You may (or may not) have heard rumours surrounding certain green veggies being a problem for your thyroid because they contain goitrogens, or are “goitrogenic”. Your doctor may (or may not) have asked you to avoid the brassica family – broccoli, cauliflower, kale, brussel sprouts, cabbage, but not explained why. Either way, you may well be feeling confused and wondering what these goitrogens are if they’re really an issue for your thyroid.
Here’s the science behind goitrogens: When you chew a raw brassica veggie, e.g. broccoli, there is a chemical reaction between the enzyme myrosinase and the phytochemical glucosinolate (both found within the broccoli) which results in the formation of several different active compounds. Many of the active compounds are beneficial – sulforaphane for example, is anti-carcinogenic. But some of the active compounds are less beneficial – progoitrin in particular degrades in the body to form goitrin which can block iodine uptake in the thyroid. Iodine is required by the thyroid in order to produce the thyroid hormones, so blocking iodine update results in less thyroid hormones being produced, and may also result in a goitre forming (enlargement of the thyroid gland). This is known as a goitrogenic effect, and is the reason brassica veggies have their goitrogenic label.
Historically, results from scientific research has suggested that these goitrogenic compounds should be avoided completely, in order to prevent thyroid issues. So if your doctor asked you to avoid brassica veggies, he probably got his advice from this research.
But here’s the one thing you really, really need to know about the goitrogens in the brassica family: A 2016 review of all those historical studies has concluded that average consumption of raw broccoli, cauliflower, and cabbage is perfectly safe, and that you would need to eat over 1kg a day of raw brussel sprouts (about 100!!) or raw kale for several months, to cause any thyroid issues. Now that’s a lot of raw brussel sprouts!
If you’re still not sure, then the good news is that if you cook your brassica veggies, even just for one minute, the goitrogenic compounds breakdown and are no longer an issue.
Which is fortunate if you really, really like brussel sprouts!