Ten different levothyroxine products manufactured by six companies were analyzed by high-performance liquid chromatography. Although nine products met the United States Pharmacopeia requirements, one product was found to have only 47% of expected potency. Until these products become more uniform, we would not recommend interchangeability of levothyroxine preparations.
Stoffer, Sheldon S. and Szpunar, Walter E.
"Potency of Current Levothyroxine Preparations Evaluated by High-Performance Liquid Chromatography,"
Henry Ford Hospital Medical Journal
: Vol. 36
Available at: https://scholarlycommons.henryford.com/hfhmedjournal/vol36/iss1/17