FGF21 and obesity due to melanocortin 4 receptor deficiency: Currently, the findings of high FGF-21 levels in those patients conflict with the beneficial effects of exogenous FGF-21 administration in patients with obesity and those with diabetes, which might be attributed to possible FGF-21 resistance, suggesting that high levels of FGF-21 are a compensation mechanism in response to metabolic stress in those patients.