Thyroid peroxidase and lactoperoxidase are capable of producing oxidized bromine species. Thus investigations into bromination reactions with tyrosine and thyroglobulin were undertaken in order to gain insight into possible formation of brominated thyroid hormone analogues. A reversed-phase high-performance liquid chromatographic method was developed for the separation of bromine/iodine-substituted tyrosines and used as a basis for these investigations combined with ultraviolet absorption and electrochemical detection. The results indicate that in vivo bromination of tyrosyl residues in thyroglobulin might be of some importance in cases of either iodine deficiency or excessive bromide intake.