Thyroid . Patient guide
TPO Antibodies (TPO-Ab)
Also known as: Thyroid peroxidase antibodies, Anti-TPO, Antithyroid microsomal antibodies
What is TPO-Ab
TPO antibodies are autoantibodies targeted at the enzyme thyroid peroxidase, which the thyroid uses to make thyroid hormone. A raised TPO antibody level is the most specific single marker of autoimmune thyroid disease, particularly Hashimoto's thyroiditis.
This biomarker entry is being clinically reviewed by our team. The factual content draws on UK guidance (NICE, NHS, Royal Colleges and the relevant speciality society where cited).
Reference range
Reported in IU/mL. Final reports always carry the issuing laboratory's range, which is what your clinician will interpret against.
| Group | Range | Note |
|---|---|---|
| Negative | under 35 | IU/mL; threshold varies between UK labs |
| Weakly positive | 35 to 100 | |
| Strongly positive | over 100 |
What it is
Thyroid peroxidase is the enzyme that incorporates iodine into thyroglobulin to make thyroid hormones T4 and T3. In autoimmune thyroid disease the immune system produces antibodies against this enzyme, gradually impairing thyroid function over months to years. TPO antibodies are present in around 90 to 95 percent of patients with Hashimoto's thyroiditis and in around 50 to 80 percent of patients with Graves' disease.
Why a clinician would order it
TPO antibodies are requested when TSH is raised but symptoms or family history suggest autoimmune cause, in subclinical hypothyroidism (mildly raised TSH with normal T4) to assess progression risk, in unexplained fatigue or hair loss with normal thyroid function tests, in recurrent miscarriage, and in women planning pregnancy with any thyroid history.
If your level is outside the range
Symptoms of low TPO-Ab
- No symptoms attributable to absent TPO antibodies
What low can indicate. Autoimmune thyroid disease is unlikely. A normal TPO does not exclude other causes of thyroid dysfunction.
Symptoms of high TPO-Ab
- Symptoms relate to thyroid dysfunction if present: fatigue, hair shedding, weight change, cold or heat intolerance, mood changes. Many patients with raised TPO antibodies feel well for years before thyroid function changes.
What high can indicate. Hashimoto's thyroiditis (most common cause), Graves' disease, postpartum thyroiditis, or low-titre positivity in healthy individuals (around 10 percent of women have a low-positive TPO without ever developing thyroid disease). Raised TPO with normal thyroid function indicates ongoing autoimmune process and warrants annual TSH monitoring.
Testing tips
No fasting required. Antibody levels do not fluctuate significantly day-to-day, so timing is flexible. Always interpret alongside TSH and free T4. In pregnancy planning, raised TPO antibodies even with normal TSH increases the risk of postpartum thyroiditis and merits closer follow-up.
Where you can get this tested
TPO Antibodies is included in the following WMG Health panels. Same-day appointments at our Harley Street clinic, with results clinician-reviewed.
Want a specific combination of markers we do not have a panel for? Build a custom panel and our clinicians will design one for you.
Looking to book a test?
Comprehensive private thyroid blood test
TSH, Free T4, Free T3 + TPO antibodies. NICE-aligned interpretation by GMC-registered doctors. From £180.
Symptoms often investigated with TPO Antibodies
TPO Antibodies is commonly tested when patients present with the following symptoms. If any of these resonate with you, the linked guides explain what to look for and which test pathway is appropriate.
Sources
UK guidance our clinicians use when interpreting this marker.
This page is general patient information, not personal medical advice. A GMC-registered clinician will review your results and tailor any interpretation to you. See our Editorial Policy for how we write and review content.
Common questions about TPO-Ab
What is a normal TPO-Ab range?
Negative: under 35 (IU/mL; threshold varies between UK labs). Weakly positive: 35 to 100. Strongly positive: over 100. Always interpret your own results against the laboratory range printed on your report, since assay-specific reference ranges vary.
What does a low TPO-Ab result mean?
Autoimmune thyroid disease is unlikely. A normal TPO does not exclude other causes of thyroid dysfunction.
What does a high TPO-Ab result mean?
Hashimoto's thyroiditis (most common cause), Graves' disease, postpartum thyroiditis, or low-titre positivity in healthy individuals (around 10 percent of women have a low-positive TPO without ever developing thyroid disease). Raised TPO with normal thyroid function indicates ongoing autoimmune process and warrants annual TSH monitoring.
Do I need to fast or prepare for the TPO-Ab blood test?
No fasting required. Antibody levels do not fluctuate significantly day-to-day, so timing is flexible. Always interpret alongside TSH and free T4. In pregnancy planning, raised TPO antibodies even with normal TSH increases the risk of postpartum thyroiditis and merits closer follow-up.
Can I order a TPO-Ab blood test privately in London?
Yes. WMG Health offers TPO-Ab as part of bespoke panels and several pre-built panels at our 134 Harley Street clinic. Results are clinician-reviewed by a GMC-registered doctor within 4 hours for the most common assays. Bespoke panels from £180; bookings via /contact/ or 020 3239 3378.