Postpartum thyroiditis is one of the autoimmune thyroid dysfunctions which occurs during 12 months after delivery. Transient episodes of hypo and hyperthyroidism in this disorder leads to maternal psychological and physical problems. Irreversible hypothyroidism occurs in a number of mothers. The prevalence and clinical course of the disease haven't been evaluated before, so this study was conducted to follow this object in 1998- 1999in Tehran.Materials and methods: One thousands and forth (1040) mothers visited in 5 health-center in Tehran were investigated for the symptoms of hypo and hyperthyroidism and the presence of goiter according to who classification.T3, T4, T3Ru, TSH, Anti Tg and Anti Tpo were measured in at 3, 4.5, 6 and 9 months postpartum those with abnormal thyroid function tests were included in patient group and others considered as control group. Patients in each visit underwent thyroid sonography and the results compared with control group.Results: The prevalence of thyroiditis in study population was 11.4%of who 68 were cases of hypothyroidism42 were of hyperthyroidism and 9 had transient postpartum hyperthyroidism before theoccurance of hypothyroidism. One patient was affected by Graves. Among patients, with hypothyroidism, 35 subjects underwent treatment with levothyroxine due to the severity of symptoms of whom 6 had recurrent hypothyroidism after discontinuing at 12 months postpartum.Significant goiter was present in 21.8 % and 6.7% of patient and control groups, respectively (P<0.001). The percentage of positive anti thyroid anti body in case and control groups were as follows respectively for Anti Tpo (61.5%, 19%) and for Anti Tg (58%, 6%) (P<0.0001).Special sonographic textures (hypoecogenicity and heterogenecity) were observed in more than 96% of patients and less than 7% of control group (P < 0.001). There was no significant association between the incidence of thyroiditis and, past medical history or family history of thyroid disorders, mother's age, sex of newborn, breast feeding and the number of deliveries.Conclusion: This study revealed high prevalence of postpartum thyroiditis in Tehran. Hypothyroidism mostly needs to be treated in mothers and some of them effected by irreversible hypothyroidism.