Propylthiouracil inhibits the conversion of l-thyroxine to l-triiodothyronine

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Abstract
6-n-propylthiouracil (PTU) administered to male Sprague-Dawley rats maintained on 2 and 5 μg L-thyroxine (T4)/100 g body weight resulted in a marked reduction in the rate of conversion of L-thyroxine to L-triiodothyronine (T3). These effects could not be ascribed to induced hypothyroidism since the group maintained on 5 μg T4/day had normal levels of liver mitochondrial alpha glycerophosphate dehydrogenase. In confirmation of previous studies, PTU also reduced the fractional rate of deiodination of T3. These observations provide a possible explanation of the many published observations indicating that PTU antagonizes the tissue effects of T4 but not of T3. The data suggest that monodeiodination of T4 but not of T3 is essential before hormonal effects can be manifested at the cellular level.