Abstract: Objective To investigate the effects of stable iodine and thyroxin on the hypothyroid rats treated with ¹³¹I，and provide reference in clinical ¹³¹I therapy for hyperthyroid and nontoxic goiter.Methods Fifty-four Wistar rats are equally divided into three groups，whose ¹³¹I dose was 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 10 )jmol stable iodine 24 h after ¹³¹I theament. 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 Neither cell numbers nor serum T₃，T₄ and TSH among three groups has statistically variance 15 d post treatment, With time prolonging, serum T₃，T₄ of group C decreased faster 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 than B，P<0.05). Conclusions The rats giving with euthyrox in early stage of ¹³¹I treatment can not only alleviate the damage in thyroid cells from p-rays，but also decrease the incidence of hypothyroid, and the stable iodine simultaneously probably can prolong the effective half-life of ¹³¹I，thus aggravate the damages and increase the incidence of hypothyroid.

Key words: Hypothroidism, Iodine radioisotope, Thyroxine, Stable iodine, Rat