Reductive deiodination is critical for thyroid function and represents an unusual exception to the more common oxidative and hydrolytic mechanisms of dehalogenation in mammals. Studies on the reductive processes have been limited by a lack of convenient methods for heterologous expression of the appropriate proteins in large scale. The enzyme responsible for iodide salvage in the thyroid, iodotyrosine deodinase, is now readily generated after engineering its gene fromMus musculus. High expression of a truncated derivative lacking the membrane domain at its N-terminal was observed in Sf9 cells, whereas expression inPichia pastorisremained low despite codon optimization. Ultimately, the desired expression inEscherichia coliwas achieved after replacing the two conserved Cys residues of the deiodinase with Ala and fusing the resulting protein to thioredoxin. This final construct provided abundant enzyme for crystallography and mutagenesis. Utility of theE. colisystem was demonstrated by examining a set of active site residues critical for binding to the zwitterionic portion of substrate.