Effect of thyrotropin on conversion of T4 to T3 in perfused rat liver.

The present study was undertaken to elucidate the direct effect of thyrotropin (TSH) on the conversion of thyroxine (T4) to 3,5,3'-triiodothyronine (T3) in the isolated perfused rat liver. The liver was perfused without recirculation with a synthetic medium containing 10 micrograms/dl T4 and the effect of constant infusion of bovine TSH (125 or 250 microU/ml) on the conversion of T4 to T3 was examined. T4 uptake in the perfused liver was not changed by the addition of TSH. The release of T3 (10.3 +/- 1.4 ng/g/30min, mean +/- SD), tissue T3 production (99.5 +/- 21.4 ng/g/30min), net T3 production (102.6 +/- 20.2 ng/g/30min), and the conversion rate of T4 to T3 (14.8 +/- 3.5%) in the liver perfused with 250 microU/ml TSH were significantly higher than those in controls (8.1 +/- 1.2 ng/g/30min, 69.0 +/- 6.8 ng/g/30min, 69.9 +/- 6.1 ng/g/30min, and 10.0 +/- 0.8%), respectively. These results suggest that TSH may directly enhance hepatic conversion of T4 to T3 in rats in vitro.