Soy Intake and Thyroid Health

Is Soy bad for my thyroid function? This is one of the most commonly asked questions from my patients. In the following paragraphs, I hope I can help to shine some light on this topic of confusion.

First, a brief general overview of the Soybean: Soybean is species of legume native to East Asia. Soybean is composed of protein at 40% and oil at 20%. The remainder consists of 35% carbohydrate and about 5% ash. Soybean contains significant amounts of essential amino acids required by the human body which our bodies are unable to synthesize.

Soybean also contains isoflavones which are organic chemicals with weak estrogenic effect (Phytoestrogenic). The two main isoflavones present in Soybeans are Genistein and Diadzein. Genistein, the major soy isoflavone, has a weak estrogenic effect in women as well as the ability to effect thyroid function, an activity independent of estrogenicity.

There are long standing debates as to whether or not soy is safe for thyroid function. Soy products are currently heavily marketed to women in the menopausal age group to help with symptoms such as hot flashes and night sweats. The potential anti-thyroid effect of soy might be more relevant because these women represent the age group in which overt hypothyroidism ( up to %4) and sub-clinical hypothyroidism (up to %10) are most likely to occur.

Some studies have shown that soy isoflavones are capable of effecting thyroid function via an inhibitive effect on the activity of an enzyme called Thyroid Peroxidase (TPO). TPO is one of the main enzymes inside the thyroid gland responsible for the manufacture of thyroid hormones. Current evidence shows that there are at least two factors that influence of soy on TPO activity:

  1. Such effect is dose-dependent and happens in much higher dosage than is normally consumed
  2. Additional risk factors for thyroid dysfunction are necessary before soy consumption can induce an anti-thyroid effect. Additional risk factors include:
    • Iodine Deficiency: greatly increases soy anti-thyroid effects, whereas iodine supplementation is protective. Thus, soy effects on the thyroid involve the critical relationship between iodine status and thyroid function. A link between iodine deficiency and soy-induced hypothyroidism in humans comes from studies of infants on soy formula in whom goiter was reversed upon supplementation with iodine.
    • Exposure to environmental goitrogens (causing enlargement of the thyroid gland and possibly affecting its function) present in water, food and air is another factor that could influence development of thyroid function abnormalities in presence of soy intake.

Take Home Message:

  • Soy products in high dosage, in a person with iodine deficiency and/or exposure to environmental goitrogens, have the ability to negatively affect thyroid function. The average individuals (one who neither consumes high quantities of soy products nor is iodine-deficient) should have no major concerns when it comes to enjoying moderate amounts of soy products (unprocessed)
  • “Moderate intake” translates to between 30mg and 50mg of isoflavones per day (see table below).
  • Minimum recommended daily allowance of iodine -in Micrograms (mcg): 120 for women (150 during pregnancy, 170 during lactation) and 150 for men should be adhered to. Under-consumption of iodine deprives the thyroid gland of the basic ingredient needed for manufacturing active thyroid hormones. Iodized salt as wells as other sources such as fresh ocean fish and seaweed for persons on a sodium-restricted diet are good choices to make sure you get your daily iodine intake. Please note: patients with autoimmune thyroiditis (also known as Hashimoto’s thyroiditis) should avoid taking more than the minimum recommended dosage since it will exacerbate the autoimmune response.
  • Those taking thyroid medication should separate any soy product intake and their medication by at least a few hours.

The following table is a sampling of total Isoflavone, Daidzein and Genistein Aglycone content of some common soy foods

Food Serving Total Isoflavones Daidzein (mg) Genistein (mg)
Soy protein concentrate, aqueous washed 3.5 oz 102 43 56
Soy protein concentrate, alcohol washed 3.5 oz 12 7 5
Miso 1/2 cup 59 22 34
Soybean, boiled 1/2 cup 47 23 24
Tempeh 3 oz 37 15 21
Soybean, dry roasted 1 oz 37 15 19
Soy milk 1 cup 30 12 17
Tofu yogurt 1/2 cup 21 7 12
Tofu 3 oz 20 8 12
Soybean, green boiled (Edamame) 1/2 cup 12 6 6
Meatless (soy) hot dog 1 hot dog 11 3 6
Meatless (soy) sausage 3 links 3 0.6 2
Soy cheese, mozzarella 1 oz 2 0.3 1

Additional Reading:

National Center for Toxicological Research Data on Goitrogenic and Estrogenic Activity of Soy Isoflavones (PDF)

To discover your path to wellness and schedule a health assessment, contact us today.