Notes: Thymidine-d4 is intended for use as an internal standard for the quantification of thymidine (Item No. 20519) by GC- or LC-MS. Thymidine is a pyrimidine nucleoside composed of a deoxyribose sugar and thymine base.{54328} It is phosphorylated by thymidine kinase 2 (TK2) in mitochondria to form deoxythymidine monophosphate (dTMP; Item No. 33549).{48673} Oral administration of thymidine in combination with deoxycytidine increases lifespan in a Tk2 H126N knock-in mouse model of TK2 deficiency.