Soy foods, those derived from the soybean, contain compounds known as isoflavones. In the plant the soy isoflavones are bound to a sugar molecule creating a compound known as glycosides. Once ingested, the process of digestion releases the sugar portion creating an isoflavone aglycone that can trigger responses within the body. In addition to inducing mild hormone-like activity, isoflavones can affect thyroid function.
Thyroid Gland
Stimulated by thyroid stimulating hormone, TSH, produced by the pituitary gland, the thyroid gland absorbs iodine and combines it with the amino acid tyrosine to produce the two thyroid hormones -- thyroxine also referred to as T4 and triiodothyronine, known as T3. The thyroid hormones then travel throughout the body to regulate each cell's metabolism, the process by which oxygen converts into energy. Because thyroid hormones affect all cells, and therefore all bodily systems, problems with the thyroid can cause a variety of symptoms.
Thyroid Problems
Thyroid problems are common, especially problems that cause hypothyroidism -- too little thyroid hormone, or hyperthyroidism -- too much thyroid hormone. Hyperthyroidism causes the body's metabolism to increase resulting in weight loss, irregular heartbeat, sweating, nervousness and irritability. Hypothyroidism causes the metabolism of the body to slow down leading to obesity, joint pain, infertility and heart disease.
Effects of Soy
Soy isoflavones can inhibit the activity of the enzyme known as thyroid peroxidase, or TPO. Cells in the thyroid gland need TPO to extract the iodine from foods and produce thyroid hormones. Eating large amounts of soy products can therefore inhibit the production of thyroid hormones leading to hypothyroidism. In addition, the presence of isoflavones can cause the thyroid to become inflamed, a condition known as goiter, to allow for additional blood to flow through in an attempt to extract more iodine. However, the Linus Pauling Institute indicates that eating soy products fails to produce a negative effect on the thyroid gland as long as iodine intake remains sufficient.
Iodine Intake
In order to prevent soy foods from negatively affecting your thyroid be sure to consume enough iodine in your diet. The main source of dietary iodine is iodized table salt. Since the addition of iodine to table salt the incidence of iodine deficiency became rare in the United States and other developed countries, according to the University of Maryland Medical Center. The National Institutes of Health Food and Nutrition Board recommend that adults consume 95 mcg of iodine per day.
Soy and Thyroid Medications
If you suffer from hypothyroidism your doctor will likely prescribe synthetic thyroid hormones to boost the missing hormone levels and reduce symptoms. The soy isoflavones in soy food products may interfere with the absorption of the medication. To prevent this interaction, MayoClinic.com suggests taking the thyroid hormone on an empty stomach and waiting at least four hours before consuming soy products.
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