Formation of 3,3′-Diiodothyronine and 3′,5′,3-Triiodothyronine (Reverse T3) in Thyroid Glands of Rats and in Enzymatically Iodinated Thyroglobulin

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Abstract
Labeled thyroxine (T4) comprised less than 2% of the total 131I in the thyroids of severely I-deficient rats labeled with 131I for 18-24 h, a much lower value than had previously been reported for I-deficient rats. This low value was attributable to 2 factors: the use of a diet low enough in I content to produce extreme I deficiency, and the use of a paper chromatography system that successfully separates T4 from the minor iodothyronines, 3,3''-diiodothyronine (T2) and 3'',5'',3-triiodothyronine (reverse T3; T3''). Formation of the minor iodothyronines, while low, becomes appreciable in relation to T4 formation in severe I deficiency. In this study, formation of labeled T2 was significant only in I deficiency, and the highest values were observed in the most severely I-deficient rats. In the latter labeled monoiodotyrosine (MIT) comprised approximately 60% of the total 131I in the thyroid, and increased formation of T2 could be attributed to the increased probability of coupling between 2 molecules of MIT. The formation of labeled T3'', was significant in thyroids from both I-deficient and I-sufficient rats. In thyroglobulin iodinated in vitro with thyroid peroxidase to varying levels of iodination, formation of T2 was evident only at lower levels of iodination, whereas formation of T3'' was significant at all levles of iodination. The comparison of relative T3'' and T4 formation in enzymatically iodinated thyroglobulin with corresponding values reported for the intermolecular (DIHPPA [3,5-diido-4-hydroxyphenylpyruvic acid]) model for T4 formation, indicates that the peroxidase model system simulates much more closely the relative formation of T3'' and T4 seen in vivo.

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