The thyroid peroxidase antibody test is a blood test. Sometimes it's done to help find out if a person has thyroid disease that's caused by an immune system condition. The test isn't used alone to diagnose thyroid disease.

Thyroid peroxidase (TPO) is a type of protein called an enzyme. It's found in a small but important gland in the neck called the thyroid. TPO plays a key role in getting the thyroid to make hormones that are crucial to health. But in some people who have thyroid disease, the immune system mistakenly targets parts of the thyroid and TPO. The immune system does this by making proteins called antibodies that work to get rid of TPO. So if other tests suggest that you have thyroid disease, you may need a TPO antibody test to check for the antibodies.

TPO antibodies in the blood may mean you have thyroid disease due to an immune system condition called Hashimoto's disease. With Hashimoto's disease, the immune system makes antibodies that attack healthy tissue by accident. Antibodies that attack the thyroid can cause swelling, called inflammation, in the gland. And the thyroid may not be able to work as well as it should.

You also may need a TPO antibody test if you're pregnant and have an immune system condition that involves the thyroid. Pregnant people with TPO antibodies have a higher risk of thyroid disease after pregnancy than do those without the antibodies.

Some people with TPO antibodies may not have thyroid disease. But the presence of TPO antibodies may raise the risk of future thyroid disorders.