From a reader: I recently was switched from Synthroid to generic levothyroxine for my thyroid condition. After a few weeks on the new form of thyroxine, I felt crummy. My doctor switched me back to Synthroid and after several days felt well. Is there any difference between the thyroid drugs I was given that would account for how I felt?
Levothyroxine is a synthetic (man-made) version of the main thyroid hormone (thyroxine, or T4) that is made and released by your thyroid gland. Levothyroxine is prescribed for people with low thyroid, a condition known as hypothyroidism.
There are four well-tested, brand name preparations of levothyroxine available in the United States for the treatment of people with hypothyroidism (low thyroid) – Levothroid, Levoxyl, Synthroid, and Unithroid. Although there are differences in how these products are manufactured, such as the use of coloring dyes and fillers, each of these brand-name medications is reliable.
The U.S. Food and Drug Administration (FDA) has approved the use of generic levothyroxine made by a number of different pharmaceutical companies. The various brand name and generic tablets of levothyroxine may differ in how much levothyroxine they contain; how much of the levothyroxine is absorbed into your body; and, how much levothyroxine circulates in your body.
If your doctor has switched you from a brand name levothyroxine – such as Synthroid – to a generic version, you may experience symptoms of too much medication or not enough medication.
In addition, you may be given a different generic preparation of levothyroxine each time you have your prescription refilled. Practically speaking, this means your doctor may need to perform more frequent blood tests and perhaps change the dose of your medication.
Although your symptoms of feeling crummy may be a coincidence or due to some other health problem, it is possible that your body was reacting to a difference between Synthroid and generic levothyroxine. It’s important that you discuss these changes with your physician.
According to the American Association of Clinical Endocrinologists, you should “use the same brand of thyroid medication throughout your treatment. 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 always stay the same.”
Green WL. “New Questions Regarding Bioequivalence of Levothyroxine Preparations: A Clinician’s Response.” Journal of the American Association of Pharmaceutical Scientists. 2005 7:E54-E58. 09 Jul 2008.
Joint Position Statement on the Use and Interchangeability of Thyroxine Products. American Association of Clinical Endocrinologists, The Endocrine Society, American Thyroid Association. Accessed July 09, 2008. www.aace.com/pub/pdf/guidelines/AACE-TES-ATA-ThyroxineProducts.pdf