m-Maleimidobenzoyl-L-thyroxine Methyl Ester, can be used in the synthesis of a conjugate containing the residues of vitamin H (biotin) and thyroid hormone thyroxine. Biotinylated thyroxine as a bifunctional ligands can be used in the development of a test system for a lanthanide immunofluorometric assay for free thyroxine in human serum.