Pharmacological influences on T4 to T3 conversion in rat liver.

Five groups of 8 rats were treated with normal diet, diphenylhydantoin, dexamethasone, thyroxine (T4) and phenobarbital respectively for 14 days. After this period plasma triiodothyronine (T3), T4 and reverse T3 (rT3) concentrations as well as the T4 to T3 converting activity in the liver homogenates were determined. Plasma T3 was low after dexamethasone, T4 and phenobarbital. Plasma T4 was low after dexamethasone and elevated after T4. rT3 plasma concentrations were at the lower limit of detection in all groups. The converting activity in the liver homogenate was significantly increased after diphenylhydantoin and very much depressed after dexamethasone. These results support the concept that different drugs may influence the peripheral thyroid hormone metabolism, especially the T4 to T3 conversion. However other pathways might be affected also because plasma T3 levels did not always reflect the converting activity in the liver.