Thyroid Peroxidase Antibodies (TPO)

TPO antibodies are immune proteins that mistakenly target an enzyme your thyroid needs to make hormones. Their presence is the most common marker of autoimmune thyroid disease, and they often appear years before thyroid function itself changes.

Cutoffs vary considerably between labs, with some reporting negative below 35 IU/mL, so compare to the range on your own lab report.

What TPO antibodies measure

Thyroid peroxidase is the enzyme that attaches iodine to make thyroid hormone. TPO antibodies are produced when the immune system treats that enzyme as a threat and begins attacking thyroid tissue.

Unlike TSH or Free T4, which measure how the thyroid is currently performing, TPO antibodies measure whether an autoimmune process is present. That makes this a risk and cause marker rather than a function marker, and it is why it is typically tested once rather than tracked every draw.

What high or low levels mean

A negative result means no meaningful antibody activity was detected, which makes autoimmune thyroid disease unlikely as a cause of any thyroid symptoms.

A positive result indicates autoimmune activity against the thyroid. It is the defining marker of Hashimoto's thyroiditis, the leading cause of hypothyroidism, and is also frequently present in Graves' disease. Importantly, a positive result does not mean your thyroid is currently malfunctioning. Many people have detectable antibodies with entirely normal TSH and Free T4. What it does mean is a meaningfully higher chance of developing thyroid dysfunction over time, which is a reason to monitor thyroid function periodically rather than to treat.

Antibody levels do not track disease severity, so there is limited value in repeating the test to watch the number move.

TPO antibodies FAQ

I have positive antibodies but normal TSH. Do I need treatment?

Generally no. Positive antibodies with normal thyroid function is a common finding and is not treated on its own. It does raise your future risk, so clinicians typically recommend periodic TSH monitoring rather than intervention.

Should I retest my antibodies?

Usually not. Antibody levels fluctuate and do not correlate with how well your thyroid is working, so once you know your status, monitoring TSH and Free T4 is more informative than repeating the antibody test.

Can antibodies go away?

Levels can fall over time and occasionally become undetectable, particularly with dietary changes in people who also have celiac disease. A drop does not reliably indicate the autoimmune process has resolved.

Read alongside Thyroid-stimulating hormone and Free T4 to see both thyroid function and its likely cause.

Sources

• MedlinePlus Medical Encyclopedia, Antithyroid microsomal antibody.

• Cleveland Clinic, Thyroid Antibodies Test.