Determination of thyroxine release in vivo: a simple, specific and sensitive method in mice

When thyroid hormone secretion in vivo was studied by the release of 125I from thyroid to blood after TSH in mice preloaded with the isotope, only 54.4 +/- 2.0% (+/- SE) of plasma 125I was butanol extractable, indicating low specificity for thyroid hormones. To improve specificity plasma 125I was bound to anti-T4 rabbit antibodies and precipitated with goat antirabbit serum. The intraassay coefficient of variation was 2.5%, and anti-T4 bound radioactivity correlated well with that extractable with butanol (r = 0.95, P less than 0.001). The effect of 40-1000 microU TSH intravenously in mice pretreated with 1 microgram T3 3 times during 2 days to suppress endogenous TSH was evaluated 2 hrs after the injection of TSH by 1) conventional radioimmunoassays of total and free stable T4 in plasma (no preloading with 125I), 2) total blood 125I in mice preloaded with 125I, and 3) anti-T4 bound 125I in plasma in mice preloaded with 125I. Stable T4 increased with TSH doses of 70 microU or higher, but the slope was low and the relation between error variance and slope did not permit a useful bioassay. Total blood 125I responded significantly to 40 microU TSH and showed a favourable relation between error variance and slope, but the specificity may be questioned. Anti-T4 bound 125I in plasma also responded significantly to 40 microU TSH and showed a very good relation between error variance and slope. The new technique of measuring anti-T4 bound radioactivity seems to combine the specificity of radioimmunoassay with a precision and sensitivity well comparable to that of the conventional radioiodine release technique.