Anti-Thyroid Peroxidase Antibodies (Anti-TPO)
Anti-TPO antibodies attack the enzyme that makes thyroid hormones. They are the marker of autoimmune thyroid disease (Hashimoto's and Graves').
What it measures
Thyroid peroxidase (TPO) catalyses the synthesis of T4 and T3 inside the thyroid follicle. In autoimmune thyroid disease the immune system produces antibodies against TPO, gradually destroying the gland. A positive anti-TPO confirms autoimmune disease as the cause when TSH and Free T4 are off: in Hashimoto's thyroiditis (commonest cause of hypothyroidism in iodine-replete countries) and in 70% of Graves' disease (hyperthyroidism). Levels do not correlate with disease severity, and they do not need repeating once positive.
What a high value can mean
- Hashimoto's thyroiditis — positive in 90–95% of cases; almost always alongside elevated TSH and low/normal Free T4.
- Graves' disease — positive in ~70%; alongside suppressed TSH and elevated Free T4.
- Postpartum thyroiditis — predictive: positive antibodies during pregnancy roughly double postpartum risk.
- Type 1 diabetes, coeliac, vitiligo — autoimmune clustering; isolated low-positive may indicate baseline autoimmune predisposition.
- Subclinical thyroid disease — positive antibodies raise the risk that subclinical hypothyroidism will progress to overt disease.
What a low value can mean
- Generally favourable. A negative test in someone with proven thyroid disease points to non-autoimmune cause (subacute thyroiditis, drug-induced, post-surgical).
When to discuss with a doctor
An isolated positive anti-TPO with completely normal thyroid function does not require treatment, but it does increase the lifetime risk of overt hypothyroidism — annual TSH monitoring is reasonable. In overt disease the antibody result confirms the autoimmune nature and shifts the management discussion. Mediora.AI shows anti-TPO alongside TSH and Free T4 for the full thyroid picture; treatment decisions belong with a clinician.