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Synthroid and other Levothyroxine Drugs Have Stability and Potency Problems

FDA Requires Manufacturers to Demonstrate Safety, Efficacy and Consistency


Updated June 11, 2014

September 1997 -- If you are taking one of the thyroid hormone replacement drugs that -- like Synthroid, Levoxyl, and others -- contain levothyroxine sodium, then the following news is of major importance to your health.

According to the Federal Register, no currently marketed orally administered levothyroxine sodium product has been shown to demonstrate consistent potency and stability and, thus, no currently marketed orally administered levothyroxine sodium product is generally recognized as safe and effective.

The government found that often levothyroxine sodium drugs do not remain potent through their expiration dates, and tablets of the same dosage strength from the same manufacturer have been found to vary in potency from lot to lot in terms of the amount of active ingredient present. This lack of stability and consistent potency has the potential to cause serious health consequences to those of us taking these drugs.

Levothyroxine sodium was first introduced into the market before 1962, without an approved "New Drug Application" (NDA), apparently in the belief that it was not a new drug.

Since that time, almost every manufacturer of orally administered levothyroxine sodium products, including Synthroid, has regularly reported recalls that were the result of potency or stability problems.

In some cases, problems result from the fact that levothyroxine sodium is unstable in the presence of light, temperature, air, and humidity. Just since 1991, there have been no less than 10 recalls of levothyroxine sodium tablets involving 150 lots and more than 100 million tablets. In all but one case, the recalls were initiated because tablets were found to be subpotent or because their levothyroxine tablets lose potency before their expiration dates. The remaining recall was initiated for a product that was found to be too potent. During this period, FDA also issued warnings to a manufacturer regarding a levothyroxine sodium product that lost potency when stored at the higher end of the recommended temperature range, and one whose potency ranged from 74.7 percent to 90.4, instead of the required 90 percent to 110 percent.

Problems also stem from formulation changes. Because these products are marketed without NDA's, manufacturers have not had to file for FDA approval each time they reformulate their levothyroxine sodium products. Manufacturers have changed inactive ingredients, physical form of coloring agents and other product aspects, resulting in significant changes in potency, in some cases increasing or decreasing potency by as much as 30 percent. As a result, in some cases, people on the same dosage for years became toxic on the same dose. There is evidence that manufacturers continue to make these sorts of formulation changes which affect potency.

Now, 35 years after their introduction, the U.S. Food and Drug Administration (FDA) issued notice (Federal Register, August 14, 1997) that orally administered drug products containing levothyroxine sodium are officially classified as "new drugs" and will need to go through the NDA process because of the stability and potency problems that have recently come to light.

In order to continue marketing these drugs, manufacturers will need to submit an NDA with documented evidence that each company's product is safe, effective, and manufactured in a way to ensure consistent potency. Because the drug is necessary to millions of Americans, the FDA is allowing manufacturers to continue to market these products without approved NDA's until August 14, 2000, in order to give the companies enough time to conduct the various research studies and to submit their NDAs.

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