Is Kale Bad for My Thyroid Health?

By: Brierley Wright, M.S., R.D.  |  Wednesday, October 15, 2014

Kale’s über-healthy reputation is in part thanks to the cancer-fighting compounds it boasts called glucosinolates. But there’s a compound within glucosinolates that interferes with your thyroid function—and some may worry that eating too much kale could hurt their thyroid and possibly even cause hypothyroidism (underactive thyroid). (Why does that matter, you ask? Your thyroid regulates many body functions—and top on the list is metabolism.)
But there’s more to the story.
Dig a little deeper and you’ll learn a chain of reactions has to happen for those thyroid-interfering compounds—called thiocyanates—to be released. Cooking kale stops that chain of reactions from happening. However, chopping raw kale for a salad or chewing it does allow thiocyanates to form. But the quantity of thiocyanates in a few ounces of raw kale (what you’d probably eat) is minimal. The odds are low that even true kale lovers will harm themselves. “You’d have to eat a couple of large bunches of kale a day to get enough [thiocyanates] to have an impact—and you’d have to eat it raw,” explained Jeffrey Garber, M.D., past president of the American Association of Clinical Endocrinologists and associate professor of medicine at Harvard Medical School.
Thiocyanates are found in all cruciferous vegetables, not just kale, and rutabaga is the vegetable with the most. Runners-up—alongside kale—are turnips, cabbage and Brussels sprouts, but the rest of the cruciferous family has very little.
If you’re still worried, ask your doctor to check your thyroid. It’s a simple blood test. If you’re taking thyroid hormone for an underactive thyroid or have a predisposition to thyroid problems, you are more susceptible to the effects of thiocyanates. Garber’s advice: “Eat as many cruciferous vegetables as you want—go to your limit—and do this for a number of weeks; then ask your doctor to check and see if it’s had an effect.”
The Bottom Line
Don’t banish kale. It delivers healthy amounts of bone-strengthening vitamin K, eye-healthy vitamin A, plus some fiber and cancer-fighting compounds. Any concern about your thyroid is truly tiny.