FGF21 and obesity due to melanocortin 4 receptor deficiency: Due to the interest in FGF21 as an anti-obesity substance, we also examined the effect of FGF21 on body temperature control in mice that had been HFD-fed (at thermoneutrality) and were thus obese (≈50 g versus chow-fed ≈30 g body weight) (Figure 1B).