Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1235 page)

   At least 6 weeks should elapse after thyroidectomy or iodine-125 treatment before a Tg test. Some reports have indicated that Tg levels may remain elevated for several weeks following successful treatment. In this case, serial determinations assessed relative to a posttreatment baseline established for the patient may still be of value in monitoring.

   Many technical pitfalls in Tg measurement include between-method variability, in appropriate reference ranges, suboptimal functional sensitivity, hook effects, HAMA interferences. RIA method is relatively resistant to TGAB and HAMA influences.

   A newer HPLC-MS method is offered by many commercial labs and can be used in suspicious TGAB interference cases.

THYROID AUTOANTIBODY TESTS

   Definition

   Antithyroid peroxidase (TPO) antibodies are autoantibodies directed against the peroxidase enzyme. This enzyme catalyzes the iodination of tyrosine in thyroglobulin (Tg) during the biosynthesis of T
₃
and T
₄
. Historically, these antibodies were referred to as antimicrosomal antibodies (AMAs) because the antibodies bind to the microsomal part of the thyroid cells. Recent research has identified thyroid peroxidase as the primary antigenic component of microsomes. Measurement of TPO antibodies has essentially replaced the measurement of antimicrosomal antibodies. In virtually all cases of Hashimoto disease and in the majority of cases of Graves disease, anti-TPO antibodies are elevated. High levels of anti-TPO antibodies, in the context of the clinical presentation of hypothyroidism, confirm the diagnosis of Hashimoto disease. Tg autoantibody measurements are most useful for evaluating samples submitted for Tg measurements because Tg autoantibodies can interfere with both competitive immunoassays and immunometric assays for Tg.

   
Normal range:

   Tg antibodies: <40 IU/mL

   TPO antibodies: <35 IU/mL

   Use

   To assess the thyroid autoantibody status in patients with thyroid disease

   To distinguish subacute thyroiditis from Hashimoto thyroiditis, as antibodies are more common in the latter

   Occasionally useful to distinguish Graves disease from toxic multinodular goiter when physical findings are not diagnostic