Catalyzes the reduction of 7,8-dihydrofolate (DHF) to 5,6,7,8-tetrahydrofolate in a NADPH-dependent manner (PubMed:12096917, PubMed:15039552, PubMed:17569517, PubMed:19196009, PubMed:19478082, PubMed:21876184, PubMed:9719595). Key enzyme in folate metabolism. Contributes to the nuclear and mitochondrial de novo thymidylate biosynthesis pathway (PubMed:21876188, PubMed:22235121). Catalyzes an essential reaction for de novo glycine and purine synthesis, and for DNA precursor synthesis. Binds its own mRNA and that of DHFR2. {ECO:0000269|PubMed:12096917, ECO:0000269|PubMed:15039552, ECO:0000269|PubMed:17569517, ECO:0000269|PubMed:19196009, ECO:0000269|PubMed:19478082, ECO:0000269|PubMed:21876184, ECO:0000269|PubMed:21876188, ECO:0000269|PubMed:22235121, ECO:0000269|PubMed:9719595}. This is the function of DHFR (dihydrofolate reductase, ENSG00000228716).