Endogeneous versus synthetic thiouracil: discrimination based on in vivo monitoring

The use of thiouracil (TU) in meat industry is forbidden in the European Union. However, positive urine samples are frequently reported within National Monitoring Programs and it is questioned whether these positive results origin from abuse or from a natural TU source (e.g. Brassicaceae-rich diet). In an attempt to answer this research question, an in vivo (pilot) cross-over study with 2 calves and 2 cows was performed, each of them alternately receiving Brassicaceae-enriched feed (30 %) (= endogenous TU) and synthetic TU (1g/animal) (=abuse). Urine was sampled at fixed time-intervals and meat and thyroid tissues were collected after slaughter. For each cow, TU levels (thyroid, meat, urine) were detected (UHPLC-MS/MS) and compared between treatments. Generally, the highest synthetic TU levels were recorded in calves (related to body weight), with mean urinary concentrations of 29.56 ± 14.77 ppm compared to 8.07 ± 2.84 ppm in cows. Urinary endogenous TU levels were 7.48 ± 4.92 ppb for calves and 10.48 ± 7.99 ppb for cows, which however did not differ from levels monitored at the start of the study (calves: 7.79 ± 3,93 ppb; cows: 8.33 ± 1.64 ppb; after 1 week acclimatization). Additionally, no residue levels of TU in meat or thyroid were detected following Brassicaceae-enriched feed administration