A new extractant for serum thyroxine by enzymatic digestion of thyroxine binding proteins.

The use of a pepsin solution to digest serum proteins results in quantitative release of thyroxine from carrier proteins. This extraction method greatly simplifies the original Murphy and Pattee (1) competitive protein-binding assay for serum thyroxine. The simplicity and efficiency of this extractant suggest that it may have application in radioimmunoassay and competitive protein-binding assay for nonpeptide ligands in serum.