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What do thyroid supplements contain?

The body makes two major types of thyroid hormone: triiodothyronine (T3) and thyroxine (T4). While T3 is more potent, your body makes a lot more T4, which makes up about 80 percent of the body's thyroid hormone. Since the body automatically converts the T4 to T3 as needed, you need only take T4 supplements for a thyroid deficiency. Although some people report that they feel better when taking a product containing T3, most medical research has found little additional benefit from supplementing with T3.

Today, most people take synthetic thyroid supplements containing a version of T4 called levothyroxine. Common brand names are Synthroid, >Levoxyl, and >Levothroid. These have largely replaced the older, natural thyroid supplements, such as >Armour Thyroid, which contain both T3 and T4. Compared with natural thyroid supplements, levothyroxine causes fewer side effects and has a lower risk for allergic reactions. It's also easier for manufacturers to guarantee potency in synthetic supplements.

A few people take liothyronine (brand name >Cytomel), a man-made version of T3, for hypothyroidism. Liothyronine may be more likely than levothyroxine to cause side effects, including rapid or irregular heartbeat. Doctors may also find it more difficult to use standard thyroid function tests to monitor people on liothyronine.

Sources:
Micromedex® Healthcare Series. Thomson Micromedex, 2006.
Pharmacotherapy: A Pathophysiologic Approach. Appleton and Lange, 1999.
Wilson, J.D. Williams. Textbook of Endocrinology. Saunders Company, 1998.
Braverman, L.E. Werner and Ingbar's The Thyroid. Lippincott-Raven Publishers, 1996.
Drug Facts and Comparisons. Facts & Comparisons, 2006.

This answer prepared 5/25/2000.
This information updated 12/6/2006.