Fig. 1-7. Postulated NADP oxidation-reduction cycle in thyroid. Four mechanisms of NADPH oxidation are outlined: the reduction of any intermediate X by an NADPH-linked dehydrogenase, the deiodination of iodotyrosines released by thyroglobulinolysis, the generation of H2O2 by THOX (thyroid H2O2 generating enzyme), and the reduction of H2O2 through GSH peroxidase. TG and TGI: uniodinated and iodinated thyroglobulin. +, activation. (From: Dumont JE (187)).