Rationale—Microsomal antibodies have been related to postpartum thyroid dysfunction and postpartum depression. Objectives—To detect the value of microsomal antibodies during gestation in a random population, as a risk factor for thyroid dysfunction and depression during the postpartum period. Main findings—The presence of microsomal antibodies was investigated in a random population of 293 women at 32 weeks' gestation. At the same time, postpartum thyroid function was assessed repeatedly in all women every six weeks up to 34 weeks' postpartum. Postpartum thyroid dysfunction, defined as the presence of abnormal TSH, in combination with abnormal fT4 and/or fT3 values, occurred in 21 women (7.2%) during the postpartum period. Depression was assessed using the Research Diagnostic Criteria without knowing the results of biochemical thyroid function tests. At 32 weeks' gestation there were 27 (9.2%) women with elevated microsomal antibody titres. Compared with microsomalantibody negative women at 32 weeks' gestation, these women had an RR of 20 for developing postpartum thyroid dysfunction and an RR of 1.7 for developing postpartum depression. Conclusions—Women with elevated microsomal antibody titres during gestation are particularly at risk for postpartum thyroid dysfunction, but only have a slightly increased risk for postpartum depression.