Can Goitrogens in Soy and Cruciferous Vegetables Interfere with Thyroid Function?

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A healthy lifestyle and preventable death? What are the six lifestyle factors that collectively appeared to cut the risk of dying prematurely in half? Not smoking, not drinking heavily, walking an hour a day, sleeping around seven hours a night, not being overweight, and the sixth is the one dietary quality indicator they used—eating your greens. Most people don’t eat enough, but is it possible to eat too many?

I’ve talked about the risks of getting too many oxalates by eating too many of certain greens, like spinach and Swiss chard, but what about low-oxalate greens like kale or collards?

This cabbage family of vegetables has natural goitrogenic compounds that can interfere with thyroid function. They can compete with our thyroid’s uptake of iodine. But the answer is not to avoid these super healthy foods, but rather to get enough iodine in our diet, as I’ve talked about before. There is, however, an extreme threshold we don’t want to cross.

How much is too much? From the New England Journal of Medicine: coma induced by the ingestion of raw bok choy. A woman had been eating about three pounds (1.30 kg) a day, every day, which means 15 cups (~450 g) a day is too much. In the two months before she was admitted to the intensive care unit in respiratory failure with a diagnosis of severe hypothyroidism, she had consumed at least 1,000 cups of raw bok choy. The reason raw was specified is that the enzyme that creates these iodine-blocking compounds when we bite into them or chop them up is rapidly deactivated by cooking, so we don’t have to worry as much about eating too many cooked crucifers.

There are also goitrogenic compounds in soybeans, though there is little evidence that this presents a problem for people with normal thyroid function who are getting enough iodine. Now if you have hypothyroidism and are taking thyroid hormone replacement, soy may impair the absorption of the drug. But that is not unique to soy—that’s why hypothyroidism patients are told to take their Synthroid before breakfast or at least four hours after a meal.

There have been over a dozen randomized, controlled trials on the effect of soy on thyroid function. Researchers found that soy has no effect on thyroid hormone levels, but there was a 10 percent elevation in TSH. That may not mean anything for people with healthy thyroid glands, but if your thyroid function is already suboptimal, that could potentially tip you over the edge.

About one in every 23 Americans has what’s called subclinical hypothyroidism. This is when our thyroid gland continues to produce normal levels of thyroid hormones but seems to have a more difficult time than usual, as evidenced by higher levels of thyroid stimulating hormone. This means our body has to stimulate our thyroid more than usual to produce normal levels of hormones, suggesting it may be starting to fail.

Okay, so 60 patients with subclinical hypothyroidism were randomized to eight weeks of either the equivalent of about two servings of soy foods a day or nothing. Six patients developed overt hypothyroidism while on the soy, which is three times the rate that would have been expected. But the soy group had a bunch of benefits, like less insulin resistance, lower inflammation, and better blood pressures. However, the thyroid findings were so concerning that the same research group ran the study again, but this time with four times the dose of soy, and this time there was no increase in overt hypothyroidism. What are we to make of all this?

Soy industry consultants concluded that these follow-up results call into question the initial findings. I mean, if four times the amount of soy appears to have no effect, then maybe the earlier findings were a fluke. But the study populations were different. In the later study, the patients started out with TSH levels that were only barely elevated. Normal levels go up to 4.5 or 5, so they were just over the line, whereas in the earlier studies they started out closer to 8, meaning the patients were further along, and therefore potentially more sensitive. If you follow people with subclinical hypothyroidism over time, most patients who start out with TSH levels over 6 end up with overt hypothyroidism within a decade, but none of those who started out with levels between 4 and 6 suffered the same fate. So, if your TSH is really elevated, it might be reasonable to avoid soy, but we’d need more research to know for sure.

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Motion graphics by Avo Media

• blood pressure
• collard greens
• cruciferous vegetables
• greens
• inflammation
• insulin
• iodine
• kale
• soy
• soybeans
• spinach
• Swiss chard
• thyroid