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.

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 (<CCα, 15 642 μg L-1) compared to rapeseed supplementation (<CCα, 65.8 μg L-1), however, with overlapping values. TU was not detected in the edible meat; however, concentrations between the CCα and 10 μg kg-1 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-1) and cows (16.9-36.7 μg kg-1) after synthetic TU administration. These results strongly point toward thyroid analysis as a discriminatory tool.