Can I eat kale with a thyroid problem?
Q: I have an underactive thyroid and I’ve heard I shouldn’t eat broccoli or kale. Is this true? Why is this? Many thanks, Joan
A: Broccoli and kale are in the brassica family of vegetables; also known as cruciferous vegetables. Other vegetables such as cabbage, cauliflower and brussels sprouts are part of this group, too.
The brassica family of vegetables remain some of my favourites due to their anti-cancer properties and liver detoxification support. They are extremely nutrient-dense and they contain a superhero component called sulforaphane. Sulforaphane is an antioxidant and stimulator of natural detoxifying enzymes, and it may reduce the risk of breast, bladder and prostate cancers.
Brassica family vegetables naturally contain substances that are known as goitrogens. Goitrogens interfere with iodine uptake by the thyroid gland and iodine is needed for the production of thyroid hormones.
With that said, research tells us that goitrogens generally do not have any negative impacts on thyroid function in people without an iodine deficiency. It’s also important to note that goitrogens in vegetables such as broccoli and kale are inactivated by high temperatures, which means that cooking these vegetables will significantly decrease, or potentially even eliminate, any goitrogenic effects.
So if you have an underactive thyroid and are concerned about goitrogens, you may prefer to eat these vegetables mostly in their cooked forms. Lightly steaming or stir-frying them is a good option, as overcooking – especially boiling in large amounts of water – can destroy and remove other important nutrients from the vegetables, particularly vitamin C.
It’s also important to consider what is at the heart of your underactive thyroid, as the road in will be the road out.
Knowing the cause is so essential for determining the best path forward for you, and in clinical practice, this is something that I always get to the bottom of before advising someone nutritionally.
For example, hypothyroidism can be caused by nutritional deficiencies (primarily iodine, but also selenium and iron), so if this was the case for you, fixing the nutritional deficiencies would be key in addressing your low thyroid function, and ensuring you are not consuming excessive amounts of raw brassica family vegetables (that is, consuming large amounts of active goitrogens) would become more important.
However, hypothyroidism can also be caused by an autoimmune condition called Hashimoto’s thyroiditis. In this case, iodine deficiency is not the mechanism causing the thyroid to be underactive, however concurrent iodine deficiency could exacerbate this.
So to summarise, I wouldn’t recommend a raw broccoli- and kale-only diet (and not just because of the goitrogen content – we need to consume a wide variety of foods daily to meet our nutrient needs), but enjoying these vegetables regularly is very unlikely to cause you any issues (in fact, it’s much more likely they will do you good), especially if you mostly eat these vegetables cooked. The health benefits of consuming brassica family vegetables are not to be underestimated, so I never want anyone to avoid these vegetables unnecessarily.
Dr Libby is a nutritional biochemist, best-selling author and speaker. The advice contained in this column is not intended to be a substitute for direct, personalised advice from a health professional. See drlibby.com
Kale is part of the brassica family of vegetables.