High performance liquid chromatography (HPLC) was used in combination with an amperometric and mass spectrometric detection to elucidate and quantitate the degradation products and contaminants of the photo-sensitive Na-thyroxine. Using HPLC with amperometric detection, seven decomposition compounds were separated. These products, which occur mostly as contaminants, were then identified by a developed liquid chromatography-mass spectrometry technique. The same HPLC method was also employed to analyze Na-thyroxine and its degradation products in three commercially available brands of Na-thyroxine tablets.