ObjectivesThyroid autoimmunity is considered as the most prevalent autoimmune condition in women in fertility age. There are different clinical evidences indicating the association between thyroid autoimmunity and increased risk of RPL. This study aimed to analyze the association of Tregs and Th17 cells development factors and anti–thyroid peroxidase (anti-TPO) antibodies in RPL patients. Healthy controls (n = 36), TPO + controls (n = 25) and TPO + RPL (n = 32) participated in this study. After blood sampling, the frequency of Th17 and Tregs was evaluated using flow cytometry. Real-time PCR and ELISA was used to assess the status of Tregs and Th17 related transcription factors and cytokines in mRNA and protein level, respectively.ResultsTPO + RPL group showed a higher Th17 frequency compared to healthy controls and TPO + controls groups (p = 0.0002 and p = 0.04, respectively). Additionally, mRNA expression levels of RORγT and IL-17 were significantly higher in TPO + RPL compared to healthy controls and TPO + controls groups. In contrast, Foxp3 and TGFβ expression was lower in TPO + RPL. ELISA findings also indicated a significantly higher IL-17 and lower TGFβ secretion in TPO + RPL compared to healthy controls and TPO + controls. Thyroid autoimmunity should intensely be controlled specially in patients with RPL history.