Biological Activity of 3,3′,5′-Triiodo-DL-Thyronine

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Abstract
Experiments have been conducted to evaluate the thyromimetic activity of 3,3′,5′-triiodo-DL-thyronine in rats as determined by its ability to slow the rate of loss of thyroidal radioactivity after administration of I131 and its suppression of thyroidal 24-hour I131 uptake. The administration of this substance in doses up to five times that of L-thyroxine failed to change the thyroidal I131 release rate under conditions in which the latter compound was strongly inhibitory. Similar results were obtained whether or not the recirculation of I131 was blocked by the administration of methimazole. In suppressing thyroidal I131 uptake it appeared to have 75% the activity of thyroxine, but inorganic iodide in comparable doses was observed to be equally effective. These results are interpeted as demonstrating a lack of significant thyromimetic activity in this compound, less than 10% that of thyroxine.
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