TPO Stands for Thyroid Peroxidase
TPO is the abbreviation for Thyroid Peroxidase, a critical enzyme in thyroid hormone production and a major autoantigen in autoimmune thyroid diseases. 1
Biochemical Function
TPO is a membrane-bound enzyme essential for thyroid hormone biosynthesis, catalyzing the oxidation and incorporation of iodine into thyroglobulin to produce thyroid hormones T3 and T4. 1, 2
The enzyme is a heme-containing protein that requires iron for its catalytic activity, specifically through hydroxylases involved in thyroid hormone synthesis. 1
TPO exists as a dimeric structure with two active sites, making it an unusual enzyme with complex structural characteristics. 3
Clinical Significance in Autoimmune Disease
Anti-TPO antibodies are the strongest predictor of progression to hypothyroidism, identifying autoimmune thyroid disease (primarily Hashimoto's thyroiditis). 4, 5
Patients with positive TPO antibodies have a 4.3% annual risk of developing overt hypothyroidism compared to 2.6% in antibody-negative individuals. 4, 5
TPO antibodies are present in 12-26% of euthyroid subjects in the general population, though most will not develop clinical disease. 2
Diagnostic Applications
TPO antibody testing identifies the autoimmune etiology of thyroid dysfunction and helps predict future thyroid failure even when TSH remains in the normal range. 4, 6
The presence of TPO antibodies correlates with the degree of lymphocytic infiltration in thyroid tissue and TSH levels, even within the normal reference range. 2
TPO antibody measurement must be interpreted alongside TSH and free T4 levels, as antibody presence alone is not diagnostic of thyroid dysfunction. 6
Important Clinical Context
In the 2022 urticaria guidelines, an erratum clarified that "anti-TPO" should be spelled out as "anti-thyroid peroxidase" and not "anti-thrombopoietin," highlighting the importance of proper terminology. 1
TPO antibodies are more predictive than thyroglobulin antibodies for future development of hypothyroidism. 5
The enzyme's structural integrity and conformational epitopes are critical for autoantibody recognition in autoimmune thyroid disease. 7, 3