Fig. 1.

Principles of the immunometric assay used for TSH measurement (upper panel), and the competitive binding assay used for the measurement of T3 and T4 (lower panel). In the immunometric assay, the interference of biotin from the patient sample results in a spuriously low concentration of TSH, whereas the opposite is seen with the competitive binding assay measuring a spuriously high concentration of T4 or T3. B = Biotin; R = ruthenium; SA = streptavidin; MAB = monoclonal antibody; Ab = antibody; ANS = 8-anilino-1-naphthalene sulphonic acid (to dissociate T4 from binding proteins); TBG = thyroxine-binding globulin; fT4 = free T4.