ObjectiveTo make a comparative study on effect of iodine and iodide excess on thyroid function and morphology in Wistar rats without iod ine deficiency.MethodsSerum TSH was measurd with solid phase radioimmun oassay(IRMA) and TT 4,TT 3,rT 3 and thyroid TT 4 with radioimmunoassay(RIA). Cer-Arsenite colormetric method were used to measure median urinary iodine(MUI). Thyroid morphological changes were observed under optical and electronic microsc ope.Metamorph imaging system was used to measure the height of thyrocytes and th e areas of thyroid follicular cavities.ResultsIntake of 840?μg/L iodine for 90 days could inc rease serum TSH without significant difference;Serum TT 4 values were markeldy higher than those in DDW group,P<0 05.Serum TT 4 was not significanlty di fferent from the groups of iodine and iodide with the same dosage.In the iodine excess groups,the following morphological changes can be seen under microscope;i ncrease of the area of follicular cavity,flat follicular cells,darker nucleus st aining,broken and combined follicles,formation of giant follicles,reduction of c apillary,enlargement of endoplasmic reticulum of thyroid follicular cells,increa se of the number of secondary lysosomes,chromatin condensation,cytoplasmic vesic ularisation,reduction of microvilli.When iodine intake was >840?μg/L for 90 d ays,the height of thyrocytes was makedly lower than those of DDW group and the a reas of thyroid follicular cavities were markedly larger than those of DDW group P<0 05.ConclusionIodine intake of 840?μg/L for 90 days may in crease serum TT 4 and inhibit the function of most of thyroid follicular cell.N o significant difference was found between the effect of iodine and iodide exces s on thyroid.