Soy is often referred to as a goitrogenic food or as containing goitrogens, but what does this mean?
Goitrogens are chemical substances found naturally in certain foods that can suppress thyroid function and potentially cause a goitre, which is an enlargement of the thyroid gland.
Aside from soy, the Brassica vegetables including cabbage, kale, broccoli, cauliflower, Brussels sprouts and kohlrabi also contain goitrogenic chemicals, but luckily, they are easily inactivated by heat, so they pose no threat to thyroid function if the vegetables are cooked. If you do have low thyroid function, I would advise against consuming large quantities of the Brassica vegetables raw.
With regard to soy products, the isoflavones in soy can also inhibit one of the enzymes involved in the production of thyroid hormones (thyroid peroxidase or TPO), but this only occurs in people who are deficient in iodine. The Recommended Daily Intake (RDI) for iodine is 150mcg. If you’re on a plant-based diet, the best sources of iodine are vegan multivitamins (which generally contain 100% of the RDI), iodised salt and sea vegetables (such as nori, wakame and dulse flakes). Interestingly, excessive iodine from certain sea vegetables is also classified as goitrogenic, so I would recommend a modest serving of sea vegetables 3 times per week only. As a public health initiative to reduce the incidence of iodine deficiency, bread is required by law to contain iodised salt, however, 2 slices only contains around 30mcg (20% of the RDI), and organic breads are exempt from having to use iodised salt. Fresh vegetables should be a good source of iodine, but unfortunately in Australia our soil is very low in iodine, which means that our fruits and vegetables don’t contain much iodine at all.
If you have been diagnosed with borderline low thyroid function (subclinical hypothyroidism), there is some evidence that soy products can increase the progression of subclinical hypothyroidism to true hypothyroidism (12) so if you know your thyroid function is low, it may be wise to limit your intake of soy foods to 1 serve per day (1 cup soymilk, 100g firm tofu or tempeh or ½ cup edamame or cooked soybeans), and ensure you’re getting enough iodine.
If you have overt hypothyrodisim and take thyroid hormone medication (thyroxine), soy products can interfere with the absorption of the medication, so it’s wise to separate any soy foods from taking these medications by 3 hours or so.
- Ensure you have a reliable daily source of iodine providing 150mcg iodine per day, e.g. iodised salt or a vegan multivitamin
- If you have borderline low thyroid function, minimise raw Brassica vegetables only and limit your soy intake to 1 serve per day (1 glass soy milk, 100g firm tofu or tempeh or ½ cup edamame or cooked soy beans). Remember that kale is a Brassica family vegetable so use baby spinach in your green smoothie instead of kale!
- If you have hypothyroidism and are on thyroxine, separate taking the medication from soy products by at least 3 hours