I periodically receive emails from readers asking me to write a post reviewing this or that thyroid book. There are a few reasons why I haven’t done it, but the overarching explanation is: thyroid books are generally worthless. Like The Bachelor(ette) television franchise, these books tend to follow a formulaic arc, promising “the most dramatic improvement in your health, ever!” First, they seek to identify with the reader – are you sick and tired of being sick and tired and getting nothing but shrugs from your doctor? Second, they overstate the prevalence of clinically meaningful undiagnosed thyroid disorders in the population, leading you to believe if you haven’t yet been diagnosed, you can rest assured that it’s definitely your thyroid. Third, they launch into a lengthy discussion/diatribe about how mainstream physicians refuse to order the testing that could diagnose your thyroid condition. Fourth, they regurgitate a bunch of filler that’s covered elsewhere on the web about diet, exercise, sleep, supplements, etc. Finally, they offer their own “protocol” for defeating your hypothyroidism and regaining control of your health.
After all that, you are left either no better off or, perhaps worse off, due to spinning your wheels following advice that is clinically unsound. Along those lines, after years of Chris Harrison (host of The Bachelor) promising me “the most dramatic season ever” – every season – I have finally cut the cord and stopped watching. It got to the point where I was watching The Bachelor only to get to know the contestants well enough to appreciate the drama of Bachelor in Paradise, the more ridiculous yet slightly more entertaining spinoff show. Like a lousy thyroid book, I’ve slowly learned to accept that every season of The Bachelor is exactly the same. Each episode is edited such that what airs is merely different people spouting precisely the same platitudes as their predecessors from previous seasons. So yeah, I’m equating wasting my time watching reality TV drivel with wasting my time reading and then writing about thyroid books that proffer the same nonsense we’ve all already heard. If I ever write a thyroid book, you can bet your rear end that it will be the Survivor* of thyroid books.
You Can’t Eat for Your Thyroid
Although I don’t plan to review a specific thyroid book today, I thought I’d do a deep dive into one of the pervasive myths found in these books – that you can eat/avoid certain foods to optimize your thyroid health. Like most of the nonsense that litters the internet about the thyroid, much of this starts with a kernel of truth and then veers off into the woo-o-sphere. With that preamble, let’s debunk!
Myth: You must avoid cruciferous vegetables.
If you try to avoid cruciferous vegetables, you might as well say, “I’m just going to stop eating vegetables.” Okay, that’s a bit hyperbolic, but veggies from the Brassica genus include broccoli, kale, turnip, Brussels sprouts, Chinese cabbage, cauliflower, and a host of others. [HD: Warning, this gets dense for a few sentences!] The “problem” with Brassica vegetables is that they contain substances known as glucosinolates. Glucosinolates are acted upon by an enzyme called myrosinase – found in adjacent plant cells – typically during the process of chewing. This enzymatic reaction leads to the formation of isothiocyanate, some of which breaks down to form a permanently charged thiocyanate ion. In addition, there is a glucosinolate called progoitrin that breaks down to goitrin. Read on to learn the mechanisms for why thiocyanate and goitrin could be bad for the thyroid.
Thiocyanate is a competitive inhibitor of the sodium/iodide symporter on the basolateral membrane of the thyroid follicular cell. Because the thyroid needs the symporter to grab iodine in order to make thyroid hormone, and thiocyanate competes with iodine for uptake by the thyroid, this could reduce thyroidal uptake of iodine and lead to decreased synthesis of thyroid hormone.
The question is, does that actually happen in a clinically meaningful way? There aren’t many human studies, but one actually added thiocyanate to milk consumed by human volunteers for 12 weeks. They were able to show an almost doubled blood level of thiocyanate, but unchanged levels of T4, T3, and TSH. It turns out that the amount of thiocyanate increase in the blood you would see from consuming 100g (3.5 oz) of many Brassica vegetables is only about 10 μM. Since human control subjects have baseline thiocyanate levels of 40-121 μM, it seems very unlikely that a normal serving of these vegetables would bump thiocyanate levels enough to interfere with synthesis of thyroid hormone.
