The present study aimed to evaluate the vitamin 25(OH)D, ROS, and IL‐21 cytokine levels in the RA patients’ blood, and to determine the association of IL‐21 with ROS and vitamin 25(OH) D. These described parameters were additionally compared in patients and further categorized on the basis of the severity of the disease and the rheumatoid factor (RF) presence or absence. The gene discussed is IL21; the disease is rheumatoid arthritis.