Soy and Thyroid Health: What You Need to Know

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Soy, often promoted as a healthy food, becomes controversial when the subject of the thyroid is involved. For almost two decades, there's been a continuing debate on whether soy can negatively affect the thyroid, and this debate continues. So, here are some dos and don'ts for soy consumption, keeping your thyroid health in mind.

About Soy

Soy (or soybeans) are a type of legume, high in protein, that contain phytoestrogens, which are plant-based estrogens.

In recent years, soy has become popular and can now be found not only in its traditional food forms such as miso, tempeh, tofu, and edamame, but also processed into burgers, protein bars, protein powders, shakes, soy milk, and nutritional supplements.

Soy's Pros

  • There is limited evidence that soy can help some women slightly alleviate menopausal symptoms such as hot flashes.
  • Substituting soy for protein sources higher in saturated fat may result in a small reduction in your "bad" LDL cholesterol.
  • Some studies of soy and the thyroid show that if you have normal thyroid function and sufficient iodine levels, eating soy foods or ingesting soy isoflavones does not adversely affect your thyroid function.

Soy's Cons

  • There is no evidence to support soy's claims of having specific benefits for heart health, weight loss, or cancer prevention.
  • Soy is a goitrogen, a food that promotes an enlarged thyroid known as a goiter. Consuming large quantities of soy, or high concentrations of isoflavones, can have an antithyroid effect, slow your thyroid function, and in some cases, trigger thyroid disease.
  • A number of studies of soy and the thyroid have found that soy has various negative effects your on thyroid function. This is most likely to occur if you are iodine deficient.
  • In some cases, soy can inhibit your ability to absorb thyroid medication properly.
  • Much of the soy available in the United States is genetically modified "GMO" soy. The physiological effects of genetically modified soy and other foods are controversial, but some experts theorize that GMO soy may pose health risks.

    Tips on Eating Soy for People with Thyroid Disease

    Until we have definitive, rigorous, high-quality studies on soy toxicity and the effects of soy on thyroid function, you can't assume that soy is universally safe for your thyroid health. But if you want to include soy in your diet, here are some guidelines.

    • Be sure that you are not iodine deficient. This is tricky, however, because the only way to truly assess if you are deficient in iodine is to have a urinary iodine clearance test. Don't supplement with iodine without clearly establishing that you are deficient because just as iodine deficiency can trigger a thyroid problem, excess iodine can also aggravate and worsen your thyroid problems. If you are iodine deficient, however, proper iodine supplementation may help your thyroid function, and minimize the potential risks that soy may have on your thyroid function.
    • Consider avoiding soy. If you are a thyroid patient with optimized thyroid treatment, and you're still suffering from hypothyroidism symptoms, consider eliminating the soy from your diet to see if that helps relieve symptoms.
    • Avoid GMO soy. If you are eating soy foods, you may want to avoid GMO soy foods until the debate over their safety has been definitively resolved.
    • Choose fermented soy. If you are going to eat soy, select fermented and food forms of soy, for example, tofu, tempeh, and miso. Avoid processed soy products such as soy powders, protein shakes, and other manufactured forms of soy.
    • Don't overconsume soy. It's probably safe to include some soy in your diet, but a daily diet of soy milk, edamame, tofu, tempeh, miso, soy burgers, soy bars, soy ice cream, and soy protein shakes is going overboard. Generally, you'll want to limit soy and isoflavone consumption to less than 30 mg per day, at most. The negative effects of soy have most often been observed when consumption levels exceed 30 mg.
    • Avoid soy supplements. You may want to avoid soy or isoflavone supplements, as they tend to have very high isoflavone levels, and there is little evidence of their benefits.
    • Keep soy apart from your thyroid medication. Do not eat soy foods within three to four hours of taking your thyroid hormone replacement medication, to avoid any interference with your thyroid medication.
    • Limit raw soy foods. Be careful about the overall quantity of goitrogenic foods that you consume raw, especially if they are in addition to soy foods, which are known goitrogens.

    A Word from Verywell

    Keep in mind that soy is one also a common allergy-triggering food, so even if it is not affecting your thyroid specifically, it may be triggering symptoms of an allergic response, which can include acne, swelling, a stuffy nose, diarrhea, stomach pains, heart palpitations, skin rashes, itching, hives, swelling in the throat, fatigue, and episodes of low blood pressure.

    Also, remember that if you do not have a thyroid gland due to congenital hypothyroidism or surgery, or you have a totally non-functioning gland due to radioactive iodine ablation (RAI) treatment, you don't need to be concerned about the effects of soy on your thyroid gland. Soy can, however, still interfere with absorption of your thyroid hormone replacement medication, so be sure to take your medication at least three hours apart from soy foods.

    Sources:

    D'Adamo CR. "Soy foods and supplementation: a review of commonly perceived health benefits and risks." Altern Ther Health Med. 2014 Winter;20 Suppl 1:39-51. 

    Messina M. "Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature." Thyroid. 2006 Mar;16(3):249-58.

    Šošić-Jurjević B et al. "Soy isoflavones interfere with thyroid hormone homeostasis." Toxicol Appl Pharmacol. 2014 Jul 15;278(2):124-34. doi: 10.1016/j.taap.2014.04.018. Epub 2014 Apr 29.

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