Tea is so popular, according to the Linus Pauling Institute, that only water is consumed more often. Green tea is one of several varieties of tea; the others are black, white and oolong. Although green tea extract is sometimes used for a low thyroid condition, green tea flavonoids have been found to cause thyroid gland enlargement and decrease thyroid hormones.
Teas
All of the beverages commonly called "tea" are derived from the leaves of the Camellia sinensis plant. After the leaves are picked, they are withered and then treated in ways to cause oxidation, sometimes incorrectly called fermentation. Oxidized tea develops characteristic colors, tastes and fragrances before it is steamed and dried into the substance most people use in a tea cup. The oxidation process decreases a substance in the tea called catechins.
Catechins and Flavonoids
Different teas contain varying amounts of bioactive chemicals, called flavonoids, and because green tea is the least oxidized tea except for white, its level of flavonoids is higher relative to black or oolong teas. Tea flavonoids are also called catechins, and they are antioxidants, which help protect the body from free radicals -- molecules caused by normal body processes or toxins such as cigarette smoke. Green tea also contains caffeine -- about half as much, cup for cup, as coffee.
Research
Research reported in the August 2010 issue of "Human and Experimental Toxicology" showed that green tea extract caused a significant decrease in the level of thyroid hormones T3 and T4 circulating in the blood. Researchers administered black and green tea flavonoids to rats and then examined their thyroid glands. Rats given low doses of green tea extract had enlarged thyroid glands when compared to rats given black tea extract. Green tea extract also elevated the level of TSH, or thyroid stimulating hormone.
Hypothyroidism and Green Tea
If you have the form of thyroid disease called hypothyroidism, or low thyroid, you may need to take thyroid extract. This medication can be synthetic or an extract from the thyroid glands of pigs. If you have hyperthyroidism, the University of Maryland Medical Center reports that decaffeinated green tea extract is a recommended supplement, in doses from 250 to 500 mg a day.
Green Tea and Thyroid Medications
Although green tea can interact with some medications, there is no evidence that it interacts with thyroid medications. The prescribing information for the two most common thyroid medications, Synthroid and Armour Thyroid, do not carry any cautions or warnings about the use of green tea. If you have questions or concerns about your thyroid condition or the use of green tea, consult a health care professional.
References
- University of Maryland Medical Center; Green Tea; David Zieve, M.D., M.H.A., et al.; September 2010
- Mayo Clinic; Hypothyroidism; June 2010
- "Human and Experimental Toxicology"; Effect of Different Doses of Un-Fractionated Green & Black Tea Extracts on Thyroid Physiology; A.K. Chandra, et al.; August 2010
- Linus Pauling Institute; Tea; J. Higdon, Ph.D., et al.; January 2008
- University of Maryland Medical Center; Hyperthyroidism; Steven D. Ehrlich, NMD.; February 2010
- Forest Laboratories; Armour Thyroid; January 2010
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