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Is your healthy diet working against your thyroid gland? Many plant foods we think of as healthy contain risky molecules that can disrupt hormonal and metabolic balance.
The main job of the thyroid gland, a small butterfly-shaped structure located at the base of the throat, is to combine iodine (a naturally occurring salt) with the amino acid tyrosine (from protein) to make thyroid hormone. Thyroid hormone is critical to the metabolism of nearly every cell in the body and is critically important to the proper development of the brain in early life.
When the thyroid gland has a hard time making enough thyroid hormone, it can become stressed and can even grow bigger to try to do its job better, forming a goiter (enlarged thyroid).
People who don’t get enough protein or iodine in their diets are at high risk for goiter. In the developed world, where high-quality, protein-rich foods are plentiful and iodine is often added to salt and processed foods, we don’t typically need to worry about protein malnutrition or iodine deficiency. However, the rest of the world is not so lucky. More than two billion people suffer from hypothyroidism due to iodine deficiency, which is the number one cause of intellectual disabilities in the world.
Yet there is a third little-known risk factor for hypothyroidism that people in every country need to know about: dietary goitrogens. Goitrogens are natural substances in plants that interfere with normal thyroid function and therefore have the potential to cause goiter, particularly when protein and/or iodine are in short supply. A staggering variety of plant foods contain goitrogens. Here are five goitrogenic foods you should know about.
1. Soy
Soybeans contain two goitrogens: genistein and daidzein. These are often referred to as soy flavonoids or soy isoflavones, which we are usually told are good for us, yet it has long been known that these compounds can cause hypothyroidism and goiter. Soy flavonoids reduce the activity of thyroid peroxidase, the enzyme that inserts iodine into the thyroid hormone.
There is strong clinical evidence demonstrating the anti-thyroid effects of soy products on infants, children, and adults. Studies have found that infants fed soy formula can develop hypothyroidism or — in more severe cases — goiter, which is usually reversed when the soy formula is discontinued. After the 1960s, manufacturers reportedly began adding iodine to formulas to reduce this risk. When a baby is born with hypothyroidism, thyroid hormone supplements are prescribed to correct the deficiency. Babies fed soy formula require 25% higher doses of thyroid hormone than babies fed soy-free formula to bring thyroid hormone levels into the normal range.
Adults are at risk as well. Consider this interesting clinical study: 60 patients with borderline hypothyroidism were given either 2 mg of soy isoflavones (the amount found in the typical omnivore’s diet) or 16 mg of soy isoflavones (the amount estimated to be in a typical vegetarian diet). The “vegetarian” dose of soy isoflavones was three times more likely to cause patients to convert from borderline (“subclinical”) hypothyroidism to full-blown (“overt clinical”) hypothyroidism.
Soy goitrogens cannot be destroyed by cooking.
2. Cassava
Cassava root is an important staple starch for two billion people, particularly in developing areas of sub-Saharan Africa, Asia, and Latin America, where it is eaten boiled, mashed, or ground into flour. Here in the U.S. we used to use cassava almost exclusively to make tapioca, but it has recently invaded the booming plant-based junk food industry, finding its way into all manner of chips and snacks.
When crushed, cut or chewed, raw cassava root generates cyanide, which the human body converts into far less toxic thiocyanates—sulfur-containing goitrogens found in a wide variety of vegetables.
Thiocyanates make it harder for the thyroid gland to absorb iodine because they compete with iodine for entry into the gland. This effect can be minimized by supplementing the diet with iodine; the excess iodine can then crowd out the thiocyanate and win the competition.
Thiocyanates also weaken the activity of the enzyme thyroid peroxidase, which is required to insert iodine into thyroid hormone. This effect can also be greatly reduced by iodine supplementation. Thiocyanates easily cross the placenta and can cause thyroid dysfunction in newborns, especially if the infant is not getting enough iodine.
Modern methods of starch extraction completely remove the cyanide from cassava, but be aware that if you purchase fresh cassava root you must peel it and boil it thoroughly (taking care to discard the boiled water) to remove the cyanide and substantially reduce thiocyanate levels. This journal article provides more details about how to reduce cyanide levels in cassava.
3. Cruciferous vegetables
All cruciferous vegetables such as broccoli, cauliflower, kale, and brussel sprouts contain thiocyanates. Therefore excessive consumption—particularly of raw cruciferous vegetables — should be avoided.
Raw rutabagas and turnips also contain a very powerful goitrogen called goitrin. Unlike most other goitrogens, this chemical can cause goiter even if there is plenty of iodine in the diet. Goitrin significantly weakens the activity of the enzyme thyroid peroxidase. It is very important to cook rutabagas and turnips before eating to avoid exposure to goitrin.
For a complete list of cruciferous vegetables and ways to reduce their thiocyanate content, please see my short post “Is Broccoli Good for You?”
4. Millet
Millet is most familiar to us in the U.S. as birdseed, but it is also a staple grain eaten by people in developing countries because it grows well in hot places with poor quality soil. Lately, this grain is gaining in popularity in the U.S. as a gluten-free alternative to wheat.
Millet contains three goitrogenic flavonoids which greatly reduce the activity of thyroid peroxidase. Millet flavonoids also (quite rudely) push thyroid hormone off of carrier proteins in the bloodstream.
Cooking does not destroy millet flavonoids.
5. Onions
Onions contain two different goitrogens: quercetin and propyl disulfide. Quercetin reduces the activity of thyroperoxidase as well as the activity of a second enzyme — hepatic deiodinase — a liver enzyme required to activate the thyroid hormone. Although onions are particularly rich sources of quercetin, quercetin is found in a wide variety of fruits and vegetables.
Boiling reduces the amount of quercetin in foods to some extent.
The Bottom Line
If you have an underactive thyroid, you may want to consider reducing your intake of plant goitrogens. Pregnant and nursing women may want to be especially careful about goitrogenic foods, as may caregivers of very young children whose brains are still developing. For more information and lists of other goitrogenic foods including sweet potato, cranberries, and asparagus, please see my more detailed article "Foods That Cause Hypothyroidism."
