RADIOIMMUNOASSAY OF 3,3′-DI-IODOTHYRONINE IN UNEXTRACTED SERUM: THE EFFECT OF ENDOGENOUS TRI-IODOTHYRONINE

Abstract

The development of a highly sensitive and specific radioimmunoassay for 3,3′-di-iodothyronine (3,3′-T₂) is described. The assay was applied to the measurement of 3,3′-T₂ in unextracted human serum and used 8-anilino-l-naphthalene-sulphonic acid to inhibit the binding of 3,3′-T₂ to serum transport proteins. The lower limit of detection of the assay was 2 fmol 3,3′-T₂ per tube, which corresponded to 10 pmol 3,3′-T₂/l serum. The mean concentration of 3,3′-T₂ in normal serum was found to be 23 pmol/l, which is considerably lower than most values reported previously. Evidence is presented which suggests that the cross-reactivity of tri-iodothyronine with the antiserum to 3,3′-T₂ is an important factor in the measurement of serum concentrations of 3,3′-T₂ by radioimmunoassay.