Thyroid peroxidase antibodies (anti-TPO)

Cut-offs depend on the assay (for example below 34, below 60 or below 100 kIU/L), so the lab's own printed cut-off is the one to use.

Thyroid peroxidase is an enzyme inside the thyroid gland that helps make thyroid hormone. Anti-TPO antibodies are immune proteins directed against it. Their presence shows that the immune system is reacting to the thyroid, which is the most common background to an underactive thyroid in the UK.

Doctors request it when TSH is outside range to see whether autoimmune thyroid disease is the likely background, in people with a family history of thyroid problems, and sometimes in pregnancy planning or recurrent miscarriage clinics. It is often included in private thyroid panels.

• Autoimmune thyroid disease, both the underactive (Hashimoto) and overactive (Graves) forms
• Other autoimmune conditions such as type 1 diabetes, coeliac disease and pernicious anaemia
• Female sex and older age, in the general population
• A family history of thyroid disease
• Some medicines, including amiodarone, lithium and interferon

• A value below the cut-off is the expected finding
• Antibody levels can fall over years in some people with treated thyroid disease
• Levels vary between assays, so a change between laboratories may not be real

• Around one in ten healthy adults, more often women, have a positive result with a normal thyroid
• Different assays give different numbers, so results from two labs are not directly comparable
• Biotin supplements can interfere on some analysers

No fasting needed. Once positive there is usually little value in repeating it, as the number does not track how the thyroid is performing.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.