Potency of levothyroxine products
S. S. Stoffer and W. E. Szpunar
Recently, the company that manufactures the levothyroxine product that we
use most often in our practice switched to high-pressure liquid
chromotography (HPLC) to monitor their product. As a result, the brand name
product now contains 100% of expected potency as determined in our
laboratory by radioimmunoassay. Previous studies in our laboratory
demonstrated that this product had only 78% of expected potency. This year
our patients have a higher mean serum free thyroxine index (FTI) than they
have had in the past. In addition, there were more elevated serum FTI
levels and fewer elevated serum thyroid-stimulating hormone levels this
year. Physicians should be aware of possible product changes being made by
manufacturers of levothyroxine products over the next year or so as more
companies switch to the HPLC method. The thyroid function studies of
patients receiving levothyroxine products should be rather carefully
monitored during this transitional period. We suspect that the new products
will be more consistent and have a more reliable shelf life.
