Obese patients with thyroid cancer show a very aggressive phenotype in terms of tumor size, tumor stage, and LN metastasis, which suggests that the factors mediating the effects of obesity on tumor progression are thyroid hormones, insulin resistance, and adipokines [44,45]; however, no previous studies have investigated the relationship between serum levels of FGF21 and tumor aggressiveness. Here, FGF21 is linked to obesity due to melanocortin 4 receptor deficiency.