Effect of chronic excessive iodine on thyroid morphology of iodine-deficient rats

[Objective] To study the effect of chronic excessive iodine on thyroid morphology of iodine-deficient rats. [Methods] Iodine-deficient rat models were made by giving 4-week old rats iodine-deficient fodder for 3 months. These iodine-deficient rats were then supplemented for 8 months 1-, 3- and 6- fold iodine which corresponded to the 3 levels of iodine intake found in our epidemiological study in 3 rural communities, the latter two levels were iodine excessive. Then changes in thyroid morphology after iodine supplement were observed. [Results] After supplement of 1-, 3- and 6-fold iodine to iodine-deficient rats for 8 months. Thyroid iodine content increased to the level higher than non-iodine-deficient rats. thyroid relative weight decreased, but was higher than non-iodine-deficient ones. The ultrastructure injures of swelling mitochondria and dilating endoplasmic reticulum in epithelial cells of iodine-deficient thyroid did not recover, but became worse with iodine supplement increase. [Conclusion] Iodine supplement alone could not correct the injures resulting from iodine deficiency with iodine accumulating in thyroid and goiter remaining. A sub-pathological state of thyroid might occur.