The possible leaching of diethyl phthalate into four currently marketed brands of levothyroxine sodium tablets was investigated. Several strengths of levothyroxine sodium tablets and sizes of containers were used. Samples were analyzed by high-performance liquid chromatography (HPLC) to determine the levothyroxine sodium content and to determine if any unidentified compounds were present. The packaging for the four brands of tablets was also analyzed by using the same HPLC system to determine if any extractable compounds could be detected in the tablets. The potencies of the four brands of tablets were comparable. The tablets from the 100-count container of one brand (brand A) were the only tablets found to contain an unidentified peak in the chromatogram. The desiccants from the bottle showed the same unidentified compound, while the bottle and closure did not yield the peak. Thin-layer chromatography and HPLC identified the peak as diethyl phthalate, a plasticizer in the desiccant. Tablets, bottles, closures, and desiccants for the 1000-count brand A product and all sizes of the other brands were negative for the presence of diethyl phthalate. The desiccants in those containers were from a different manufacturer than the desiccant in the brand A 100-count bottle. Diethyl phthalate in the desiccant in 100-count bottles of brand A levothyroxine sodium tablets appeared to have leached into the tablets.