OBJECTIVE: The aim of this study is to investigate the alterations in the thyroid functions in preeclampsia and to compare the prevalence of thyroid autoantibodies in preeclamptic pregnant women with that of normal pregnants'. STUDY DESIGN: This study was carried out in Zekai Tahir Burak Women's Research and Teaching Hospital between august and december 2003. 49 preeclamptic pregnant women hospitalized in our high risk pregnancy clinic and 36 normotensive pregnancies followed in antenatal outpatient clinic were enrolled as the study groups. Serum TSH, Free T3, Free T4, Total T3, Total T4, Anti TPO (Antithyroid peroxidase) and Anti TG (Anti antithyroglobulin) levels were determined in both groups. RESULTS: Both groups were controlled for mean age, gravida, parity, the number of living offspring. Significant difference in Anti TPO antibody was found between preeclamptic and normotensive groups (44.22±17.7 vs. 31.42±11.49, respectively) (p<0.001). Similarly, there was a significant difference in the levels of Anti TG antibody between preeclamptic and normotensive groups (341.53±158.35 vs. 193.14±57.9, respectively). When Total T3 levels were compared, values were found to be higher in preeclampsia group (3.38±1.25 vs. 2.61±0.81). Similarly, total T4 was found to be significantly higher in preeclampsia group than in normotensive group (145.45±46.8 vs. 112.94±29.23) (p<0.001). When two groups were compared in terms of Free T3 and T4, no significant difference was found between preeclampsia and normotensive groups (3.08±1.04 vs. 3.40±1.22) and (0.1±0.55vs.0.95±0.15), respectively (p>0.05). TSH levels were not significantly different between preeclampsia and normotensive groups (2.28±1.73 vs. 1.73±0.87, respectively). However, as p= 0.055, this value was very close to significance. CONCLUSION: In the light of the laboratory findings, subclinical hypothyroiditis was observed in preeclamptic group. In this group, thyroid autoantibodies increased significantly as well. It was thought that thyroid functions were impaired in preeclamptic patients. Therefore thyroid functions should be monitored more closely, making it possible to prevent the possible effects of hypothyroiditis and postpartum thyroiditis in the newborn.