Fibroblast growth factor 21 (FGF21) is a novel metabolic regulator that holds promise as a therapeutic target in the treatment of type 2 diabetes and obesity. The regulation of FGF21, however, is only emerging. Most of the existing evidence is derived from animal studies – particularly mice – where repeated measurements of hormones are difficult. Over the last decade, several studies evaluating the regulation of FGF21 in human physiology have emerged in the literature. These have shown that plasma FGF21 is an acutely regulated hormone and that the glucagon-to-insulin ratio seems to be pivotal in the regulation of FGF21. In the present narrative review, we evaluate the regulation of FGF21 in humans with a focus on the regulatory role of the glucagon-to-insulin ratio.