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Soybeans and cruciferous vegetables like broccoli, cauliflower and cabbage are potentially goitrogenic foods, that is, they can disrupt the function of the thyroid gland.
Concerning cruciferous vegetables, it is documented that the risk that their ingestion could alter the thyroid function is low if one’s iodine intake is adequate. So there is no need to avoid them. It should also be noted that heating cruciferous vegetables inactivates the potential goitrogenic effects.
For soybeans, if there is no abnormality in the thyroid gland, there is little risk that their ingestion could affect thyroid function. However, the effect of soy consumption on people with hypothyroidism remains controversial. One study found a three times higher risk of developing hypothyroidism with a vegetarian diet (16 mg per day of soy phytoestrogens) compared to a western diet (2 mg per day with soy phytoestrogens) in patients with subclinical hypothyroidism. However, in a more recent study, a very high pharmacological dose of soy phytoestrogens (66 mg per day) did not affect thyroid function in patients with subclinical hypothyroidism. More studies are therefore needed to better understand the effects of soy consumption on thyroid function in people suffering from hypothyroidism. In the meantime, those affected with the condition should eat soy foods in moderation. It should also be noted that in the case of medicated hypothyroidism, it is recommended to wait a few hours after taking thyroid hormone replacement (Synthroid) before consuming soy.
Vitamin D deficiency is associated with hypothyroidism. In addition, untreated hypothyroidism can lead to bone loss which can be exacerbated by the vitamin D deficiency commonly seen in people with hypothyroidism. Vitamin D is mainly synthesized by the skin when it is sufficiently exposed to the ultraviolet rays of the sun. It is found naturally in very few foods. The only foods that contain it in its natural form are oily fish, shiitake mushrooms and egg yolks. The other sources of vitamin D in our diet come from fortified foods such as milk, margarine and plant based drinks. In North America, during the winter months, the skin is not sufficiently exposed to the sun to produce enough vitamin D and it is very difficult to meet one’s vitamin D needs through food only. Thus, a vitamin D supplement is recommended during the winter months for adults of all ages, and all year round for people aged 50 and over.
Selenium is an essential trace element that is an integral part of thyroid function. Selenium requirements for adults are 55 μg per day. The selenium content of plant foods depends on the amount of selenium in the soil of a given region. In North America and Europe, selenium supplementation is seldom necessary since our needs are largely met through food. Wheat grown in North America is a good source of selenium. In Canada, all farm animals receive a selenium supplement in their diet. Fish, seafood and certain nuts (Brazil nuts and walnuts) are also excellent sources of selenium. For example, 6 to 8 Brazil nuts provide about 840 μg of selenium, 90 g (3 oz) of canned tuna contains 70 μg, and a slice of whole wheat bread contains 10 μg.
Studies show that people with hypothyroidism are more at risk for vitamin B12 deficiency. It is therefore recommended to have an annual blood test to check your B12 blood level. This vitamin is mainly found in foods from animal sources (meat, poultry, fish, dairy products and eggs).
Soybeans, coffee, as well as calcium and iron supplements may interfere with the absorption of drugs for hypothyroidism (Synthroid). Thus, it is recommended not to consume them at the same time.