Misleading High Thyrotropin Results Obtained with a Two-Site Immunometric Assay Involving a Chimeric Antibody (Technical Briefs‪)‬

Serum thyrotropin (TSH) measurements are widely used in the diagnosis of thyroid dysfunction and to monitor l-thyroxine ([T.sub.4]) replacement therapy in primary hypothyroidism. The accepted TSH reference interval in serum is 0.4-4.0 mIU/L, and a TSH concentration between 0.5 and 2.0 mIU/L is generally considered as the optimum therapeutic target during replacement therapy (1). Because of technical problems such as interference from heterophilic antibodies reacting with assay anti-mouse antibodies, immunometric assays may give falsely increased TSH values (1). Chimeric antibodies have been introduced in some assays with a view to eliminate interference from anti-mouse antibodies (2). We report here for the first time two cases of misleading high TSH results obtained with an immunometric assay involving a chimeric anti-TSH antibody. The first case (patient 1), a 56-year-old woman on long-term l-T4 replacement therapy, was referred to the laboratory for biological thyroid follow-up. Her serum TSH concentration had been slightly above the reference interval for 2 years. The TSH value (5.4 mIU/L) measured with the Elecsys immunometric assay performed on the Elecsys 2010 analyzer (Roche Diagnostics) confirmed this finding. This slightly increased TSH value was in agreement with the previous TSH results but disagreed with the free thyroxine ([FT.sub.4]) concentration (20.5 pmol/L) measured on the Elecsys platform, which was within the reference interval (12.2-23.2 pmol/L). As expected, in the serum from this adequately treated patient who was clinically euthyroid, [FT.sub.4] was in the upper third of the reference interval (1), but surprisingly, TSH remained slightly increased.