Radioimmunoassay of 3,3'-di-iodothyronine in unextracted serum: the effect of endogenous tri-iodothyronine.

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-1-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.