Among the various autoantibody tests applied in research and clinical practice, the determination of thyroid microsomal (TMAb) and thyroglobulin antibodies (TgAb) still retains its strong value in the screening for thyroid autoimmunity. The presence in the serum of TMAb is almost invariably associated with thyroid autoimmune disease or focal thyroiditis. The appearance of TMAb together with elevated serum-TSH in subclinical autoimmune thyroiditis strongly suggests progression to overt hypothyroidism. Pregnant women with positive TMAb and/or TgAb run an increased risk for post-partum painless thyroiditis with transient thyrotoxicosis and subsequent hypothyroidism. After delivery also a relapse of previously unrecognized Graves' thyrotoxicosis may occur. Thyroid antibody determination is not a valuable tool to discriminate autoimmune thyroiditis from thyroid malignancies. TMAb and TgAb determination helps to recognize individuals with thyroid autoimmunity among patients with non-thyroid autoimmune diseases such as Addison's disease and Type I diabetes mellitus.