Background: Iodide has been known to control the function and the growth of the thyroid gland, and to be used as a substrate of thyroid hormone. Moreover, it has been suggested that excessive iodide stimulates the thyroid autoimmune responses. To evaluate the effects of iodide on thyrocytes, we investigated cell function and proliferation, or thyroid autoantigen expression after administering iodide to rats or FRTL-5 cells. Methods and Results: Ten-weeks-old Sprague-Dawley rats were sacrificed after 7 days of NaI treatment. The expressions of thyroid autoantigens were examined by northern blot analysis. Chronic administration of iodide resulted in no effect on TSH receptor(TSHR) and thyroperoxidase (TPO) mRNA expression, while it increased thyroglobulin (TG) and diminished sodium-iodide symporter (NIS) mRNA expression. FRTL-5 cells were also treated with various concentrations of NaI. The generation of cAMP or iodide uptake was decreased, and the cellular growth was also inhibited by iodide. However, the expressions of all thyroid autoantigens(TSHR, TG, TPO, MHC class I and class II) except NIS were unchanged for 72 hours after iodide administration. The expression of NIS was mildly increased after 24 hours. Conclusion: Iodide resulted in decreased cell proliferation and cellular function of cAMP generation and iodide uptake. Chronic administration of iodide increased TG and diminished NIS mRNA expression in vivo but not in vitro(J Kor Soc Endocrinol 17:69~78, 2002).