Autoimmune thyroid diseases (AITD) may lead to increased cardiovascular risk; however, the underlying mechanism for this is unclear. The aim of our study was to investigate serum asymmetric dimethylarginine (ADMA), vitamin B12, folic acid and homocysteine (HCY) levels in patients with and without AITD. A total of 180 individuals were enrolled in this study, encompassing 45 patients with Hashimoto’s Thyroiditis (HT), 45 patients with Graves’ disease (GD), 45 individuals with multinodular goiter (MNG) and 45 healthy controls who were admitted to Selçuk University Medical Faculty’s Department of Endocrinology. All statistical analyses were conducted using the R Statistical Software (version 4.2.1). We found increased levels of HCY and ADMA, and decreased levels of folic acid in AITD compared to the control group (P=0.021, P=0.04 and P=0.017, respectively). Vitamin B12 levels were similar between groups. HCY was negatively correlated with vitamin B12 and folic acid (r=-0.028, P=0.002 and r=-0.368, P < 0.001, respectively). Our study revealed that a rise in ADMA and HCY levels in patients with AITD can provide insight into how AITD may contribute to endothelial dysfunction and cardiovascular risks.