Iodothyronine Metabolism in Liver and Kidney Homogenates from Hyperthyroid and Hypothyroid Rats*

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Abstract
The effect of hyperthyroidism and hypothyroidism on conversion of T4 [thyroxine] to T3 [triiodothyronine] and the degradation of rT3 [reverse T3] was studied in rat liver and kidney homogenates. Administration of T4 (5 .mu.g/100 g BW[body weight]/day for 5 days) to normal rats resulted in significant increases in the rates of both reactions in liver homogenates; the increase was no greater after T4 doses of 10 and 20 .mu.g/100 g per day. Responses also were no greater after 12 than after 5 days of T4 treatment. After thyroidectomy, T3 production declined gradually to 30% of control in 6 wk; rT3 degradation initially decreased more rapidly, but at 4 and 6 wk, the percentage of fall was similar for both reactions. No significant changes in either reaction were found in kidney homogenates from T4-treated rats. In kidney homogenates from thyroidectomized rats, both reactions were significantly impaired at 6 wk. 5''-Deiodination of T4 and rT3 apparently change in parallel directions after various experimental manipulations. A single 5''-deiodinase catalyzes T4 to T3 conversion and rT3 degradation in the liver and perhaps the kidney as well.