Anti-thyroid peroxidase (TPO) antibodies attack the enzyme responsible for iodination in thyroid hormone synthesis.
Mechanism: Elevated levels indicate autoimmune thyroiditis. Most commonly Hashimoto's, sometimes Graves'. The antibodies progressively damage the thyroid gland over years.
Lab range: Below 35 IU/mL standard. Any elevation is meaningful; values can range from 50 to 1000+.
Caveats: Positive antibodies often precede clinical thyroid dysfunction by years. Worth checking even with a normal TSH and fT4. Strictly speaking it's an antibody, not a hormone, but it's part of the thyroid panel.