Anti-TPO (anti-thyroid peroxidase) antibodies are immune proteins your body makes against thyroid peroxidase, an enzyme the thyroid gland needs to produce thyroid hormone. A positive result indicates autoimmune thyroid disease — most often Hashimoto's thyroiditis — but it is a marker of that underlying process, not a measure of how well your thyroid is currently functioning.
What a positive result does and doesn't mean
You can have positive anti-TPO antibodies with completely normal TSH and T4, sometimes for years, if your thyroid is still compensating. A positive result raises the chance that your TSH will rise over time, which is why it usually leads to closer monitoring rather than immediate treatment. Treatment decisions are still based on TSH and T4, not the antibody titer itself — there's no standard medicine that treats the antibody level directly.
Why it matters in pregnancy and fertility
Anti-TPO positive status, even with a currently normal TSH, is associated with a higher chance of TSH rising during pregnancy (when thyroid hormone demand increases) and has been linked to a higher risk of miscarriage and preterm birth in some studies. This is why doctors often monitor TSH more closely through pregnancy in women known to be anti-TPO positive. See Anti-TPO antibodies high with normal or high TSH: treat or watch, and can antibodies be lowered? for the full picture.