BACKGROUND: The prevalence of thyroid dysfunction in pregnancy and in the first postpartum year (postpartum thyroid dysfunction (PPTD)) in women with diabetes mellitus type 1 (DM1) is known to be higher than in the general population. To assess prevalence, incidence and risk factors in The Netherlands we performed a prospective cohort study. DESIGN: From 1994 to 1998, 126 women with DM1 from eight Dutch clinics were included. TSH, free thyroxine, free tri-iodothyronine and anti-thyroid peroxidase antibodies (TPO-ab) were measured pre-pregnancy, in the first and last trimester of pregnancy and at 1.5, 3, 6, 9 and 12 months after delivery. RESULTS: Eighty-two women completed the study. Thyroid dysfunction during pregnancy was observed in 22.5% (first trimester) and 18.4% (third trimester), and mostly consisted of subclinical hypothyroidism. Baseline characteristics of women with thyroid dysfunction in pregnancy did not differ from those without thyroid dysfunction. Overt PPTD was seen in 15.9%. Incidence of PPTD was 10%. Patients with PPTD were slightly older than those without PPTD and the prevalence of TPO-ab was higher in these women. CONCLUSION: In women with DM1 the prevalence of thyroid dysfunction during pregnancy and overt PPTD is 3-fold higher than in the general Dutch population. Risk factors are age and TPO-ab. Given the possible impact on psychomotor development of the offspring and on well-being of the mother these data suggest there is a case for screening (pre-)pregnant women with DM1 for TSH and TPO-ab.