Specific-nuclear triiodothyronine binding sites in rat liver and kidney

Injection of increasing quantities of L-triiodothyronine (T₃) resulted in a progressive decrease in the proportion of T₃ bound to the nuclear fraction of rat liver and kidney cells. A similar dose-response relationship could not be demonstrated for other ultracentrifugal fractions. Only a modest reduction in nuclear binding of thyroxine was observed after injection of large amounts of this iodothyronine. The results indicate the presence of relatively specific binding sites for T₃ in the nuclei of rat liver and kidney.