The present study is divided into two groups ,one is the patients with subclinical hypothyroidism who has TPO antibody positive women with TSH concentrations > 2.5 mU/L and below the upper limit of the pregnancy specific reference range ,another is TPO antibody negative with TSH concentrations greater than the pregnancy specific reference range and below 10.0 mU/L.The two groups are randomized into either the LT4 treatment group or control group by the use of a computer-generated list.