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What is the difference between natural and synthetic thyroid supplements?

Thyroid supplements are drugs that treat hypothyroidism, a condition where the body doesn't make enough thyroid hormone on its own. Both natural and synthetic (man-made) thyroid supplements are available by prescription.

Natural thyroid products were available first. Examples are Armour Thyroid and >Thyroid USP. They're made from animal thyroid glands and contain two thyroid hormones, T3 (triiodothyronine) and T4 (thyroxine). Natural thyroid products tend to be less consistent from batch to batch than the synthetic versions. In fact, some people have developed serious side effects from natural thyroid products that contained greater-than-expected amounts of T3. Natural thyroid supplements are also more likely to cause allergic reactions than the synthetics.

Today most people take synthetic thyroid supplements, which contain a synthetic version of T4 called levothyroxine. Common brand names are >Synthroid, >Levoxyl, and >Levothroid. Advantages of these products include low cost, low risk of allergic reactions, and standardized potency. Synthetic thyroid supplements come in several different tablet strengths, which makes it easier to get a precise dose.

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.
Drug Facts and Comparisons. Facts & Comparisons, 2006.

This answer prepared 11/17/2000.
This information updated 2/7/2007.