The DIASTAT® anti-thyroglobulin (anti-Tg) test is a quantitative/qualitative enzyme-linked immunosorbent assay (ELISA) for the detection of autoantibodies specific for thyroglobulin in human serum or EDTA, Li-Heparin or citrated plasma. It is intended to aid in the diagnosis of autoimmune thyroid disorders and is not definitive in isolation. Autoantibody levels represent one parameter in a multicriterion diagnostic process.
Thyroglobulin (Tg) and thyroid peroxidase (TPO) are considered the major autoantigens associated with chronic autoimmune thyroiditis. Clinical diagnosis is usually based on the presence of serum or plasma autoantibodies to Tg and TPO. Thyroglobulin, a 660kD water-soluble glycoprotein, is the precursor of the thyroid hormones tri-iodothyronine and thyroxine. The presence of anti-Tg autoantibodies in patients with Hashimoto’s thyroiditis was first demonstrated in 1956.
Anti-Tg autoantibodies are predominantly of the IgG class. They are detected, often with TPO autoantibodies, in the majority of Graves’ disease and Hashimoto’s, including other variants of chronic primary hypothyroidism such as myxoedema and asymptomatic thyroiditis. They have also been detected in spontaneous or post-partum painless thyroiditis, in thyroid autoimmunity with rheumatoid arthritis, and in non-thyroid autoimmune diseases such as Addison’s disease and Type 1 diabetes mellitus. Depending upon methodology and assay cut-off, they are also detected in 1.3% to 14.6% of apparently healthy asymptomatic subjects. The role of circulating Tg autoantibodies is unclear; they may simply be indicators of disease as, unlike TPO autoantibodies, they do not appear to be pathogenic.
The wells of the microtitre strips are coated with purified human Tg antigen. During the first incubation, specific autoantibodies in diluted serum or plasma bind to the antigen-coated surface. The wells are then washed to remove unbound components. In the second incubation, the Conjugate, enzyme-labelled antibodies to human IgG, binds any surface-bound autoantibodies. After further washing, specific autoantibodies are traced by incubation with the Substrate. Addition of Stop Solution terminates the reaction, resulting in a coloured end-product. The amount of Conjugate bound is measured in absorbance units. In the qualitative protocol, the amount of Conjugate bound by the sample is compared with that bound by the Reference Control. In the quantitative protocol, the concentration of anti-Tg autoantibody can be estimated by interpolation from a dose-response curve based on Standards. The Standards are referenced against the International Reference Preparation of anti-thyroglobulin 65/93.