The results demonstrated the beneficial effects of the TNF-α antagonist, i.e., etanercept, which not only improved the clinical condition of children with JIA but also positively influenced the metabolism of both adipokines and tenascin C. Conclusions: The obtained results suggest the potential use of adiponectin, leptin, and tenascin C as biochemical markers of the effectiveness of etanercept therapy in inhibiting the progression of degenerative joint changes in children with JIA treated with TNF-α inhibitors. The gene discussed is LEP; the disease is juvenile idiopathic arthritis.