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Levothyroxine Q&A

Information for Patients about Synthroid, Levoxyl, Unithroid and Levothroid


Updated December 13, 2003

Are All Levothyroxine Products the Same?

According to a study published in the April, 1997 issue of the Journal of the American Medical Association ([link url=http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=910334 4&dopt=Abstract]See PubMed Reference), Synthroid was found to be "bioequivalent" to Levoxyl and two other generic levothyroxine products. (Bioequivalent means that a drug has the same biological effects of that to which a medicine was compared.) The research was conducted by Synthroid's former manufacturers, Boots Pharmaceuticals and Knoll Pharmaceuticals, but results were not published for seven years. The company claimed it had concerns about the methodology of the research itself had commissioned, however critics claimed that the research was a disappointment to the company, which had been intended to show that Synthroid was superior in quality and therefore reason to justify prices two to three times higher than competitive levothyroxine products. The research was published despite the manufacturer's protest. (The publication of the research also led to a class action lawsuit filed against Synthroid's manufacturer in 1997, claiming that patients were overcharged during the period the research was not published, paying more for a produt that was allegedly superior, when the research disproved this. This class action lawsuit has ostensibly been settled in late 2000, however payments to patients have yet to be disbursed.)

At present, it could be argued that the levothyroxine products are not "the same" because only two of the four brand name products are FDA-approved. As the FDA wrote in its August 2000 approval notice for Unithroid, the first levothyroxine approved: ""Although oral levothyroxine drugs products have been marketed in the United States since the 1950's, the approval of Unithroid represents the first time that a single ingredient oral levothryoxine product has been approved by the FDA. The unapproved thyroid hormone replacement products that have been on the market have been associated with stability and potency problems. These problems have resulted in product recalls and have the potential to cause serious health consequences to the public. With the approval of the NDA for Unithroid, patients and physicians now have available to them an oral levothryroxine sodium drug product that has been determined to be safe and effective by the FDA and that also meets FDA standards for manufacturing processes, purity, potency, and stability."

The approval of Levoxyl, which took place in May of 2001, also added a second approved levothyroxine to the list of FDA-approved drugs.

Since Levothroid has filed but is still awaiting approval, and Synthroid has yet to file for approval, they are still considered unapproved thyroid hormone replacement products, and have yet to receive official FDA approved status, which determines them to be safe and effective and that also meets FDA standards for manufacturing processes, purity, potency, and stability.

Is It Hard to Switch Thyroid Medicines? Should You Switch?

People react differently to different thyroid medications. For example, some people are allergic to the fillers and dyes in one brand or dosage size of a levothyroxine product, but not to another. In other cases, if you have been taking a product that has had fluctuating potency, you may have had to switch dosages numerous times in response to fluctuating TSH levels and symptoms. And even in getting a refill of the same brand, or in switching from one brand to another, you may encounter batches of levothyroxine that have differing potencies, despite being the same stated "dosage."

Thyroid patients who require levothyroxine for the treatment of their thyroid condition may find that it is in their best interest to be taking a product that has been shown, through the rigorous government FDA approval process, to be a stable, consistent, potent levothyroxine drug that has minimal variations in potency, and assured stability, and will be consistently available without any risk -- however remote -- of interruption or regulatory difficulties.

Ultimately, your doctor is the best person to advise you regarding any potential changes in your thyroid hormone replacement.

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