FGF21 and obesity due to melanocortin 4 receptor deficiency: Serum samples from 405 participants (33 with normal weight and normoglycemia (NG), 156 with obesity and NG, 157 with obesity and impaired glucose tolerance (IGT), and 59 with obesity and T2D) aged 47 ± 13 years, were recruited to study the serum GDF15 and FGF21 concentrations and their relationship with obesity, T2D, and aging.