To determine the prevalence of thyroid dysfunction in Thai postpartum women, we evaluated thyroid function and thyroid autoantibodies in 812 consecutive unselected women at 1.5 months post partum. At 3.5 months post partum 570 women without previous thyroid abnormality returned for a second set of thyroid function test. The prevalence of thyroid dysfunction was 1.1%, which was less than that reported from other countries. Various types of postpartum thyroid dysfunction, namely, transient thyrotoxicosis followed by transient hypothyroidism, transient thyrotoxicosis or hypothyroidism occurring alone or permanent hypothyroidism were encountered. Eight out of 9 patients with thyroid dysfunction had thyroid autoantibodies. Fine needle aspiration biopsy of the thyroid was done in 4 patients and all showed lymphocytic thyroiditis. Even though Bangkok is an iodine surfeit area, iodine intake is relatively lower than in other areas where the prevalence of postpartum thyroid dysfunction is much higher. The discrepancy in geographic prevalence of postpartum thyroid dysfunction may result from the interaction of immunogenetic heterogeneity of different ethnic background, environmental iodine intake, and other unidentified environmental factors.