Labeled metabolites appearing in human serum after 125I-triiodothyronine (T3) administration: a quantitative reappraisal.

The appearance in human serum of labeled iodothyronines arising from 3,5,3'-triiodothyronine (T3) catabolism was measured after bolus administration of 125I-T3. The use of column chromatography made it possible to separate in the plasma samples iodoproteins, iodide, T3 and a fourth peak ("pre-T3") eluting just before T3. The radioactivity associated with this pre-T3 peak was found to be 0.5% of T3 activity 30 min after injection, and reached a plateau value of 5.6% +/- 1.2 (mean +/- SD) from the 10th hr onward. From these data, we calculated that a maximal 5% underestimation in T3 metabolic clearance rate is inherent in those analytical methods that do not completely separate pre-T3 from T3 radioactivity. The MCR of 3,3'-diiodothyronine (T2) was also measured from the plasma disappearance curve after single injection of 125I-3,3'-T2. From these data and the mean disappearance curve of T3, the appearance curve of 3,3'-T2 in plasma was reconstructed by convolution under the assumption of a 100% conversion of T3 to 3,3'-T2. A plateau value of 4.6% of T3 activity was computed, very comparable to the experimentally determined 5.6%. This suggest that, if labeled 3,3'-T2 is the main component of the pre-T3 peak, the conversion into 3,3'-T2 represents a major pathway of T3 metabolism in man.