Abstract: Objective To investigate the effects of stable iodine and thyroxin on the hypothyroid occurrence of rats treated with ¹³¹I thus to provide reference in clinical ¹³¹I therapy for hyperthyroid and nontoxic goiter. Methods 54 Wistar rats are equally divided into three groups, whose ¹³¹I dose can be calculated by their mass. Group A accept only ¹³¹I as contrast, group B accept Euthyrox (200 pg/d, 30d) after ¹³¹I treatment, group C accept 10μmol stable iodine 24 h after ¹³¹I theatment. Serum triiodothyronine (T₃), thyroxine (T₄), thyroid-stimulating hormone (TSH) are measured and cell numbers per high power lens(4×100)are counted in 15, 60, 90 d following ¹³¹I treatment respectively. All the data are analysed by Fisher test and q test. Results 15 d posttreatment, neither cell numbers nor serum T₃, T₄ and TSH among three groups has statistically variance. With time prolonging, serum T₃, T₄ of group C decreased quicker than the other two groups (in 60 d, T₄ of group C is less than group B, q=5.16, P<0.05; in 90 d, T₄ of group C is less than group A and B, q value is 3.31 and 7.69 respectively, P<0.05). Conclusions Giving the rats euthyrox in time post ¹³¹I treatment can not only alleviate the damage in thyroid cells from β-rays, but also decrease the incidence of hypothyroid. On the other hand, giving the rats stable iodine simultaneously probably can prolong the effective half-life of ¹³¹I, thus aggravate the damages and increase the incidence of hypothyroid.