Objective To observe the effects of casein on iodine metabolism in blood,urine and thyroid of mice under excessive iodine.Methods A 2 by 3 factorial design was used in the experiment.The levels of iodine and casein were 50 and 600 μg/L in drinking water and 0( Ⅰ ),10％( Ⅱ ),20％( m ) in food,respectively.After six and twelve months,iodine content in serum,thyroid gland and urine was determined by arsenic-cerium catalyzed spectrophotometry.Results In six months,the levels of serum iodine in 50 Ⅰ,50 Ⅱ,50Ⅲ,600 Ⅰ,600 Ⅱ,and 600Ⅲ groups were (85.59 ± 8.78),(64.59 ± 9.06),(72.53 ± 11.69),(110.04 ± 9.37),(81.06 ± 9.94),(86.63 ± 19.59)μg/L,respectively; the levels of iodine content in thyroid gland were (0.21 ± 0.09),(0.29 ±0.08),(0.24 ± 0.05),(0.50 ± 0.10),(0.37 ± 0.13),(0.42 ± 0.12)g/kg,respectively; the levels of urinary iodine median were 87.5,68.1,105.5,746.5,828.3,1014.2 μg/L,respectively.The variance analysis results of factorial design showed iodine and casein significantly influenced the serum iodine level(F =27.95,18.52,all P ＜0.05),however,there was no interaction between the two(F =0.81,P ＞ 0.05); iodine significantly influenced the iodine content in thyroid gland(F =31.35,P ＜ 0.05),the presence of iodine interacted with casein(F =3.34,P ＜0.05).In twelve months,the levels of serum iodine were (88.54 ± 12.33),(72.45 ± 7.73),(72.93 ± 13.61),( 106.26 ± 12.00),(90.03 ± 7.90),( 104.88 ± 11.67)μg/L,respectively; the levels of iodine content in thyroid gland were (0.58 ± 0.12),(0.40 ± 0.14),(0.69 ± 0.16),(0.84 ± 0.13),(0.89 ± 0.13),(1.02 ± 0.11 )g/kg,respectively;the levels of urinary iodine median were 104.8,121.5,102.7,829.1,1080.8,895.2 μg/L,respectively.The variance analysis results of factorial design showed iodine and casein significantly influenced the iodine content in serum and thyroid gland (F =42.78,7.42 and 66.62,7.90,all P ＜ 0.05),however,there was no interaction between the two(F =1.93,2.31,all P ＞ 0.05).Conclusions Long-term intake of iodine can significantly increase the iodine content in serum,thyroid gland and urine,but casein may accelerate iodine excretion and partly reduce the effect.