Immunoradiometric assay of thyroglobulin in patients with differentiated thyroid carcinomas: Need for thyroglobulin recovery tests

As interference from thyroglobulin autoantibodies appears to have been overcome in new commercial thyroglobulin assays by the use of monoclonal antibodies, the need for thyroglobulin recovery tests became uncertain. Sera (n=45) from patients with differentiated thyroid carcinomas were selected on the basis of a thyroglobulin recovery value below 70 % in the Dynotest Tg immunoradiometric assay (Brahms) routinely used in our laboratory. Serum thyroglobulin levels were then measured using three other commercial immunoradiometric assays: thyroglobulin ERIA (Pasteur), HTGK (Sorin) and ELSA HTG (Cis Bio International). Thyroglobulin autoantibodies were measured using the Thyrak assay (Brahms). Although many patients were thyroglobulin antibodies-negative (< 200 U/ml, n=26), most immunoradiometric assays failed to detect thyroglobulin in patients with evidence of recurrence. Low thyroglobulin values associated with low thyroglobulin recovery in thyroglobulin antibody-negative patients appear to be more biologically relevant than a single low thyroglobulin value, which can lead to lack of medical intervention. We conclude that the thyroglobulin recovery test is a prerequisite for the correct interpretation of serum thyroglobulin levels determined with immunoradiometric assays in the follow-up of thyroglobulin autoantibody-negative patients treated for differentiated thyroid carcinomas.