Abstract
The correlation between T4 5'-deiodinase activity and thyroid hormone concentration in rat tissues was sought. Studies with normal and thyroidectomized rats indicated a significant correlation of the hepatic T3 concentration with the enzymeactivity in liver homogenate (r=0.80, p<0.01) or in liver microsomes (r=0.08, p<0.01).The correlation between the hepatic T4 concentration and activity of the enzyme was less significant (r=0.55, p<0.05 for homogenate, and r=0.66, p<0.01 for micrsomes). The degree of T4 5'-deiodinase activity in various organs was found to increase in line with tissue concentration of T3. It seems likely that the level of T4 5'-deiodinase activity is controlled by the tissue concentration of T3.
