Anti-TPO Antibodies: What a Positive Result Means

Thyroid peroxidase (TPO) is an enzyme responsible for the addition of iodine to thyroglobulin, a critical step in thyroid hormone synthesis. In certain autoimmune conditions, such as autoimmune hypothyroidism (also known as Hashimoto’s thyroiditis), the body makes antibodies against TPO. More than 90% of people with Hashimoto’s thyroiditis have anti-TPO antibodies (Mariotti et al., 1990), and testing for these antibodies can help to confirm the diagnosis. Approximately 10% of people with normal thyroid function tests (TSH, T4, T3) also have anti-TPO antibodies, which increase the risk of developing hypothyroidism in the future.

For people with hypothyroidism, a positive anti-TPO antibody titre can be used to confirm the diagnosis of autoimmune (Hashimoto’s) thyroiditis. For people with normal thyroid function tests, a positive anti-TPO antibody titre provides an indication of your risk of developing hypothyroidism in the future. People with anti-TPO antibodies have a 1-2% risk of developing hypothyroidism each year (Vanderpump et al., 1995).

The reference range NiaHealth has adopted for anti-TPO sets the at-risk threshold at 35 kIU/L, which is the commonly accepted cutoff for a positive anti-TPO titre. However, in a population-based cohort study, a cutoff of 29 kIU/L was found to predict the progression to subclinical hypothyroidism amongst individuals who were euthyroid at baseline (Walsh et al., 2010).