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 < 0.001). The effect of 40-1000 μU TSH intravenously in mice pretreated with 1 μg 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 μU 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 μU 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 μU 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. [ABSTRACT FROM AUTHOR]