Catalyzes the reductive methylation of 2'-deoxyuridine-5'-monophosphate (dUMP) to 2'-deoxythymidine-5'-monophosphate (dTMP) while utilizing 5,10-methylenetetrahydrofolate (mTHF) as the methyl donor, and NADPH and FADH(2) as the reductant. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + dUMP + H(+) + NADPH = (6S)-5,6,7,8-tetrahydrofolate + dTMP + NADP(+) Binds 4 FAD per tetramer. Each FAD binding site is formed by three monomers. Pyrimidine metabolism; dTTP biosynthesis. Homotetramer. Belongs to the thymidylate synthase ThyX family.