Goitrins also have the potential to interfere with thyroid hormone production, as seen in many animal studies. There is no data on what constitutes a normal level of goitrins in human blood, so we really don’t know how to define the safe dietary threshold. Again, there isn’t much high-quality human data, but one study found that it took 77 μM of progoitrins to suppress thyroidal uptake of a diagnostic dose of radioiodine. As you will recall from the preceding paragraph, a 100g serving of broccoli has only 10 μM, so it seems far-fetched that a normal serving of vegetables would supply enough goitrins to cause a huge problem.
The only journal article reporting a negative effect of Brassica vegetables on thyroid function was a case report of an 88 year-old woman with diabetes, who was ingesting 1.5 kilograms of raw bok choy daily for several months. Temporarily tabling the fact that I can’t imagine eating that much cooked bok choy, let alone raw bok choy, why would anyone do this? Did her naturopath recommend it? The paper didn’t say, but she believed that it would help control her diabetes. In her defense, if she wasn’t eating much else – I don’t see how she could, with all that bok choy clogging up her stomach – her blood glucose probably looked awesome. Unfortunately, she presented to the hospital in myxedema coma, a state of profound hypothyroidism.
Fortunately for all you bok choy lovers, cooking Brassica vegetables inactivates the myrosinase enzyme that leads to the formation of thiocyanates and goitrins. Unless you are eating truckloads of raw Brassica vegetables on a daily basis, your thyroid is going to be just peachy. Not only that, but glucosinolates are known to be cancer-fighting compounds, so I would not recommend restricting vegetables that contain them (though raw vegetables may have more potent cancer-fighting ability than cooked, for the reason stated above). If you want to read more about cruciferous vegetables and the thyroid, check out this great review article.
Myth: You must avoid soy.
Soy has garnered quite a bit of negative attention when it comes to hypothyroidism. People got all bent out of shape back in the 1950s – 1960s when several cases of infant goiter were reported with soy formula. However, it soon became clear that this risk was eliminated with the addition of iodine to the formula.
The pathophysiology stems from the fact that soy is particularly rich in isoflavones, when compared to other plant foods. In rats and in test tubes, these isoflavones have been shown to inhibit thyroperoxidase (TPO) enzyme activity. Because TPO catalyzes iodination of T4 and T3 in the formation of thyroid hormone, interfering with it could lead to hypothyroidism. Isoflavones also compete with tyrosine for iodination by the thyroid, and tyrosine is used in the formation of thyroid hormone. But, research has shown that this competition is negligible and clinically irrelevant.
Despite their potential for causing problems, at least 13 clinical trials have shown that isoflavones have no effect on the thyroid function of euthyroid people. In other words, if your thyroid function is normal, eating soy isn’t going to cause it to slow down. Admittedly, trial data is tough to interpret and extrapolate, as almost all trials use isolated soy protein (ISP) as opposed to less processed food like tofu. But, since the isoflavone content of ISP is probably going to be a little higher than the tofu you find at the supermarket, it is likely safe to say: if ISP in the trials didn’t cause hypothyroidism, tofu and other soy products won’t either.
On the flip side of the coin, there was one Japanese trial showing that consumption of soy in the study subjects led to goiter. Unfortunately, this study was of lower quality than the other 13 trials above. There was no control group, and the soy content of the product administered to study subjects was not described in detail. In addition, based on the way the authors did describe their intervention, it seemed like the soy content of the study medication may have actually been lower than the amount of soy euthyroid Japanese people typically consume on a daily basis. Combining this fact with the fact that no other trial showed results of this nature, it makes their results highly implausible.
Now, there was one British study that actually looked at giving soy to patients with subclinical (borderline) hypothyroidism. They used pretty high doses of a soy phytoestrogen and found that several subjects progressed to overt hypothyroidism.
