The function of FGF21 (fibroblast growth factor 21, ENSG00000105550) is as follows. Stimulates glucose uptake in differentiated adipocytes via the induction of glucose transporter SLC2A1/GLUT1 expression (but not SLC2A4/GLUT4 expression). Activity requires the presence of KLB. Regulates systemic glucose homeostasis and insulin sensitivity. {ECO:0000250|UniProtKB:Q9JJN1, ECO:0000269|PubMed:15902306, ECO:0000269|PubMed:17623664}.