Thyroid Peroxidase Antibodies (TPO): What Your Result Means

What is Thyroid Peroxidase Antibodies (TPO)?

Thyroid peroxidase (TPO) is the enzyme the thyroid uses to make hormones. When the immune system wrongly produces antibodies against it, the thyroid slowly wears down — typically leading to hypothyroidism. Measuring TPO antibodies is the standard way to confirm an autoimmune thyroid condition, and it is also the marker most closely linked to postpartum thyroid problems.

Typical reference interval: Reported as negative/positive, often with a titer and lab-specific cutoff (commonly around 34–40 IU/mL for the quantitative assay). Low-positive values are common and often incidental; strongly positive results carry the most weight.. Laboratories differ — the interval printed on YOUR report always governs.

If your Thyroid Peroxidase Antibodies (TPO) is high

Common reasons Thyroid Peroxidase Antibodies (TPO) runs above the reference interval:

• Hashimoto's thyroiditis — the classic cause — the immune system attacking the thyroid through TPO antibodies, usually leading to underactive thyroid over time
• Postpartum thyroiditis — antibodies present before or during pregnancy can flare after delivery, causing thyroid swings
• Other autoimmune conditions — people with one autoimmune disease often carry others — TPO positivity can appear alongside them
• Subclinical autoimmune tendency — mildly positive TPO with normal thyroid function is common and may reflect a future risk rather than current disease

If your Thyroid Peroxidase Antibodies (TPO) is low

Common reasons Thyroid Peroxidase Antibodies (TPO) runs below the reference interval:

• Negative result — the expected finding — most people do not have an autoimmune thyroid process
• No current autoimmune activity — a quiet immune system keeps TPO antibodies at baseline

Symptoms worth knowing

TPO antibodies cause no symptoms. The symptoms they predict are those of hypothyroidism — fatigue, weight gain, cold intolerance, depression, and hair changes — once the thyroid has been damaged enough. Some people carry the antibody for years feeling perfectly well.

What usually happens next

1. A positive TPO result with a rising TSH is the classic Hashimoto's pattern — a clinician will usually confirm and plan thyroid management
2. Positive TPO with normal thyroid function is a watch-and-monitor situation, not an immediate treatment trigger
3. Pregnant or postpartum women with TPO antibodies warrant thyroid follow-up with their obstetric or primary care team

Frequently asked questions about Thyroid Peroxidase Antibodies (TPO) results