Thyroid peroxidase test is a test that measures the level of an antibody that is directed against thyroid peroxidase (TPO). Autoantibodies to thyroid peroxidase (TPOAb) are produced within the body.The presence of TPOAb in the blood reflects a prior attack on the thyroid tissue by the body's immune system.PO is found in thyroid follicle cells where it converts the thyroid hormone T4 to T3. The thyroid gland is located in the lower part of the neck, below the Adam's apple, wrapped around the trachea (windpipe).