Thymidine is a pyrimidine nucleoside. It consists of the pyrimidine base thymine attached by a covalent bond to a deoxyribose sugar (pentose sugar). Thymidine is found only in the DNA double helix (not in RNA), where it pairs with adenine via complementary base pairing.
Thymidine can be phosphorylated by a kinase enzyme, resulting in the addition of 1, 2 or 3 phosphate groups. Phosphorylation can form molecules of dTMP (deoxythymidine monophoshate), dDTP (deoxythymidine diphosphate) or dTTP (deoxythymidine triphosphate).