Quantitation of extrathyroidal conversion of l-thyroxine to 3,5,3′-triiodo-l-thyronine in the rat

Abstract

Studies of the rate of extrathyroidal conversion of thyroxine (T4) to 3,5,3′-triiodo-L-thyronine (T3) were carried out in rats. Total body homogenates were prepared and extracted with ethanol 48, 72, and 96 hr after the intravenous injection of ¹²⁵I-T4. ¹³¹I-T3 was added, and the paper chromatographic purification of T3 was effected by serial elution and rechromatography in three paper and one thin-layer cycles. The ratio of ¹³¹I-T3 and ¹²⁵I-T3 counting rates in the final chromatograms, which was identical in three different paper chromatography systems, was used to calculate the proportion of ¹²⁵I-T3 to ¹²⁵I-T4 in the original homogenates. In order to discount the effects of in vitro monodeiodination of T4 during extraction and chromatography, we killed control animals immediately after injection of ¹²⁵I-T4 and processed them in a similar fashion to the experimental groups. The average ratio of ¹²⁵I-T3 to ¹²⁵I-T4 in carcass extracts of animals killed between 48 and 96 hr after isotopic injection was 0.08 whereas the average ratio of ¹²⁵I-T3 to ¹²⁵I-T4 in chromatograms of control animals was 0.01. On the basis of the proposed model, calculations indicated that about 17% of the secreted T4 was converted to T3. Assuming values cited in the literature for the concentration of nonradioactive T3 in rat plasma, these findings would suggest that about 20% of total body T3 is derived by conversion from T4. Moreover, since previous estimates have suggested that in the rat, T3 has about 3 to 5 times greater biologic activity than T4, these results also raise the possibility that the hormonal activity of T4 may be dependent in large part on its conversion to T3. A necessary assumption in calculating T4 to T3 conversion in this and other studies is that the 3′ and 5′ positions are randomly labeled with radioiodine in phenolic-ring iodine-labeled T4. Evidence supporting this assumption was obtained in the rat by comparing the amount of labeled T3 produced after injection of phenolic and nonphenolic-ring iodine-labeled T4.