To accommodate local TH requirement

To accommodate local TH requirements, additional systems are in operation. One such system is the control of TH entry into the cell through active transmembrane transporters.9 Another is the activation of the hormone precursor thyroxine (T4) by removal of the outer ring iodine (5′-deiodination) to form triiodothyronine (T3); alternatively, T4 can be inactivated by inner ring (5-deiodination) to form reverse T3 (rT3) and T2 (Figure 1B). Changing concentrations of deiodinases in various cell types allows an additional local regulation of hormone supply.10