Phosphorylates thymidine, deoxycytidine, and deoxyuridine in the mitochondrial matrix (PubMed:11687801, PubMed:9989599). In non- replicating cells, where cytosolic dNTP synthesis is down-regulated, mtDNA synthesis depends solely on TK2 and DGUOK (PubMed:9989599). Widely used as target of antiviral and chemotherapeutic agents (PubMed:9989599). {ECO:0000269|PubMed:11687801, ECO:0000269|PubMed:9989599}. This is the function of ENSG00000166548 (TK2, thymidine kinase 2).