Thyrotropin (TSH) Analytical Background

Radioimmunoassay (RIA), a technique in which radioactively-labeled analyte competes with native analyte from the patients specimen for a limited number of antibody binding sites on a reagent antibody specific for the analyte being measured, provided the first generation of clinical assays for thyrotropin, or thyroid-stimulating hormone (TSH) more than forty years ago. First-generation TSH assays made possible clinical diagnosis of hypothyroid conditions by demonstrating TSH values greater than 5 mIU/L (milli-International Units per liter), but the analytical sensitivity of approximately 1.0 mIU/L was insufficient to characterize hyperthyroid conditions, including situations in which exogenous thyroxine replacement dosages may have been too high¹.

Approximately twenty years ago, the second generation of TSH assay became available through the development of highly-specific monoclonal antibodies and new technologies employing antibody excess, including immuno-radiometric assay (IRMA) brought a 10-fold improvement in analytical sensitivity allowing accurate detection of TSH values as low as 0.1 mIU/L. For the first time, the second generation TSH assays, then called “highly-sensitive TSH”, were capable of aiding clinical diagnosis of hyperthyroidism as well as hypothyroidism. Second-generation TSH became the single, recommended screening test for thyroid dysfunction in otherwise normal individuals not receiving exogenous thyroid replacement². Yet the imprecision of these assays at the low-end of the analytical curve did not allow for effective monitoring of the effectiveness of replacement thyroid hormone therapy.

Within the past decade, new technologies in signal amplification in fully-automated chemilluminescent immunoassays brought about the advent of “third-generation”, or “ultra-sensitive TSH” procedures with more than a 100-fold improvement in analytical sensitivity, now making possible accurate quantification of TSH values as low as 0.005 mIU/L. Since a 2-fold change in total thyroxine has been shown to result in a 100-fold inverse change in TSH concentrations, third-generation TSH procedures now provide the analytical sensitivity and precision at low concentrations sufficient to effectively evaluate and monitor virtually all subnormal thyrotropin conditions³.

1. Utiger RD. Radioimmunoassay of human plasma thyrotropin. J Clin Invest. 1965;44:1277-1286
2. Ross DS. Serum thyroid-stimulating hormone measurement for assessment of thyroid function and disease. Endocrinol Metab Clin North Am. 2001;30:245-264
3. Wehmann RE, Rubinstein HA, Nisula BC. A sensitive, convenient radioimmunoassay procedure which demonstrates that serum hTSH is suppressed below the normal range in thyrotoxic patients. Endoc Res Commun. 1979;6:249-255