Lesson 1. Healthy Foods That Affect Your Thyroid Function

Nutritional guide for Hashimoto’s disease series

What’s good for one person is not necessarily good for another. This is especially true for thyroid patients. Some foods that are considered to be healthy and we eat every day could interfere with thyroid hormone production, cause goiter and contribute to hypothyroidism.

Goitrogens are substances that can block thyroid hormone formation by interfering with iodine uptake. Certain foods, drugs (amiodarone) and chemical compounds (perchlorate) contain goitrogens and may reduce thyroid function.

Consumption of goitrogenic foods with a diet can result in the enlargement of the thyroid gland (goiter) and/or hypothyroidism especially if a person is iodine deficient.

Goitrogenic Foods

Soy foods contain isoflavones which reduce thyroid hormone production. The isoflavones can disturb the thyroid activity involved in the iodination and affect coupling reactions required for T4 hormone synthesis.

Soy foods with isoflavones include:

Cruciferous vegetables contain isothiocyanates which have been linked to decreased thyroid hormone production. For example, in certain areas of India a high incidence of goiter occurs because the diet consists primarily of cabbage.

Cruciferous vegetables include:

Other foods that contain goitrogens include:

 Are These Foods Really Dangerous?

Soy and soy isoflavones have shown to have both risks and benefits. However, soy induced goiter and other hypothyroid indicators have been also observed in humans.

Goiter and high normal TSH levels in healthy iodine sufficient adults occurred as early as 1 month after consuming a diet that included 30 g of pickled soybeans per day. Furthermore, no changes in serum thyroid hormone T3 and T4 levels were found indicating a tendency to early stage of subclinical hypothyroidism with high normal or elevated TSH. After 1 month off of the soy diet, TSH decreased to the pre-treatment levels and goiters were diminished in size.

The major isoflavone synthesized by the soybean is genistein. Genistein is particularly harmful for people who have pre-existing low or borderline low thyroid function. A daily dose of genistein as low as 30 mg can affect normal thyroid function. Mega consumption of isoflavone containing food such as soy burgers can bring the total daily genistein intake to over 200 mg.

Soy products lose their goitrogenic properties after a long cooking or fermentation process. In the traditional Japanese diet, soy has been eaten as a condiment after it has undergone a long time fermentation (for months or sometimes years).

Fermented soy products such as miso (soup base), tempeh, and natto (fermented soy bean preparation) do not have the negative properties of unfermented soy.

Modern processed soy products, including soy burgers and soy cheese are not the same as fermented soy in the traditional Japanese diet. They are unfermented and include tofu and soy protein which do not provide the same benefits as fermented soy products.

Unfermented soy such as soy milk is also the second most common allergen. About 1% of the population in United States has dairy allergies including intolerance of cow’s milk. Most of these people also show an allergy reaction to soy milk.

Additional Risk Factors Of Hypothyroidism

It appears that additional risk factor(s) besides diet rich in soy may be also necessary to induce hypothyroidism. Consumption of soy products and other goitrogenic foods combined with iodine deficiency has a synergetic effect on inducing hypothyroidism and incidence of goiter putting the population living in the iodine deficient areas at risk. For example, the goitrogen in African cassava is widely consumed in central Africa where the iodine intake is low resulting in high incidence of goiter.

Iodine deficiency is an emerging concern in elderly Americans. Consumption of iodized salt, one of the main sources of dietary iodine may decrease with the desire or need to reduce the possible hypertensive effects of high salt intake. Those postmenopausal and elderly women who consume large amounts of soy products in conjunction with low sodium diet may be at higher risk for arising thyroid problems.

Groundnut (Arachis hypogaea) is a well known goitrogenic agent even in the presence of large amounts of iodine in the diet. Diet poor in essential ingredients like vitamins causes increased goitrogenic effect if groundnut consumed on a regular basis. This is of special significance to India, where raw or fried groundnut is used in large amounts in the human diet in various parts of the country.

Goitrogens in your diet

Most goitrogenic foods contain a lot of vitamins and minerals and provide a high nutritional value. Based on this fact, it is not advisable to completely exclude them from the diet.

The influence of goitrogenic foods on the patients with underactive thyroid function depends on how much of the thyroid gland is still involved in hormone production. If the thyroid gland is still partially functioning then the high consumption of goitrogenic foods could still affect its performance and decrease hormone output.

In the patients who have had their thyroid removed or are on a high dose of thyroid medication that practically completely substitutes the hormone production, eating goitrogenic foods should not make any difference. The thyroid hormones are not naturally produced and therefore the goitrogens should not be of any interference. The TSH has been normalized through prescription medication and patients are not at risk for developing a goiter.

For most cruciferous vegetables a relatively large quantity is necessary to affect the thyroid function. In addition, both isoflavones found in soy products and isothiocyanates contained in cruciferous vegetables are heat sensitive. Cooking will destroy about one third of these substances.

To avoid the negative properties of unfermented soy, it is wise to replace it with fermented soy product such as miso, tempeh and natto.

P.S.

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References:

The Cleveland Clinic Guide to Thyroid Disorders. M. Skugor, J.B. Wilder, Kaplan Publishing, 2009

Goitrogenic and estrogenic activity of soy isoflavones. D.R. Doerge, D.M. Sheehan, Environmental Health Perspectives, vol. 110, suppl. 3, June 2002

Studies of goitrogenic agents in food. Part I. Goitrogenic action of groundnut. V. Skinivasan, N.E. Moudgal, P.S. Saema, J Nutr.1957 Jan 10;61(1):87-95.