Abstract: The 3 h thyroid 131 I uptake rate and the content of serum TT 3,TT 4,TSH are measured in 63 patients of premature hypothyroidism (consiting of 33 provisional pypothyroids and 30 perpetual hypothyroidism) before and after thyroxine substitutes treatment for six moths. The results show that there is obvious difference in 131 I uptake rate compared provisional hypothyroidism with perpetual hypothyroidism, and no difference in the content of serum TT 3,TT 4,TSH before the treatment. Compared with normal conditions, there is no difference in 131 I uptake rate of provisional hypothyroidism, but the 131 I uptake rate of perpetual hypothyroidism has obvious decrease before and after the treatment. Therefore the type of patients who suffer from premature hypothyroidism can be distinguished according to the 131 I uptake rate:if the 3 h thyroid 131 I uptake rate is normal,it is provisional hypothyroidism;if not,it is perpetual hypothyroidism.