Radiometric enzyme assay for thyroid peroxidase (TPO), based on the ability of TPO to oxidize added radio-labeled [¹³¹I]-iodide in the presence of H₂O₂, generated in situ by the glucose oxidase system, and to catalyze subsequent iodination of specific tyrosyl residues in the added thyroglobulin.

The values of specific TPO enzyme activity, determined in microsomal fractions of the thyroid glands of rats maintained for up to 56 days on (A) the iodine-sufficient diet B or (B) the iodine-deficient diet R, in dependence on the extent of bromide intake. (Contrary to literature data, the relationship between the rats’ bromide intake and peroxidase activity in their thyroids was rather complex: it was biphasic, and it also depended on the level of iodine supply to the animals).