Thyroid hormones have an established role in many life-sustaining processes essential for normal growth, development, metabolism and reproduction. Thyroid deficiency, or hypothyroidism, during infancy or later in childhood can stunt physical growth and impair hearing, motor control and intelligence. Consuming soy products can increase risk of thyroid problems.
Goiter and Low Thyroid Function
People who eat large amounts of soy-based products can develop goiter. However, susceptibility to soy-induced goiters may not be substantial unless you are consuming an iodine-deficient diet. Goiter usually occurs as a result of prolonged iodine deficiency. The principle isoflavones in soy, or genistein and daidzein, inhibit the uptake of iodine by the thyroid gland.
Refuting Evidence
In 2003, Harvard Medical School researchers examined the effects of soy isoflavones on thyroid gland size and thyroid hormone biosynthesis. Their findings were published in the journal "The American Journal of Clinical Nutrition" in December 2010. The double-blind, randomized, controlled trial included 38 healthy postmenopausal women who were either given 90 mg of soy isoflavones or placebo tablets. There was no significant change in thyroid hormone levels reported within the six-month time period of this study.
Soy and Hypothyroidism
Soy isoflavones have been shown to partially inactivate thyroid peroxidase, a special enzyme involved in thyroid hormone production. Thyroid peroxidase, or TPO, converts inorganic iodine to active iodine, which is taken up into the thyroid gland for use. Iodine is an important trace element for the healthy production of thyroid hormones. Excessive consumption of soy foods can make an iodine deficiency worse. This leads to thyroid gland underfunctioning, known as hypothyroidism.
Soy and Thyroid Medication
Thyroid hormone replacement is the recommended therapy for clinical hypothyroidism. Your doctor will prescribe synthetic thyroid hormones that you will take daily. According to the NYU Langone Medical Center, soy protein can reduce the absorption of perorally administered T4, or thyroxine. This causes the drug to lose its therapeutic value, requiring a larger dose to offset the loss.
References
- Mayo Clinic; Hypothyroidism (Underactive Thyroid); June 12, 2010
- "The Anti-Estrogenic Diet"; Ori Hofmekler and Rick Osborn; 2007
- "Biochemical Pharmacology"; Anti-thyroid Isoflavones From Soybean; R.L. Divi et al.; Nov. 1997
- Harvard Medical School HealthBeat; Do soy products cause thyroid problems?; April 26, 2006
- "Nutrition Noteworthy"; Soy and the Thyroid: Can This Miracle Food Be Unsafe?; Monica Ralli; 2003
- NYU Langone Medical Center; Thyroid Hormone; February 2011
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