TPO and thyroiditis: Thyroid peroxidase (TPO) is a key enzyme in the production of thyroid hormones (Taurog et al. 1990) and a major thyroid autoantigen (Czarnocka et al. 1985) in addition to thyroglobulin (Roitt et al. 1956) and the thyroid-stimulating hormone receptor (Rees Smith & Hall 1974).