Regulation of rat liver type 1 iodothyronine deiodinase mRNA levels by testosterone

To investigate the underlying mechanisms of sex-related differences in liver type 1 iodothyronine deiodinase (ID1), we studied the sex-related differences and roles of sex steroids in liver ID1 mRNA levels in the rat. In both euthyroid and thyroidectomized rats, liver ID1 activity and ID1 mRNA levels in female rats were less than those in male rats. A positive correlation was observed between liver ID1 activity and ID1 mRNA levels. Liver ID1 activity and ID1 mRNA levels in male rats decreased after orchiectomy, and were increased to control levels by testosterone administration. Ovariectomy or beta-estradiol administration did not alter liver ID1 activity or ID1 mRNA levels in female rats. ID1 mRNA levels in cultured rat hepatocytes were significantly increased by testosterone, but not by beta-estradiol. These results suggest that the sex-related differences in liver ID1 activity are attributable to differences in ID1 mRNA levels, and that testosterone plays an important role in the sex-related differences in liver ID1 mRNA levels.