|There is no reason for the vast majority of people to be concerned about the effect of soyfoods on thyroid function. Even hypothyroid patients can safely consume soyfoods|
The impact of soyfoods on thyroid function has been studied for more than 70 years so a lot is known about this relationship . But before discussing this information it is important to have some understanding of the function of the thyroid.
The thyroid is one of the largest endocrine glands (a gland that secretes a substance such as a hormone into the bloodstream) in the body. This gland is found in the neck below where the Adam’s apple is located in men. The thyroid controls how quickly the body burns energy (metabolic rate) and affects every organ in the body. It accomplishes these tasks by synthesizing two hormones called thyroxine (T4) and triiodothyronine (T3). Iodine, which is an essential mineral, is needed for thyroid hormone synthesis. The production of thyroid hormones is controlled by another hormone called thyroid stimulating hormone which is produced by the pituitary gland.
Symptoms of hypothyroidism, which occur when too little thyroid hormone is produced, include weight gain and lethargy whereas symptoms of hyperthyroidism, which occur when too much thyroid hormone is produced, often include weight loss and trouble sleeping [2, 3].
Research in rodents (rats are extremely sensitive to factors affecting the thyroid) conducted in the 1930 and 1940s showed that feeding diets containing large amounts of soy produced goiter (enlarged thyroid). And in the 1960s, several cases of goiter were identified in infants fed soy formula [4-6]. However, increasing the iodine content of the diet of the experimental animals and fortifying soy infant formula with iodine (which is the current practice) eliminated these problems.
In humans with a healthy thyroid consuming sufficient iodine there is simply no question that soyfoods don’t cause thyroid problems – more than 20 clinical (human) studies have shown this to be the case . Some of these studies have been quite long – two to three years in duration . Thus, for the vast majority of people soyfoods are nutritious additions to the diet whose consumption should be encouraged.
Soyfoods may inhibit somewhat the absorption of synthetic thyroid hormone, such as Synthroid, which is taken by hypothyroid patients . However, foods in general have this effect, as do enriched fiber foods, herbs and many drugs . This is why thyroid hormone is taken on an empty stomach and why hypothyroid patients can still consume soyfoods. If there is any small effect on absorption, the medication dose can easily be adjusted accordingly.
It was noted above that iodine intake is essential for the production of thyroid hormones. So what effect does soyfood consumption have on the thyroid of individuals whose iodine intake is inadequate? The answer to this question may never be known for certain because it would be unethical to design an experiment to examine this question. Understandably, it would be wrong to restrict the iodine intake of healthy subjects or to not inform subjects of their inadequate iodine intake. Fortunately, iodine intake is United States is in general very good because of the use of iodized salt . When an individual’s intake is inadequate, the appropriate recommendation is to increase iodine intake rather than to avoid soyfoods.
Finally, there is a condition called subclinical hypothyroidism that affects anywhere from 5 to 10% of older individuals . The effect of soyfoods on this population is currently being studied and the results should be known soon. There is no human evidence indicating soyfoods are contraindicated for subclinical hypothyroid patients and since these patients are typically being monitored by a physician, any effect of consuming soy can be detected.
The important point is that for the overwhelming majority of the population consuming soyfoods will not in any way impair thyroid function.
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