Effects of different levels of iodine nutrition on liver typeIdeiodinase and brain typeIIdeiodinase activities in rat

Wistar rats with different levels of iodine nutrition were killed after 3,6 and 12 months of experiments.Serum thyroid hormones were assayed with RIA.The activity of typeⅠdeiodinase(DⅠ)and typeⅡdeiodinase(DⅡ)was measured based on the release of radioiodide from the ~(125)Ⅰ-labeled substrate.The result showed that hypothyroidism reflected by decreased T_4 happened during the initial phase of iodine deficiency.The activity of DⅠand DⅡin rats was raised significantly in iodine deficiency groups.An excess of iodine inhibited DⅠactivity resulting in decreased serum TT_3 and FT_3.However,DⅡactivity increased in rats with iodine excess, attributing to the inactivation of T_3 and T_4 to the substrate of DⅡenzyme.