Anti-thyroglobulin antibodies (Anti-Tg) are autoantibodies produced by the immune system against thyroglobulin — a large protein stored in the thyroid gland that serves as the precursor for thyroid hormone synthesis. Their presence indicates that the immune system is targeting the thyroid.

Anti-Tg antibodies are found in approximately 60–80% of people with Hashimoto's thyroiditis and in a smaller proportion of those with Graves' disease. They are also used alongside thyroglobulin measurements in thyroid cancer monitoring — since Anti-Tg antibodies can interfere with thyroglobulin assays and give falsely low results, their presence must be accounted for when interpreting post-thyroidectomy surveillance.