The last thing I should mention about soy is that it does interfere with the absorption of thyroid hormone pills, so people with hypothyroidism do need to be careful about eating soy too close to their morning dose of levothyroxine. But, since any food in the stomach has the potential to decrease levothyroxine absorption, this effect of soy is not really unique and shouldn’t deter people with hypothyroidism from eating it. I think the only people who might run into trouble are hypothyroid folks who consume large amounts of soy throughout the day. Because there is some enterohepatic circulation of levothyroxine, the constant presence of soy in the gut could lead to decreased absorption of the ingested thyroid hormone, even many hours after the dose has been taken.
In conclusion, euthyroid people do not need to worry about soy hurting their thyroids. People with borderline hypothyroidism who consume large amounts of soy might want to consider a multivitamin containing a normal amount of iodine (no mega doses!), if they live in a region where iodine deficiency exists. That would likely mitigate the risk of developing overt hypothyroidism. People with hypothyroidism on thyroid hormone replacement therapy can eat soy in moderation, as long as they don’t eat a ton of it within several hours of taking their levothyroxine.
For more information about soy and the thyroid, check out this good article.
Myth: You should eat seaweed or take mega doses of iodine-containing supplements.
Iodine is necessary for normal thyroid hormone synthesis. However, iodine deficiency in the U.S. is not very common. Nonetheless, I see plenty of patients being “tested” for iodine deficiency by their alternative medicine providers. Clearly these providers don’t know their assay, as there is no good test for diagnosing iodine deficiency in an individual. Further, the “treatment” they initiate with mega doses of iodine is misguided.
Excessive doses of iodine can actually suppress thyroid function. You heard me…naturopaths can cause people with normal thyroids to become hypothyroid by giving them huge amounts of iodine. It’s often not catastrophic, as euthyroid people will escape from this so-called Wolff-Chaikoff effect in 1-2 weeks and return to normal. But, if a euthyroid patient has underlying autoimmune thyroid disease (Hashimoto’s thyroiditis, for example), they may not escape from the Wolff-Chaikoff effect, becoming persistently hypothyroid with continuation of the iodine supplement.
People with a predisposition to hyperthyroidism also don’t handle large iodine loads very well. In these cases, the excessive dose of iodine can precipitate hyperthyroidism (the Jod-Basedow phenomenon). Bottom line: if you have good reason to believe you are iodine deficient, consider a multivitamin with somewhere around 150 micrograms of iodine.
Myth: Avoiding gluten will heal your thyroid if you have Hashimoto’s thyroiditis.
This is one of my favorites because, to the best of my knowledge, there is no good data to suggest this is true for people without celiac disease. Even in people with celiac disease, the evidence is conflicting and controversial. Some studies suggest that a gluten-free diet in patients with celiac disease can decrease the likelihood of developing other autoimmune diseases (like Hashimoto’s). Other studies show that there is no effect of a gluten-free diet on autoimmune disease progression.
The important point here is, while celiac disease is more common than was once thought, it is still a fairly rare disease. Hypothyroidism due to Hashimoto’s thyroiditis is the most common autoimmune disease. Though an individual with one autoimmune disease is at risk for others, the vast majority of people with Hashimoto’s do not have and will never develop celiac disease. Therefore, it stands to reason that most of the people who are being counseled to avoid gluten for the sake of their thyroids are being misled.
I’m not interested in getting into a debate about so-called “gluten-intolerance,” so please don’t leave long comments about how avoiding gluten has changed your life even though you don’t have celiac disease. I concede that avoiding starchy carbs is likely to have great impact on almost everyone’s health, so it is not surprising to see people feel better and appear metabolically healthier when they eliminate gluten or any other starchy foods from their diets.
It’s your turn now. Are there any other foods or supplements you’ve seen recommended to “help” the thyroid? Comment below!
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*In my opinion, Survivor is the greatest reality show ever created. I firmly believe that, if you hate Survivor, it’s because you have negative misconceptions about the show and haven’t actually watched an entire season. It is one of the most engaging social experiments ever conducted, still resonating and innovating after 36 seasons. Jeff Probst and company, you rock.