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.
Stoffer SS, Szpunar WE. Potency of Levothyroxine Products. JAMA. 1984;251(5):635-636. doi:10.1001/jama.1984.03340290049020