What are goitrogens?
Goitrogens are naturally occurring compounds in food and the environment that can block thyroid hormone production in various ways.
When thyroid hormone production is suppressed, the thyroid will enlarge to try and compensate.
This type of enlarged thyroid is called a goiter. It is usually caused by iodine deficiency, but other factors, like goitrogens, can play a role as well (1).
There is not just one type of goitrogen. There are many types that have different effects (see below).
What foods are considered goitrogenic?
Goitrogenic foods contain goitrogens and may need to be avoided by people with hypothyroidism or Hashimoto’s, especially if they are also iodine deficient. See each food below for more details:
1. Raw cruciferous vegetables
Raw cruciferous vegetables contain bitter compounds called glucosinolates.
While glucosinolates have a lot of health benefits, they can also inhibit iodine uptake by the thyroid and may interfere with TPO activity (2).
For this reason, people with hypothyroidism may not want to consume TOO many raw cruciferous vegetables, since it could further impair thyroid function and trigger hypothyroid symptoms.
Unfortunately, there is no clear consensus on what amount classifies as “too much,” and very little research has been conducted in humans (3).
One study found collard greens, brussels sprouts, and Russian kale might have enough goitrogens to reduce thyroid function, but not broccoli, broccoli rabe, or regular kale (3). However, the amounts can vary based on growing and preparation conditions, so more research is definitely needed.
Examples of cruciferous vegetables include:
- Bok choy
- Broccoli rabe
- Brussels sprouts
- Canola (rapeseed)
- Chinese broccoli (gai-lan)
- Collard Greens
- Mustard greens and seeds
*Of note: Cooking or fermenting the vegetables deactivates the glucosinolates, so cooked cruciferous vegetables or fermented items like sauerkraut and kimchi are just fine (4, 5).
Soy contains a few goitrogenic isoflavones, including genistein, daidzein, and glycitein.
These compounds block the TPO enzyme that converts iodine to iodide, which is a required early step in thyroid hormone production.
By interfering with TPO, soy isoflavones reduce thyroid hormone production and can induce goiter in rats, especially when they are also iodine deficient (6, 7).
There have also been several reports from the 1960’s of human infants developing goiter while on soy formula that lacked iodine (but adding iodine to the formula appears to have solved the problem) (8).
Another study found that half of the people who ate 30 grams of soybeans daily for 3 months developed symptoms of hypothyroidism and goiter, even though their TSH levels were technically still “normal” (9). These effects disappeared 1 month after they stopped eating the soy.
There is also some evidence that genistein, one of the isoflavones in soy, can bind to TPO and create a new antigen that the immune system may react to, possibly triggering Hashimoto’s (10).
Cooking does NOT change the goitrogenicity of soy, so people with hypothyroidism or Hashimoto’s may want to avoid it, even when cooked (7).
Like soy, millet also contains goitrogenic isoflavones that can interfere with thyroid function by reducing iodine uptake by the thyroid and interfering with TPO function. This may lead to goiter, especially if iodine intake is already low (11, 12).
Rural villages in which millet makes up more than 70% of the calories in the diet are more likely to have high rates of goiter than villages where millet is a smaller part of the diet (~30% of calories), even when controlling for rates of iodine deficiency (13, 14).
Millet should probably be consumed in moderation by people with hypothyroidism or Hashimoto’s, so as to not worsen thyroid function.
There is no clear definition of “moderation,” but based on the study mentioned above, it only appears to be problematic at intakes much higher than most people would consume.
Tapioca contains cyanogenic glucosides, which are metabolized into thiocyanates that reduce thyroid iodine uptake (2).
Consumption mostly appears to cause goiter when iodine or protein intakes are very low, and adding iodine to the diet can reverse it (15, 16).
Interestingly, most cultures that consume cassava soak it for several days before eating it. This presumably leaches out some of the goitrogenic compounds, since cultures that soak cassava have lower rates of thiocyanate exposure than cultures that don’t (17, 18).
Older studies have found that peanuts have some anti-thyroid properties in rats, but they can be reversed with adequate iodine intake (19). Human research is needed before any real recommendations can be made.
6. Pine nuts
One study found that diets high in pine nuts may be linked to goiter at an Indian Reservation in Chile, but more research is needed (20).
7. Babassu coconut
Babassu coconut is a type of fruit commonly consumed (along with mandioca, a cassava-like tuber) in some areas of Brazil that have a high incidence of goiter, despite adequate iodine intake (21).
Further analysis found that babassu has very strong anti-thyroid properties in rats, but human studies are needed (21).
8. Lima beans
Lima beans also contain cyanogenic glucosides, which interfere with iodine uptake in the thyroid. However, since lima beans are typically boiled for long periods of time, the amounts left intact in well-cooked beans might be negligible (22).
9. Flax (linseed)
Flaxseeds contain cyanogenic glucosides that impair iodine uptake by the thyroid.
A few studies have found that feeding animals diets of flax can cause goiter (23). However, this is easily prevented by adding supplemental iodine to the diet. No human studies have been done.
10. Raw bamboo shoots
Fresh bamboo shoots have been found to be rich in cyanogenic glucosides, and high intakes have been shown to reduce TPO activity and induce hypothyroidism within 3 months in rats (24).
What other substances can act as goitrogens?
Nitrates are used in the curing of many meat and fish products.
Feeding rats diets with 3% nitrates for 4 weeks was enough to reduce thyroid function in rats (25).
In humans, infants with higher levels of urinary nitrates tend to have higher TSH levels as well (26).
However, not all studies have found a link between nitrates and goiter (27).
Since bromine, chlorine, and fluoride are halogens like iodine, they can compete with iodine for space in the thyroid, reducing thyroid function (28).
Using a water filter that removes chlorine (including in the shower) and using non-fluoridated toothpaste/water are two ways to reduce exposure.
Perchlorate also inhibits the transport of iodine into the thyroid.
It is often found in drinking water, but it is unclear at what level it may start having goitrogenic effects. It may not be as big of a factor as thiocyanates and nitrates (29, 30, 31).
4. E.coli contaminated drinking water
Several goiter outbreaks are believed to have been caused by water infected with E. coli that produced anti-thyroid compounds (32, 33, 34)