Thyroid autoimmunity (TAI) is common in women of reproductive age. There is a relationship between TAI and recurrent pregnancy loss and infertility. In pregnant patients with thyroid autoimmunity, the T helper-1 (Th1)/T helper-2 (Th2) ratio may shift to a Thl -type response and these activated T lymphocytes may lead to implantation failure. The aims of this study were to investigate the serum levels of Thl-, Th2-, and T-helper-17-(Th17)-associated cytokines in pregnant patients with TAI, and to evaluate how these cytokines change with L-thyroxin treatment during pregnancy. Twenty pregnant women with TAI diagnosed in the first trimester of pregnancy who were not on L-thyroxine treatment, 14 pregnant women with known TAI before pregnancy already been on L-thyroxine treatment, and 19 pregnant patients without TAI were included in this study. Thyroid function tests, thyroid autoantibodies, and cytokine levels were measured at the first and the second trimesters. In pregnant patients who were diagnosed with TAI in the first trimester, both serum IL-2 levels and IL-17 levels were significantly higher than those of the control group. There were no significant differences between groups for serum IL-4, IL-6, IL-23, IL-10, and IFN gamma levels. In the second trimester, no significant differences were found between groups for all the cytokines measured. There are significant differences in Thl - and Thl 7-associated cytokine levels between patients with TAI and the control group in the first trimester. In the second trimester cytokine levels were similar among all groups. This pattern may be associated with the clinical benefits of L-thyroxine treatment. (C) 2015 Elsevier Ireland Ltd. All rights reserved.