Objective To investigate the cumulative incidence of persistent hypothyroidism in patients who were diagnosed as postpartum thyroiditis ( PPT),and to determine the factors associated with the development of persistent hypothyroidism in those patients.Methods The present study was performed as the continuous study followed by the former epidemiological survey on PPT,in which 58 patients with PPT (35 overt PPT and 23 subclinical PPT) were diagnosed.The 58 patients were followed up at 12th month postpartum,and then for every 6 months until 24 months postpartum.Fasting blood samples were taken for testing serum TSH,thyroid peroxidase antibody ( TPOAb),and thyroglobulin antibody ( TgAb ).Free T3 ( FT3 ),free T4 ( FT4 ),and TSH receptor antibody ( TRAh ) were detected if TSH was abnormal.50 healthy postpartum women were used as control group.Results Of the total 58 PPT patients,91,4％ ( n =53 ) were successfully followed.Five patients with overt PPT and 6 patients with subclinical PPT developed persistent hypothyroidism,and the cumulative incidence of persistent hypothyroidism in the studied PPT patients was 20.8％.Among 15 PPT patients who had a classical biphasic course (a thyrotoxic phase followed by a hypothyroid phase),persistent hypothyroidism was seen in 26.7％ (n =4 ).Among 11 PPT patients with hypothyroidism only,persistent hypothyroidism was seen in 63.6％ ( n =7).On the contrary,none of the patients with thyrotoxicosis only had persistent hypothyroidism.All of the patients who developed persistent hypothyrodism had a higher TSH levels than 4.8 mU/L at 6th month postpartum.Before delivery,TSH levels of the patients developed persistent hypothyroidism were significantly higher than those of the patients with transient hypothyroidism,and this was the case at the 12th month postpartum ( all P＜0.01 ).PPT patients maintained a relatively higher rate of thyroid autoantibodies.The positive rate of TPOAb at the 12th,18th,and 24th month postpartum was 56.6％,50.9％,and 52.8％,respectively; and the positive rate of TgAb being 35.8％,30.2％,and 30.2％,respectively.Both the positive rate and titer of TPOAb in patients with overt PPT were higher than those in patients with subclinical PPT at the 18th and 24th month postpartum (P＜0.05).Conclusions 20.8％ patients with PPT developed persistent hypothyroidism at the 24th month postpartum.Whether a patient with PPT would develop persistent hypothyroidism depends on his clinical feature and TSH level.