Potency of brand name and generic levothyroxine

The tablet thyroxine content of a brand name and two generic products were studied. The mean thyroxine content was significantly different in the generic tablets when compared with that in the brand name product. The coefficient of variation of tablet thyroxine was lowest in the brand name, although this product had only 78% of expected potency. When these products were tested in three hypothyroid patients, two serum free thyroxine indexes were well outside the normal range when the most potent generic product was substituted for the brand name drug. Manufacturers should be required to measure the thyroxine content of their tablets. Until such time that levothyroxine products become more uniform, physicians, pharmacists, and legislators should not encourage product switching.