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Thyroid Drug Potency is Being Tightened: FDA Changes Specs for Levothyroxine

Synthroid, Levoxyl, Levothroid, Unithroid, and the Generics Are All Affected


Updated October 04, 2007

Thyroid Drug Potency is Being Tightened: FDA Changes Specs for Levothyroxine
Ultimately, however, in 2009 when the new specifications go into effect, the narrowing of the potency range for levothyroxine should result in more consistent results for patients. When you switch brands or dosage size of levothyroxine, there will be less of a gap in effectiveness, and your medication should, in general, more reliably deliver levothyroxine in a dosage closer to -- and no more than 5% more or less than -- the stated dose.

And if you, like millions of Americans, take a generic levothyroxine, the narrower range will reduce the possibility of wider potency fluctuations -- and thus, overdosage, underdosage, or poor suppression of your thyroid levels -- that can occur after prescription refills. When you refill a generic, you typically receive levothyroxine from whichever manufacturer your distributor has available at that time, so potency fluctuations have been a particular concern for patients on generic levothyroxine. (Read more about concerns about generic levothyroxine based on potency standards.)

More Information from the FDA


FDA Acts to Ensure Thyroid Drugs Don't Lose Potency Before Expiration Date," Press Release, October 3, 2007, online

Mary Shomon, About.com's Thyroid Guide since 1997, is a nationally-known patient advocate and best-selling author of 10 books on health, including "The Thyroid Hormone Breakthrough: Overcoming Sexual and Hormonal Problems at Every Age," "The Thyroid Diet: Manage Your Metabolism for Lasting Weight Loss," "Living Well With Hypothyroidism: What Your Doctor Doesn't Tell You...That You Need to Know," "Living Well With Graves' Disease and Hyperthyroidism," "The Thyroid Guide to Hair Loss," "Living Well With Autoimmune Disease," and "Living Well With Chronic Fatigue Syndrome and Fibromyalgia." Click here for more information on Mary Shomon.

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