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 post-translational 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 radio-iodine therapy.

Serum thyroglobulin concentrations should be very low or undetectable following thyroidectomy for thyroid cancer. A serum thyroglobulin concentration above 1 ng/mL in an athyrotic individual is suspicious of residual or recurrent cancer. 

The presence of TgAb may cause falsely low or undetectable thyroglobulin levels that can mask the presence of cancer. A 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 antibodies 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 Access DxI instruments have a reference range of 0–4 IU/mL and Siemens has a reference range of 0-39 U/mL. A value within the reference range indicates a negative result for thyroglobulin antibodies. 

Specimen requirement is one red top gel barrier tube of blood. 

References

Saravanan P, Dayan CM. Thyroid autoantibodies. Endocrinol Metab Clin North Am. 2001;30(2):315-337.

Spencer C, Fatemi S. Thyroglobulin antibody (TgAb) methods - Strengths, pitfalls and clinical utility for monitoring TgAb-positive patients with differentiated thyroid cancer. Best Pract Res Clin Endocrinol Metab. 2013;27(5):701-712. 

Netzel BC, et al. Thyroglobulin (Tg) testing revisited: Tg assays, TgAb assays, and correlation of results with clinical outcomes. J Clin Endocrinol Metab. 2015;100(8):E1074-83.