Thyroglobulin Antibody
Thyroglobulin (Tg) is a large glycoprotein that is synthesized in thyroid follicles and serves as the precursor of thyroid hormone (T4). The extent of iodination and posttranslational modifications of Tg may be important determinants of its immunogenicity
Approximately 20% of patients with thyroid cancer have detectable thyroglobulin autoantibodies (TgAb). Following thyroidectomy, TgAb concentrations typically decrease by 50% within the first year and usually become undetectable within a median of 3 years. Serial serum TgAb measurements may be an independent prognostic indicator of cancer recurrence. Other causes of an increase in TgAb level include second surgery, fine needle biopsy or radioiodine therapy.
The presence of TgAb may cause falsely low or undetectable thyroglobulin levels by noncompetitive immunometric assays that can mask the presence of cancer. A qualitative test for TgAb should be performed on all specimens prior to thyroglobulin testing to determine the risk for interference with thyroglobulin measurement. Any level of TgAb has the potential to interfere with thyroglobulin measurement. Serial measurement of TgAb concentration can be used to monitor recurrence in patients with unreliable thyroglobulin measurement.
Nearly all patients with Hashimoto’s thyroiditis have high concentrations of antibody to Tg and thyroid peroxidase (TPO). Measurement of anti-TPO is a more sensitive test for diagnosis of thyroiditis. TgAb is no longer recommended as part of the work-up for thyroiditis. See Thyroid Peroxidase Antibody.
Reference range for TgAb varies significantly depending on the method. Beckman Coulter instruments have a reference range of 0–4 IU/mL and Siemens has a reference range of 0-39 U/mL. Specimen requirement is one red top tube of blood.
