Background: Postpartum thyroiditis (PPT) is an autoimmune disease that usually occurs in the first year after parturition and presents with periods of transient thyrotoxicosis and/or hypothyroidism, and in some cases results in permanent hypothyridism.Objective: To determine the prevalence of PPT in healthy postpartum women in Shiraz, southern Iran.Methods: Of 460 postpartum women from Shiraz 385 cases who had no history of any medical problem or signs of other autoimmune disorders were selected during 1-8 months of postpartum period, to fill a questionnaire about signs and symptoms of their possible thyroid dysfunction. TSH and Anti-Tpo Ab were measured and FT4 assay was done on samples with abnormal TSH. 100 women at reproductive age were randomly selected as control group.Result: The clinical and biochemical prevalence of PPT were 33% and 11.4%, respectively (p<0.01). Hyperthyroidism was more frequent in early months of postpartum period and hypothyroidism was more frequent in later months. Thyrotoxicosis and hypothyroidism occurred in 34 (8.8%) and 10 (2.6%) mothers, respectively. Positive Anti-Tpo Ab was found in 80% of patients and in 38% of the control group (p<0.0001).Conclusion: Our results showed a high prevalence of PPT in women in Shiraz. This may be due to the transition from low to adequate iodine intake and participation of women in earlier postpartum period. The major difference compared to other studies is the high frequency of thyrotoxicosis.