The Use and Misuse of the Sensitive Thyrotropin Assays

Since its introduction five years ago, the sensitive TSH (S-TSH) immunometric assay (IMA) has established itself as an important new thyroid test. In fact, TSH IMAs have now replaced the less sensitive first generation TSH radioimmunoassay (RIA) methods. As implied by the term sensitive, these TSH IMAs provide both improved analytic assay precision as well as clinical sensitivity for routinely distinguishing the suppressed serum TSH values characteristic of hyperthyroidism from euthyroidism (1). This improved sensitivity largely results from the IMA sandwich assay configuration in which two anti-TSH antibodies are employed, rather than the single antibody RIA methods (competitive ligand binding assays). Usually one or both of these antibodies are monoclonal and, therefore, possess a high degree of specificity for the TSH epitopes. The first antibody, which is present in excess and bound to a solid support such as a tube wall or bead, is directed at the specific β-subunit of TSH.