Compound-induced changes in hepatic microsomal enzyme activity, especially that of
thyroxine glucuronyltransferase activity, and changes in bile flow can have significant
effects on thyroid hormone metabolism. Direct measurement of plasma hormone
clearance and T4 half-life can be determined in compound-dosed rats by measuring the
plasma values of 125I-T4 following the intravenous bolus injection of radioactive
thyroxine. Either intact (Davies, 1993) or thyroidectomized (McClain et al., 1989) rats
can be used and while determining the total radioactivity of plasma is acceptable, a more accurate approach is to count specific 125I-bound-T4 by binding the hormone to T4
antibodies attached to the surface of polypropylene tubes. Following the administration of an anti-inflammatory agent known to be a liver enzyme inducer, Davies (1993), using the latter approach, demonstrated a reduction in plasma T4 half-life and an increase in the plasma clearance of thyroxine from 2.86 ml min−1 kg−1 to 3.37 ml min−1 kg−1 (p