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Thyroid peroxidase antibodies (TPO Ab) target the enzyme that catalyses thyroid hormone synthesis, and their presence is the strongest blood marker for autoimmune thyroid disease. Elevated TPO Ab are found in over 90% of people with Hashimoto's thyroiditis and in most people with Graves' disease (hyperthyroidism). Positive TPO Ab can precede thyroid dysfunction by a decade or more, identifying individuals who need monitoring and who may benefit from gluten elimination, selenium optimisation, or stress management to slow immune attack.
Also known as: Anti-Thyroid Peroxidase, Anti-TPO, Iodide peroxidase, Thyroid Microsomal, Thyroid microsomes, Thyroid peroxidase, Thyroid Peroxidase Ab, Thyroid Peroxidase Antibodies
High TPO antibodies confirm autoimmune thyroid attack (Hashimoto's thyroiditis) and predict future thyroid failure even when TSH is still normal. Low or absent TPO antibodies make autoimmune thyroiditis unlikely.
Standard threshold for positivity is above 35 IU/mL, though any detectable level above 9 IU/mL increases hypothyroidism risk. Optimal is undetectable.