Thyroglobulin Antibody

Thyroglobulin antibody (TgAb) measurement is typically used in two clinical scenarios, in the assessment of autoimmunity and in the follow-up of patients treated for differentiated thyroid carcinoma (DTC).^1-8 In thyroid autoimmunity, TgAb level can be increased. However, the presence of TgAb is not always pathogenic nor diagnostic, especially at very low levels. Because changes in TgAb levels can reflect changes in thyroid tissue mass, TgAb concentrations can also serve as a surrogate tumor marker for DTC recurrence and for monitoring changes in tumor mass in certain patients.¹ A rising, or de novo appearance of TgAb may indicate recurrence, whereas a progressive decline suggests successful treatment.³ When present, TgAb interferes with thyroglobulin (Tg) measurement, causing falsely low or undetectable Tg immunometric assay values that can mask disease. Guidelines mandate that every Tg test have TgAb measured simultaneously and quantitatively by immunoassay. The propensity and magnitude of TgAb-Tg interference relates to both Tg and TgAb concentrations and the class of Tg method used.

The United States NHANES III survey reported a TgAb prevalence of approximately 10% for the general population, measured by competitive immunoassay.¹ This study reported that 3% of subjects with no risk factors for thyroid disease had detectable TgAb without associated presence of thyroid peroxidase (TPO) antibodies.¹ TgAb prevalence has been shown to be approximately twofold higher than normal for patients diagnosed with disseminated thyroid carcinoma (~20%).^2,3 It has been suggested that low levels may represent "natural" antibody in healthy individuals or, alternatively, may represent underlying silent autoimmune thyroid disease.⁴ There is some debate over the clinical utility of serum TgAb measurement for assessing the presence of thyroid autoimmunity in areas of iodide sufficiency.^4,5 In iodide-deficient areas, however, TgAb is believed to be useful for detecting autoimmune thyroid disease, especially for patients with a nodular goiter. TgAb measurements are also useful for monitoring iodide therapy for endemic goiter, since iodinated Tg molecules are more immunogenic. Sera samples were obtained in the United States for males <30 years of age following the criteria outlined by the National Academy of Clinical Biochemists (NACB) for establishing a normal reference range for thyroid tests.^6,7 The screening criteria included serum TSH levels between 0.5 and 2.0 mIU/L, no personal or family history of thyroid disease, and absence of nonthyroid autoimmune disease. One hundred thirty-seven screened samples were tested, generating a 95% nonparametric upper reference limit below 4 IU/mL. Additionally, 519 samples were collected in the United States for both males and females ranging from 18 to 74 years of age. The screening criteria included serum TSH levels between 0.5 and 2.0 mIU/L, no personal or family history of thyroid disease, and absence of nonthyroid autoimmune disease. Of the 519 samples tested, 96% fell below 4 IU/mL.

Labcorp reports TgAb results above the limit of detection as elevated. The decision to employ this threshold is based on the fact that the presence of TgAb above the limit of detection is suggestive of the presence of thyroid tissue and can be a negative prognostic in patients treated for DTC. Also, the presence of any level of TgAb in cases where Tg testing is ordered causes the lab to employ an alternate method for that measurement; one that is not confounded by TgAb.