TNFRSF11B and obesity due to melanocortin 4 receptor deficiency: While serum OPG levels were increased in certain metabolic disorders, such as obesity, type 2 diabetes, and polycystic ovarian syndrome, that were closely related to insulin resistance [3–11], some other studies reported that serum OPG levels was not related to insulin resistance or even negative related to insulin resistance [12, 13].