Goitrogens: Do You Need to Avoid Broccoli, Kale and Soy?

No, most people don't need to avoid broccoli, kale or soy. Goitrogens are natural compounds in some foods that, in large amounts, can interfere with how a thyroid gland uses iodine. But a 2024 review of 123 studies concluded that brassica vegetables in a normal diet don't harm thyroid function, especially when you get enough iodine, and Australia adds iodised salt to most bread. The published cases of harm involve extreme amounts. If you take levothyroxine, the thing to watch with soy is timing, not the thyroid itself.

If you have Hashimoto's, an underactive thyroid, or no thyroid at all, you have probably seen lists telling you to cut out kale smoothies and tofu. Here is what goitrogens actually are, what the research in people shows, and where the real cautions lie.

What are goitrogens?

Goitrogens are natural compounds that can, in large enough amounts, make it harder for a thyroid gland to take up and use iodine to make thyroid hormone. The name comes from "goitre", an enlarged thyroid. The main sources are:

• Brassica (cruciferous) vegetables such as broccoli, kale, cabbage, cauliflower, Brussels sprouts and bok choy
• Soy foods
• Millet and cassava, which matter mainly in parts of the world where they are daily staples and iodine is scarce

What the research in people shows

The lab and animal findings are where the worry comes from. Human studies at normal intakes are far more reassuring:

• A 2024 systematic review of 123 studies concluded that including brassica vegetables in the daily diet, particularly with adequate iodine, poses no adverse effects on thyroid function.
• A 2022 review by endocrinologists in the US Endocrine Society's journal found little proven benefit in avoiding cruciferous vegetables or soy for people with thyroid conditions.
• In small studies, cooked Brussels sprouts every day for four weeks, a broccoli sprout drink for 12 weeks, and kale juice for a week did not change thyroid tests in healthy adults.

These human studies were small and mostly in healthy volunteers, so they can't rule everything out. But nothing in them suggests ordinary servings cause problems.

The case everyone quotes

The best-known case report is of an 88-year-old who ate one to one and a half kilograms of raw bok choy every day for several months and developed a severely underactive thyroid. That is many times what anyone eats in a normal diet. A 2024 review noted that cases like this involve an unusual amount and can't be turned into general rules about avoiding these vegetables.

Does cooking help?

Cooking lowers goitrogen levels. In one study, cooking cut a key goitrogen in cabbage by around 81 to 87%. It reduces them rather than removing them completely, but normal amounts are fine whether you eat these vegetables cooked or raw.

Why iodine is the real context

Goitrogens matter most where people are short of iodine, which is why the goitre problems linked to cassava and millet come from iodine-deficient regions. In Australia, most commercial bread has had to be made with iodised salt since 2009, and the latest national survey classed adults as iodine sufficient overall, although levels have slipped slightly. Getting iodine from ordinary food, such as bread, dairy, eggs and seafood, is the best background for eating brassicas freely.

That doesn't mean taking iodine supplements to "protect" against goitrogens. With Hashimoto's, extra iodine from tablets or kelp can cause problems of its own. Our guide to iodine, kelp and your thyroid explains the balance.

Soy: two separate questions

Does soy harm the thyroid?

Not when iodine intake is adequate, according to the evidence so far. A review of 18 randomised trials found soy had no effect on thyroid hormone levels and raised TSH only very slightly. You may have read that soy "triples your risk". That came from one 2011 study of 60 people with a mildly underactive thyroid who were not on medication and took a soy supplement. The same team's later trial at a higher dose found no increase.

Does soy affect levothyroxine?

Yes, but through absorption in the gut, not the thyroid. The Australian levothyroxine leaflet says soy products can reduce how much of your tablet you absorb, and that your dose may need adjusting when you start or stop soy supplements. So keep soy away from your tablet, like any food, keep your intake steady, and tell your doctor if you start or stop a soy protein powder or supplement.

Do goitrogens stop levothyroxine working?

No. The Australian levothyroxine leaflet doesn't mention broccoli, kale or cabbage at all. Take your tablet with water at least 30 minutes, and preferably 60 minutes, before food, and you can eat your vegetables as normal. Our guide to timing food and coffee with levothyroxine explains the gap.

After a thyroidectomy

Goitrogens act on a thyroid gland's use of iodine. After a total thyroidectomy, your hormone comes from your tablet, so there is little for them to act on. Small amounts of thyroid tissue can remain after surgery until radioactive iodine, and after a partial thyroidectomy the remaining side still works, but normal servings are fine either way. Our guide to what to eat after a thyroidectomy covers the rest.

Do goitrogens cause Hashimoto's?

There is no evidence that they do. Hashimoto's is an autoimmune condition, and no study has shown that broccoli, kale or soy cause it. Our guide to the Hashimoto's diet looks at what the research does and doesn't support.

Myths you can let go of

• "Never eat raw kale or broccoli." Normal raw amounts are fine; only extreme intakes have been linked to harm.
• "Goitrogens cause Hashimoto's." No evidence.
• "Soy destroys your thyroid." Trials show no meaningful effect when iodine is adequate.
• "Cooking removes all goitrogens." It reduces them, and normal amounts are fine anyway.
• "Broccoli blocks your thyroid tablet." It isn't on the label. Soy's effect on absorption is a timing issue.

