Discrimination between Synthetically Administered and Endogenous Thiouracil Based on Monitoring of Urine, Muscle, and Thyroid Tissue: An in Vivo Study in Young and Adult Bovines

Abstract

Thiouracil (TU), synthesized for its thyroid-regulating capacities and alternatively misused in livestock for its weight-gaining effects, is acknowledged to have an endogenous origin. Discrimination between low-level abuse and endogenous occurrence is challenging and unexplored in an experimental setting. Therefore, cows (n = 16) and calves (n = 18) were subjected to a rapeseed-supplemented diet or treated with synthetic TU. Significant higher urinary TU levels were recorded after TU administration (<CC, 15 642 μg L) compared to rapeseed supplementation (<CC, 65.8 μg L), however, with overlapping values. TU was not detected in the edible meat; however, concentrations between the CC and 10 μg kg were noted in thyroid tissue of calves and cows following rapeseed supplementation. The latter concentrations were significantly higher in thyroid tissue of calves (22.9-41.8 μg kg) and cows (16.9-36.7 μg kg) after synthetic TU administration. These results strongly point toward thyroid analysis as a discriminatory tool.

Discrimination between the exogenous and endogenous origin of thiouracil in farm animals, the final chapter?

“…The origin of these "background" levels in the control animals is unknown. However, this was also observed in a thiouracil treatment study for calves and cows performed by Wauters (Wauters et al 2017). This study hypothesised that thiouracil's origin in the control group could be from other feed ingredients from plants, which possibly also contained glucosinolate-like or chemicalrelated compounds, which could form thiouracil.…”

“…After treatment, in the first week, concentrations were between 35 and 4600 µg/L and during the second week between 0.8 and 47 µg/L. In the study of Wauters (Wauters et al 2017), the first week after treatment showed a swift decline of thiouracil in bovines, even below 10 µg/L. This difference could be due to the metabolism of porcine, which is different from cows.…”

“…Data for intentional abuse of thiouracil is scarce. Two different studies (Wauters et al 2017;Van Meulebroek et al 2018) were performed to determine threshold levels based on animal experiments whereby bovines were treated with thiouracil. One of these studies was also aiming to identify relevant biomarkers for thiouracil abuse (Van Meulebroek et al 2018).…”

Discovery of urinary biomarkers to discriminate between exogenous and semi-endogenous thiouracil in cattle: A parallel-like randomized design

“…Urine samples that were considered for metabolic fingerprinting have previously (after about a half year of storage) also been analyzed in a targeted fashion using a triple quadrupole instrument, whereby TU concentrations were determined using matrix-matching calibration curves [ 16 ]. TU concentrations, as discussed in this manuscript, were thus obtained by the cited targeted approach.…”

“…TU in urine and thyroid tissue [ 13 ]. Unfortunately, a systematic occurrence of urine samples, exceeding this recommended TU concentration, has been observed in recent years [ 14 , 15 , 16 ]. Therefore, national and international surveys have been conducted to set a new threshold value that is more accurate to differentiate between exogenous and semi-endogenous TU [ 5 ].…”