Anti-Thyroid Peroxidase Antibodies (Anti-TPO)

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.

• 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.

• Generally favourable. A negative test in someone with proven thyroid disease points to non-autoimmune cause (subacute thyroiditis, drug-induced, post-surgical).