Eat your vegetables. Get iodine from ordinary food. Keep soy, like any food, away from your tablet.

Frequently asked questions

What foods are goitrogens?

The main goitrogenic foods are brassica vegetables like broccoli, kale, cabbage, cauliflower, Brussels sprouts and bok choy, plus soy, millet and cassava. In normal amounts, with adequate iodine, research has not found them to harm thyroid function.

Should I avoid broccoli and kale with a thyroid condition?

No. A 2024 review of 123 studies concluded that brassica vegetables in a normal diet don't harm thyroid function, especially with enough iodine. The published cases of harm involved extreme amounts, such as more than a kilogram of raw bok choy a day for months.

Is soy bad for your thyroid?

A review of 18 trials found soy had no effect on thyroid hormones and only a tiny effect on TSH when iodine intake was adequate. If you take levothyroxine, keep soy away from your tablet and tell your doctor if you start or stop a soy supplement, because it can affect absorption.

Does cooking destroy goitrogens?

Cooking reduces goitrogens but doesn't remove them completely. In one study it cut a key goitrogen in cabbage by around 81 to 87%. Normal servings are fine whether cooked or raw.

Do goitrogens matter after a thyroidectomy?

Goitrogens act on a thyroid gland's use of iodine, and after a total thyroidectomy your hormone comes from your tablet. If some thyroid tissue remains, normal servings are still fine. The bigger issue on levothyroxine is timing food, especially soy, around your tablet.

The honest takeaway

Goitrogens sound worrying, but at normal intakes, with the iodine most Australians get from bread and dairy, broccoli, kale and soy are not a problem for your thyroid. The published harm involves extreme amounts. If you take levothyroxine, keep soy and other food 30 to 60 minutes away from your tablet, keep your habits steady, and enjoy your vegetables.

ThyroBase is a nutritional supplement and is not intended to diagnose, treat, cure, or prevent any disease. It is designed to sit alongside your thyroid medication, not replace it. This article is general information, not medical advice. Never start, stop or change a supplement or prescription medication without speaking to your doctor or pharmacist first.

References

Galanty A, Grudzińska M, Paździora W, et al. Do Brassica vegetables affect thyroid function? A comprehensive systematic review. International Journal of Molecular Sciences. 2024;25(7):3988.
Larsen D, Singh S, Brito M. Thyroid, diet, and alternative approaches. Journal of Clinical Endocrinology and Metabolism. 2022;107(11):2973-81.
Felker P, Bunch R, Leung AM. Concentrations of thiocyanate and goitrin in human plasma, their precursor concentrations in brassica vegetables, and associated potential risk for hypothyroidism. Nutrition Reviews. 2016;74(4):248-58.
McMillan M, Spinks EA, Fenwick GR. Preliminary observations on the effect of dietary brussels sprouts on thyroid function. Human Toxicology. 1986;5(1):15-9.
Chartoumpekis DV, Ziros PG, Chen JG, et al. Broccoli sprout beverage is safe for thyroid hormonal and autoimmune status: results of a 12-week randomized trial. Food and Chemical Toxicology. 2019;126:1-6.
Kim SSR, He X, Braverman LE, et al. Letter to the editor. Endocrine Practice. 2017;23(7):885-6.
Chu M, Seltzer TF. Myxedema coma induced by ingestion of raw bok choy. New England Journal of Medicine. 2010;362(20):1945-6.
Panduang T, Phucharoenrak P, Karnpanit W, Trachootham D. Cooking methods for preserving isothiocyanates and reducing goitrin in Brassica vegetables. Foods. 2023;12(19):3647.
Otun J, Sahebkar A, Östlundh L, et al. Systematic review and meta-analysis on the effect of soy on thyroid function. Scientific Reports. 2019;9(1):3964.
Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006;16(3):249-58.
Sathyapalan T, Manuchehri AM, Thatcher NJ, et al. The effect of soy phytoestrogen supplementation on thyroid status and cardiovascular risk markers in patients with subclinical hypothyroidism: a randomized, double-blind, crossover study. Journal of Clinical Endocrinology and Metabolism. 2011;96(5):1442-9.
Sathyapalan T, Dawson AJ, Rigby AS, et al. The effect of phytoestrogen on thyroid in subclinical hypothyroidism: randomized, double blind, crossover study. Frontiers in Endocrinology. 2018;9:531.
Bell DS, Ovalle F. Use of soy protein supplement and resultant need for increased dose of levothyroxine. Endocrine Practice. 2001;7(3):193-4.
Anitha S, Upadhyay S, Grando S, Kane-Potaka J. Does consumption of pearl millet cause goiter? A systematic review of existing evidence. Frontiers in Nutrition. 2024;11:1323336.
Eutroxsig (levothyroxine sodium) Product Information and Consumer Medicine Information. Australian Commission on Safety and Quality in Health Care, Medicine Finder. 2025.
Food Standards Australia New Zealand. Iodine fortification. foodstandards.gov.au.
Australian Bureau of Statistics. National Health Measures Survey 2022-24. abs.gov.au.