Species-specific lipophilicity of thyroid hormones and their precursors in view of their membrane transport properties

► The species-specific lipophilicity of thyroid hormones and their precursors were determined. ► Contributions of the zwitterionic forms to the overall lipophilicity are higher than those of the non-charged ones. ► At physiological pH thyroid hormones are strongly amphipathic. ► The results were interpreted in terms of their membrane transport properties.