Thyroxine is the tyrosine based hormone produced by the thyroid gland, containing a chiral center in its molecular structure. Synthetic thyroxine is commercially available to treat thyroid dysfunctions, but only a few direct analytical methods to determine its optical purity have been reported. This study attempted to find an optimized liquid chromatographic condition for direct enantiomer separation on crown ether type chiral stationary phase. Among various mobile phases tested, 100% methanol solution containing 10 mM $H_2SO_4$ was found to be most appropriate. This chromatographic method was validated and applied to measure the optical purity of six different L-thyroxine products from three domestic pharmaceutical companies and four commercially available D- and L-thyroxine reagents. The chromatographic results showed that every pharmaceutical product had quite high optical purity of above 97% (all but one were higher than 99%) while two of four thyroxine reagents demonstrated relatively higher enantiomer impurity (5-6%).