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Rats were treated with insulin infusion over 12 h with or without addition of cycloheximide and compared with controls. T4 to T3 deiodination in microsomal liver fraction was measured. Insulin treatment led to an increase in T3 production, whereas this effect was blunted by the inhibition of protein synthesis by cycloheximide. Neither serum concentrations of T4 nor the liver content of non-protein-bound sulph-hydryl groups explained the alterations in T3 production. It is strongly suggested that insulin directly acts on T4-5'-monodeiodination by stimulating de novo synthesis of protein.