Thyroid Peroxidase Antibody (TPO) Blood Test: Normal Ranges, Causes & What Your Results Mean

What Is Thyroid Peroxidase Antibody?

Thyroid peroxidase antibodies (TPO Ab, also called anti-TPO or thyroid microsomal antibodies) are autoantibodies directed against thyroid peroxidase — the key enzyme responsible for iodinating thyroglobulin and synthesising thyroid hormones T3 and T4. TPO antibodies are the most sensitive and specific serological marker of autoimmune thyroid disease.

Positive TPO antibodies are found in over 90% of patients with Hashimoto's thyroiditis and approximately 75% of those with Graves' disease. They are also present in 10–15% of the general population without clinical thyroid disease, though these individuals have a higher future risk of developing hypothyroidism.

Why Is TPO Tested?

• Confirming autoimmune thyroid disease — the single best antibody test for Hashimoto's thyroiditis
• Predicting hypothyroidism risk — TPO-positive individuals with mildly elevated TSH have a 4.3% annual risk of developing overt hypothyroidism (Whickham Survey)
• Subclinical hypothyroidism management — NICE recommends checking TPO to guide treatment decisions when TSH is elevated but T4 is normal
• Postpartum thyroiditis risk — TPO-positive women have 30–50% risk of postpartum thyroid dysfunction
• Recurrent miscarriage — TPO positivity is associated with increased miscarriage risk
• Monitoring during pregnancy — thyroid autoimmunity affects both mother and foetal outcomes

Normal Ranges

What Does a Positive TPO Result Mean?

• Hashimoto's thyroiditis — the most common cause of hypothyroidism in the UK; gradual thyroid destruction
• Graves' disease — autoimmune hyperthyroidism; TPO positive in ~75%
• Increased risk of hypothyroidism — even with currently normal thyroid function, annual TSH monitoring is recommended
• Postpartum thyroiditis — transient hyper- then hypothyroidism in the first year after delivery
• Associated autoimmune conditions — type 1 diabetes, Addison's disease, pernicious anaemia, coeliac disease
• Drug-induced thyroiditis — interferon, amiodarone, lithium, and immune checkpoint inhibitors

What Does a Negative TPO Result Mean?

• Autoimmune thyroid disease less likely — but not excluded; ~5–10% of Hashimoto's patients are TPO-negative
• Other causes of thyroid dysfunction — iodine deficiency, medications, thyroid surgery, or radiation

Managing Positive TPO Antibodies

• Annual TSH monitoring — if currently euthyroid, check TSH every 12 months
• Selenium supplementation — 200 µg/day has been shown in multiple trials to reduce TPO antibody levels, particularly in mild autoimmune thyroiditis
• Optimise vitamin D — levels above 75 nmol/L are associated with lower autoimmune thyroid risk
• Screen for coeliac disease — gluten sensitivity co-exists in 2–5% of autoimmune thyroid patients
• Reduce inflammatory triggers — anti-inflammatory diet, stress management, adequate sleep
• Levothyroxine — if TSH is persistently above 10 mU/L, or above 4.5 mU/L with symptoms, NICE recommends treatment
• Pregnancy planning — discuss thyroid monitoring plan with your GP or endocrinologist

When Should You Get Tested?

• Elevated TSH on a thyroid function test
• Goitre (enlarged thyroid) on examination
• Family history of autoimmune thyroid disease
• Planning pregnancy, especially with other autoimmune conditions
• After treatment with immune-modulating drugs

Which Lola Health Tests Include TPO?

TPO antibodies are available as an add-on with any Lola Health blood test. For a complete thyroid panel, combine with TSH, FT4, FT3, and thyroglobulin antibodies. Included in Peak Insights.