whether you eat kale or avoid it, it is important to support your thyroid through proper nutrition. That means getting adequate iodine in your diet.

Sea vegetables are an especially rich source of iodine. Every day, I add a teaspoon or two of dried dulse flakes to my green smoothies. One teaspoon contains 110% RDA of iodine. Kelp is also an excellent source of iodine and you can add kelp powder to green smoothies as well. Neither of these sea vegetables will make your smoothie salty or “fishy”.

Research also suggests that deficiencies of selenium, iron, and vitamin A exacerbate the effects of iodine deficiency. It’s a good thing that cruciferous greens like kale are also rich in iron and beta-carotene (which your body converts to vitamin A). As for selenium, keep some Brazil nuts on hand. Just one nut per day will provide adequate selenium in your diet.

There are nutrients and anti-nutrients (toxins) in all foods that you eat, including fruits and vegetables. As long as you eat whole foods and keep an eye on your nutrient intake, you don’t have much to worry about.

As a precaution, I also recommend rotating your greens. I wouldn’t stuff 3 or 4 bunches of kale each day into green smoothies. Over time, that could potentially cause a problem. You wouldn’t want to overload your body with one particular anti-nutrient that may contribute to a nutrient imbalance. (Read more on How and Why You Should Rotate Your Greens.)

If you do suspect that you have impaired thyroid function, be sure to get tested and work with a nutritionally trained medical practitioner who can help you establish a diet that will help support and improve thyroid function.

If your thyroid is healthy, there’s no need to eliminate (or cook) kale or other raw cruciferous vegetables.

Return to the Green Smoothie FAQ.

Tracy Russell is the creator of RESET 28: A 28-Day Program For Energy, Weight Loss & A Healthy Glow, and founder of Incredible Smoothies. She has been helping people take control of their health and well being with green smoothies, a whole foods diet, and fitness since 2009.