Correlation of T4 to T3 converting activity with T3 concentration in rat tissues.

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 enzyme activity in liver homogenate (r = 0.80, p less than 0.01) or in liver microsomes (r = 0.08, p less than 0.01). The correlation between the hepatic T4 concentration and activity of the enzyme was less significant (r = 0.55, p less than 0.05 for homogenate, and r = 0.66, p less than 0.01 for microsomes). 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.