Increased metabolism of iodothyronines in the rat after short-term cold adaptation

The effect of 2 weeks of continuous exposure to 4C on the metabolism of iodothyronines in the rat was studied. Metabolic clearances of [131I]triiodothyronine ([131]t3) and [125I[-thyroxine ([125I]T4) were increased in exposed animals. Estimated absolute turnover of T4 was increased approximately 2-fold as a result of cold exposure. Urinary and fecal clearances of labeled hormones were elevated. Intracellular total radioiodine and iodothyronine radioiodine concentrations were reduced in liver and kidney 24 h after a single injection of tracer hromones. Plasma hormonal binding was not altered. Expanded tissue spaces of [125I]T4 (liver, 52.4%, kidney, 66,7%) were measured by analysis of composite plasma and tissue disappearance curves in subgroups of animals sacrificed 6-24 h after dose injection. Enlarged tissue spaces ot [131I]T4 (liver 8.3%, kidney 26.2%) were observed in the cold-adapted groups. Fractional disappearance rates of labeled iodothyronines from the total rat were accelerated as determined in individual exposed animals and by composite disappearance curve analysis. These investigations suggest that the observed altered kinetic parameters of iodothyronine disappearance from plasma, tissue and total body pools are due to increased hormone flux, owing to increased deiodinative and fecal hormone disposition. Moreover, the data demonstrate that alterations in the peripheral metabolism of T3 and T4 become manifest early in the adaptation process to cold.