ABSTRACT
After the administration of 131I to normal animals or human subjects, labelled thyroxine and triiodothyronine, but at most traces of labelled iodotyrosines can be detected in the serum. However, several investigators using various methods claim to have found considerable amounts of one or both of these iodotyrosines when assaying the stable (non-radioactive) iodinated compounds in the serum.
Considering the available evidence as convincing for the present, an attempt has been made to explain this discrepancy. A schematic model of the thyroidal iodine metabolism is proposed, based on (a) the hypothesis that the iodotyrosines are present in the circulation in a »masked« form (i. e. protected against deiodination), and (b) the known functional heterogeneity of the thyroid tissue. This heterogeneity should be of a qualitative as well as quantitative nature.
As the physical decay rate of 131I is short in comparison with the turnover rate of the masked iodotyrosine pool, an isotope equilibrium experiment with rats was carried out, using the long-lived isotope 125I. The results of this experiment, viewed together with those of a similar investigation published by others, seem to lend support to the proposed mechanism.
The presence of non-negligible amounts of a diiodotyrosine-like compound in normal rat serum seems fairly well established.
