How does a thymidine block halt DNA synthesis?
Excess thymidine in a mitotic cell generates negative feedback on the production of deoxycytidine triphosphate from cytidine-5'-phosphate. Excess quantities of deoxyadenosine and deoxyguanosine also block progression through S-phase. However, as a reagent for the control of replication timing, thymidine has been found to work best as its blocking activity can be applied and reversed more consistently.

Xeros N. (1962) Deoxyriboside Control and Synchronization of Mitosis. Nature 194: 682-683.