Q: What are the differences in thyroxine preparations?

A: Synthroid®, Levoxyl®, Levothyroid®, and Unithroid® are the brand-name forms of thyroxine currently marketed in the U.S. In addition, there are several different generic versions of thyroxine on the U.S. market. The U.S. Food and Drug Administration (FDA) considers most of these preparations to be equivalent to each other. Therefore, in most cases your pharmacist is permitted to give you a generic thyroxine product instead of a brand-name product, unless your prescription specifies “no substitutions.”

Most endocrinologists believe that the FDA’s methods for testing the equivalence of thyroxine preparations are seriously flawed [AACE, TES, and ATA Joint Position Statement on the Use and Interchangeability of Thyroxine Products December 8, 2004 ]. One of the problems with the FDA’s methods is that they do not use TSH blood tests, the cornerstone of thyroxine monitoring, to compare different thyroxine preparations in study participants. Another concern is the many factors that can influence thyroxine absorption and availability to your body. There is variation in the thyroxine content among the various approved preparations with the same stated dose on the tablet, in some cases greater than 10%. This is a potential problem if a patient switches thyroxine preparations.”[“Generic Levothyroxine.” The Medical Letter on Drugs & Therapeutics September 27, 2004].

Thyroid disease often requires lifelong therapy and is best managed with consistent and precise treatment with the same brand of thyroid hormone. Your doctor may change your dose of thyroid hormone, but the brand of your thyroid hormone medication should be constant.

When you go to the pharmacy, do not change the brand of your thyroid medication without checking with your doctor. You should not change from one brand of thyroid medication to another, from your brand of thyroid medication to a generic product, or from one generic product to another without first checking with your doctor. Your insurance company or state aid program, however, may not pay for the cost of a brand name drug or charge a higher co-payment if you want a specific brand name drug. Repeat blood tests and visits to your doctor may be required if your thyroxine preparation is changed, and your dose may need to be readjusted. Finally, be familiar with the shape and markings on your branded levothyroxine product. The generic levothyroxine product may contain the brand name as part of the labeling, so be sure to inspect your tablets when you receive a refill.

Take a look at our Patient Web Brochure Hypothyroidism to learn more about hypothyroidism and how it is treated.

Medical Disclaimer The information contained in or made available through the American Thyroid Association Website is not intended to replace the services of a trained health professional or to be a substitute for medical advice of physicians. The user should consult a physician in all matters relating to his or her health, and particularly in respect to any symptoms that may require diagnosis or medical attention. The American Thyroid Association makes no representations or warranties with respect to any information offered or provided within or through the American Thyroid Association Website regarding treatment, action, or application of medication.
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