Pyrimidines are ultimately catabolized (degraded) to CO2, H2O, and urea. Cytosine can be broken down to uracil which can be further broken down to N-carbamoyl-β-alanine. Thymine is broken down into β-aminoisobutyrate which can be further broken down into intermediates eventually leading into the citric acid cycle.
β-aminoisobutyrate acts as a rough indicator for rate of DNA turnover.
^Nielsen, HR; Sjolin, KE; Nyholm, K; Baliga, BS; Wong, R; Borek, E (1974). "Beta-aminoisobutyric acid, a new probe for the metabolism of DNA and RNA in normal and tumorous tissue". Cancer research34 (6): 1381–4. PMID4363656.