Effect of Hypothyroidism and Hyperthyroidism on Triiodothyronine Production in Perfused Rat Liver*

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Abstract
The effect of altered thyroid states on hepatic T3 production in a functioning intact organ system, the isolated perfused liver was evaluated. Thyroidectomized rats were treated for 3-4 wk with vehicle, T4 [thyroxine] 1.5 .mu.g/100 g-1 day-1, or T4 20 .mu.g/100 g-1 day-1, to produce hypothyroidism, euthyroidism or hyperthyroidism. Livers were perfused for 1 h with medium containing T4, 10 .mu.g/dl, and T3 production was estimated by RIA [radioimmunoassay]. T3 production in the hypothyroid, euthyroid and hyperthyroid groups, respectively, was 1.61 .+-. (SE) 0.50, 5.18 .+-. 0.55, and 15.62 .+-. 1.61 ng g-1 liver h-1. These differences in T3 production resulted entirely from changes in percent conversion of T4 to T3 which were 0.87 .+-. 0.25%, 3.21 .+-. 0.38% and 12.02 .+-. 1.82% in the hypothyroid, euthyroid and hyperthyroid groups, respectively. The measured hepatic uptake of T4 decreased slightly with T4 administration from 188 .+-. 13 to 162 .+-. 7 and 144 .+-. 10 ng/g liver in these same groups. The changes in T3 production were not accounted for by differences in biliary excretion or deiodination of T3. These studies demonstrate a stimulatory effect of T4 on the conversion of T4 to T3 which is important to altering net hepatic T3 production.