Abstract: ［Abstract］Objective　To observe the change of thyroid function and treatment effect of Levothyroxine for hypothyroidism during pregnancy．Method　With high-sensitive chemical irradiance assay (CIA) technique analisis the FT3，FT4，TSH changes thyroid function of different pregnancy period and the 25～150 ug/d changes dose to treat in different dose use it．Results　The levels of TSH in groups of patients with hypothyroidism & subclinical hypothyroidism were higher than those of the normal groups during pregnancy medium period（P＜0.01）．The levels of FT4 in the subclinical hypothyroidism patients were higher than those in hypothyroidism patients（P＜0.05）．In the 3rd trimester， the levels of FT3 and FT4 in hypothyroidism patients were lower than those in the normal subjects（P＜0.05），and the levels of TSH were much higher than the normal groups（P＜0.01）．The required doses of Levothyroxine in both groups of patients were much higher the first pregnancy period（P＜0.05）．The groups of subclinical patients need more dose in first pregnancy period （P＜0.01）．The doses of both groups of patients after childbirth were lower than the groups in the 3rd trimester（P＜0.05）．Conclusions　The doses of Levothyroxine in both groups of patients are increasing in the 2rd and 3rd trimester．All mothers with thyroid disease get adequate treatment in time and remain thyroid function normal，the adverse influence may be reduced to the least. So serial monitoring and therapeutic intervention may be necessary for the prevention of undesirable obstetric outcomes.