A rapid and precise sequential saturation radioimmunoassay for thyroxine.

A radioimmunoassay for the measurement of total thyroxine in unextracted serum or plasma is described. The assay is performed according to the sequential saturation principle in less than 4 h. The entire assay is carried out at room temperature. An antiserum of high affinity was used as binder, ammonium 8-anilino-1-naphthalene-sulphonate as displacing agent, and activated charcoal as adsorbent in the separation step. The standard curve was linear in the range 0-136 nmol/l serum. The precision in terms of SD was nearly the same through-out this range. Estimates of within-assay SD varied from 1.3 to 4.1 and between-assay SD from 0 to 3.3 nmol/l. The recovery of thyroxine added to normal plasma was 100% and the sensitivity was 6 nmol/l. Mean thyroxine concentration in plasma from young bulls was 92 nmol/l. The assay appears to be very well suited for experimental purposes when high precision is essential.