9 Foods Thyroid Experts Avoid—and You Should Too
Some foods touted as nutritional superstars—like kale and millet—might not be the best choices for the health of your thyroid gland.
First, what’s a thyroid?
The main job of the thyroid, a butterfly-shaped gland at the base of the neck, is to glean iodine from your diet to produce thyroid hormones, which support almost every important bodily function you can think of—heart rate, circulation, metabolism, your internal clock, liver function—the list goes on and on. An estimated 20 million Americans (and far more women than men) have some form of thyroid disease. The gland is over- or under-active, according to the American Thyroid Association. More than half don’t know it, which is why everyone should use caution when it comes to these foods. Check out these hidden dangers of a “normal” thyroid.
Kelp and seaweed
Kelp and other seaweeds are considered green superfoods, but these ocean plants are no friend to the thyroid gland. Although extremely rich in iodine, they can throw your iodine levels out of whack. “Seaweed is the richest source of dietary iodine,” explains Richard Mack Harrell, MD, an endocrinologist with Memorial Healthcare System in Hollywood, Florida, and a spokesman for the American Association of Clinical Endocrinologists. “And in general endocrinologists are telling people with established thyroid disease to be careful about eating kelp and other seaweed.”
Oh, the irony: Here’s another popular health food that may not be the best choice for people with thyroid issues. Kale is a cruciferous vegetable, and all of the vegetables in this category (like cabbage, broccoli, cauliflower, Brussels sprouts, rutabaga) are considered goitrogens—substances that disrupt thyroid function by interfering with how the gland uses iodine. “Even though they’re good for us, cabbage and other cruciform vegetables eaten in large quantities, especially in the context of iodine deficiency or borderline iodine levels, can result in hypothyroidism. These vegetables generate an ion that competes with the uptake of iodine by the thyroid,” writes endocrinologist Christian Nasr, MD of the Cleveland Clinic. Cooking cruciferous vegetables makes them thyroid-safe, but since kale is usually eaten raw—in salads and smoothies, for instance—it may be a problem. “When people are eating big kale salads every day, especially if they’re low in thyroid hormone, it can affect thyroid function,” says Cheryl Harris MPH, RD, a dietitian and nutrition coach. Here are some health conditions you can blame on your thyroid.
Soy is another goitrogen, which can be an issue for people whose iodine levels are already compromised. “The main problem is that soy hinders the absorption of the hormones patients are taking,” writes Dr. Nasr. “Some studies show that if you eat a lot of soy, or drink a big glass of soy milk, within one hour of taking a thyroid hormone, it might affect absorption.” And avoiding soy is becoming more difficult, too.” The problem with soy is a relatively recent one. “Soy has been safely used for thousands of years as a condiment,” says Harris. “But now it’s in soy milk, vegan burgers, protein bars, and shakes. It’s not uncommon to see people eating soy at breakfast, lunch, and dinner.” And unlike the goitrogens in cruciferous vegetables, the compounds in soy can’t be destroyed by cooking.
There is a link between thyroid and celiac disease, and many people with thyroid issues think avoiding gluten is worth a try. “The idea is that there’s a pretty significant crossover between thyroid disease and celiac disease,” explains Harris. “Up to 5 percent of people with Hashimoto’s also have celiac. There is some research showing that some people with thyroid disease show improvements, better numbers, on a gluten-free diet.”
Dr. Harrell, though, is a bit more skeptical. “There’s not good data that it works, although there are a lot of people in my practice who’ve done it,” he says. “One or two felt better, but there’s no scientific research yet that says it’s something we should be doing.” Bottom line: Check with your doctor before going gluten-free. Find out the 13 silent signs of a thyroid problem that you should know.
The issue with fish is high levels of mercury, in tuna as well as swordfish, mackerel, and orange roughy. Mercury is particularly dangerous for the thyroid, according to Amy Myers, MD, author of The Thyroid Connection; Dr. Myers has Graves’ Disease, an immune condition resulting in the overproduction of thyroid hormones. “Unfortunately for your thyroid, mercury and iodine are chemically very similar to each other, so your thyroid is quick to absorb and store mercury, too.” She explains that when the thyroid stores mercury in place of iodine, it won’t have enough iodine to produce adequate levels of hormones. Another issue, according to Dr. Myers, is that high mercury levels put people at risk of developing autoimmune disorders. She cites a 2011 study that found that women with high mercury exposure were more than twice as likely to have thyroid antibodies, which could indicate Hashimoto’s thyroiditis, the most common reason for an underactive thyroid.
The sweet stuff already has a bad enough rap where your health is concerned—now you can add thyroid trouble to the list of grievances. Harris explains that people with thyroid disease are at a higher risk of developing diabetes and cardiovascular disease, and limiting sugar can help minimize your risk. But don’t rush to replace the sugar in your diet with artificial sweeteners. “There are a few recent small studies showing that artificial sweeteners can affect thyroid function,” she says. “There haven’t been large-scale studies yet, but there’s some belief that the artificial sweeteners may affect the gut microbiome.”
The issue here isn’t anything in the salts—it’s what they don’t have: A lot of the table salt available in the United States has been iodized (iodine was added) to help people get enough of the nutrient. But the popularity of kosher and sea salts means some people could be falling short. “The only known use of iodine in the body is to make thyroid hormone,” says Elizabeth N. Pearce, MD, an endocrinologist with Boston Medical Center, and president-elect of the American Thyroid Association. “And we’re probably now starting to see the reemergence of mild iodine deficiency in select proportions of the populations.” Don’t miss these 7 healthy habits that make your thyroid happy.
Millet and cassava
Not traditionally used in American cooking, both millet and cassava flours have recently become more popular. “Millet is a grain that used to be used in this country mainly in bird seed,” says Harris. “But it’s getting used a lot more now, particularly as more people go gluten-free. And cassava is a root vegetable used a lot in Central American cooking, but it’s also being used in this country now for flour, and for tortilla chips and other snacks.” Like millet, cassava is also gluten-free. . . and both are also goitrogens. But neither millet or cassava is likely to cause thyroid issues for people unless they already have low iodine levels or diagnosed thyroid conditions.
“There’s a lot of speculation about why thyroid disease is so common now, and whether the additives and chemicals in processed foods play a role,” says Harris. “There’s some belief that the additives carrageenan and polysorbate 80 are linked to many autoimmune diseases, so it may be an issue with thyroid disease, too. But it hasn’t been extensively studied.” Another potential hazard in processed foods is nitrates, chemicals commonly used as preservatives in processed foods and cured deli meats like hot dogs and bacon. “Nitrates are similar enough to iodine to competitively block its absorption, which reduces thyroid function,” according to Dr. Myers, who adds that these chemicals have also been linked to increased rates of thyroid cancer. Next, find out the thyroid cancer symptoms you shouldn’t ignore.