Thyroid peroxidase (TPO) Ab test measures antithyroid microsomal antibodies in the blood. Microsomes are small cell particles. The body produces microsomal antibodies when these small cell particles escape from damaged thyroid cells. A blood test can be done to detect and measure the amount of these antibodies in the blood. This test is done to confirm the cause of thyroid problems, including Hashimoto's thyroiditis. The test may also be used to diagnose other autoimmune disorders.