Conversion of L-thyroxine to triiodo-L-thyronine and biological activity of L-thyroxine, as measured by changes in growth hormone.

Pituitary growth hormone (GH) content and plasma GH response to pentobarbital (PB) of severely hypothyroid rats are markedly decreased compared with euthyroid rats. Both pituitary and plasma GH may be increased by single ip injections of physiologic doses of thyroxine (T4) (1.75 mug/100g BW) or triiodothyronine (T3) (0.2 mug/100g BW). Using this increase in GH of hypothyroid rats to measure the biological effectiveness of single doses of both iodothyronines, we have shown that the administration of 6-propyl-2-thiouracil (PTU) interferes with the activity of T4, but not with that of T3. It is known that deiodination of T4 and of T3 and conversion of T4 to T3 are partially inhibited by PTU. Therefore, it appears that deiodination of T4 is biologically important, whereas deiodination of T3 is not. Data presented here are thus consistent with the hypothesis that conversion of T4 to T3 in vivo plays an important role in the expression of T4 activity.