Thymidine kinase is an enzyme that catalyzes the reaction that follows (Figure 22.17):

(Deoxy)thymidine + ATP <=> dTMP + ADP

The form of the enzyme found in the cytosol acts only on deoxythymidine. The thymidine kinase found in the mitochondria acts on deoxythymidine, deoxycytidine, and deoxyuridine. The mitochondrial thymidine kinase's activity is sufficiently broad that it will also act on the anti-HIV drug, 3'-azido-2'3'-dideoxythymidine (AZT). Evidence suggest that deoxyribonucleotides of AZT interfere with mitochondrial function, possibly by inhibiting mitochondrial DNA replication or transcription, which may explain some of the side effects of cardiotoxicity observed with its use.

The ability of cells to uptake thymidine and phosphorylate it is exploited in the so-called HAT selection medium (see here), which selects for cells that have functional nucleotide salvage pathways.