BACKGROUND The association between Graves' disease (GD) and serum vitamin D levels has been studied for decades although the results were controversial. Moreover, the difference in vitamin D levels between the different stages of GD is not well studied. Therefore, this study aimed to compare the vitamin D levels between active and remission GD and to investigate the factors affecting vitamin D levels in GD patients. METHODS This cross-sectional study was performed between 1 January to 31 December 2021. The eligible patients were in either the active or remission stage of GD. The demographic and clinical data of the patients willing to participate in the study were collected, as well as their vitamin D levels. Comparisons of continuous parameters between the active and remission groups were performed using the Mann-Whitney U test, while categorical parameters were performed using the Chi-square test. RESULTS 75 patients were diagnosed with GD, with 54.7% in the active stage. The mean vitamin D level was lower in the active GD group than in the remission GD group (28.23 vs. 31.58 ng/mL, respectively, p-value 0.079). The prevalence of vitamin D deficiency (i.e., serum vitamin D level < 20 ng/mL) in the active GD group was 14.6%, and in the remission GD group was 0% (p-value 0.02). Moreover, there was a significant negative correlation between the serum vitamin D level and serum free T4 level (p-value 0.03). CONCLUSIONS In spite of non-significance, patients with active GD had lower mean vitamin D levels compared to those with remission GD. The prevalence of vitamin D deficiency was significantly higher in the active GD patients. Additionally, a negative correlation between serum vitamin D levels and serum free T4 levels was observed in this study.