Abstract: In order to investigate the gene regulatory mechanism that excess iodine damage to thyroid,an animal model of goiter was reconstructed in guinea pig with feeding them excess iodine water for 180 days.The effect of excess iodine on apoptosis and cell cycle in thyroid was detected by using by flow cytometry;the expression of Bcl-2 and Bax was detected by immunofluorescent techniques.The result showed that(1).Excess iodine may affect cell cycle and induce apoptosis in thyroid.(2)The of Bcl-2 was notably decreased in the excess groups(p<0.01),but the expression of Bax protein was notably increased in excess groups(p<0.01),the ratio of Bcl-2 to Bax in excess groups decreased(p<0.01)(3)There is a negative correlation between the expression of Bcl-2,Bcl-2/Bax and the apoptosis rate.This study suggested that excess iodine may induce apoptosis in thyroid by changing cell cycle and the expression of Bcl-2 and the ratio of Bcl-2/Bax,which may damage to its structure and function.