A TPO test is a blood test that measures antibodies against thyroid peroxidase, an enzyme your thyroid needs to produce hormones. It’s primarily used to determine whether thyroid problems are caused by an autoimmune condition, most commonly Hashimoto’s disease. If your doctor has already found signs of thyroid dysfunction through other blood work, a TPO test helps pinpoint the underlying cause.
What Thyroid Peroxidase Actually Does
Thyroid peroxidase (TPO) is an enzyme that handles a critical step in hormone production: attaching iodine to a protein called thyroglobulin. Without this step, your thyroid can’t manufacture the hormones that regulate your metabolism, growth, and brain function. The enzyme sits inside the thyroid gland, a butterfly-shaped tissue at the base of your neck.
In autoimmune thyroid disease, your immune system mistakenly identifies TPO as a threat and produces antibodies to destroy it. As these antibodies attack the enzyme, your thyroid gradually loses its ability to make hormones. That’s what the TPO test detects: the presence and level of those antibodies in your blood.
Why Your Doctor Ordered This Test
A TPO test is rarely the first thyroid test you’ll get. It typically comes after other blood work (like TSH or free T4) has already flagged something abnormal. At that point, your doctor wants to know whether the problem is autoimmune in nature, because the cause shapes how the condition is monitored and treated over time.
The most common reasons for ordering a TPO test include:
- Unexplained hypothyroidism. If your thyroid hormone levels are low and your doctor suspects Hashimoto’s disease, TPO antibodies confirm it. Over 90% of people with Hashimoto’s test positive.
- Hyperthyroidism workup. Up to 70% of people with Graves’ disease also have TPO antibodies, so the test can be part of a broader antibody panel.
- Pregnancy with known thyroid issues. Pregnant women with autoimmune thyroid conditions may be tested to assess risks to the pregnancy.
- Subclinical hypothyroidism. If your thyroid levels are only slightly off, a positive TPO result helps predict whether you’ll eventually develop full hypothyroidism.
What Your Results Mean
Results come back as either negative or positive, often with a specific number attached. The general threshold is less than 5.6 IU/mL for a normal (negative) result, according to Cleveland Clinic reference ranges. Anything above that is considered positive, meaning your immune system is producing antibodies against thyroid peroxidase.
A positive result doesn’t automatically mean you need treatment right now. It means your immune system is targeting your thyroid, but the pace of damage varies widely from person to person. Some people with TPO antibodies maintain normal thyroid function for years. Others progress to overt hypothyroidism more quickly. For women with positive TPO antibodies and mildly elevated TSH (between 2.5 and 4 mIU/L), the risk of progressing to full hypothyroidism is roughly 1% per year. At higher TSH levels, that rate climbs to 2% to 4% per year.
Higher antibody levels generally correlate with more active autoimmune inflammation, but the number alone doesn’t tell the full story. Your doctor will interpret it alongside your TSH, free T4, symptoms, and sometimes an ultrasound of your thyroid.
TPO Antibodies and Pregnancy
This test carries special significance during pregnancy. Women who test positive for TPO antibodies face a roughly 2.5 times higher risk of miscarriage compared to those without the antibodies, even when thyroid hormone levels appear normal. The risk of preterm birth is also elevated, at about 1.8 times higher. And half of TPO-positive women in one study developed postpartum thyroiditis, an inflammation of the thyroid after delivery, compared to about 22% of those without the antibodies.
Despite this evidence, routine screening for TPO antibodies in all pregnant women isn’t standard practice yet. Most guidelines recommend testing only when there’s already a known thyroid condition or strong risk factors. If you’re pregnant and concerned, it’s worth asking whether the test makes sense for your situation.
How to Prepare for the Test
The blood draw itself is straightforward, with no fasting required. But there’s one important preparation step that often gets overlooked: if you take biotin supplements (also labeled as vitamin B7 or vitamin H), you need to stop them at least 48 to 72 hours before the test. Biotin interferes with the type of lab assay used to measure thyroid antibodies and can produce falsely elevated results. This includes high-dose biotin found in many hair, skin, and nail supplements. In documented cases, biotin supplementation pushed TPO antibody readings above the normal range when the true value was normal.
What Happens After a Positive Result
If your TPO antibodies are positive and your thyroid hormones are already abnormal, treatment typically involves thyroid hormone replacement. For Hashimoto’s, this means taking a daily pill that replaces what your thyroid can no longer produce on its own. Most people feel noticeably better within a few weeks as hormone levels stabilize.
If your TPO antibodies are positive but your thyroid function is still normal, you likely won’t start medication right away. Instead, your doctor will monitor your thyroid levels periodically, often once or twice a year, to catch any decline early. The antibodies serve as an early warning system: your immune system is active against the thyroid, and over time, that may reduce its function enough to require treatment.
A positive TPO result is a permanent finding for most people. The antibodies don’t typically go away, though their levels can fluctuate. What matters more than the antibody number itself is how well your thyroid continues to function over time, which is tracked through regular TSH and hormone testing.
Cost and Access
A TPO antibody test is widely available through standard labs. Without insurance, direct-to-consumer pricing runs around $69 through services like Labcorp OnDemand, and the test is eligible for HSA and FSA funds. With insurance, you’ll typically pay less if your doctor orders it with a documented medical reason, such as abnormal thyroid levels or a family history of autoimmune thyroid disease.