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Five thyreostats (TSs),namely tapazole,thiouracil,methylthiouracil,propylthiouracil,and phenyl-thiouracil,were determined in bovine urine using ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS ) in positive electrospray ionization mode. Extraction and clean-up were achieved using a ChemElut cartridge with tert-butyl methyl ether,without a derivatization step. Separation was achieved on an Acquity UPLC SS T3 column. The mobile phase was acetonitrile and water containing 0. 2%(v/v)formic acid. The mass spectrometer was operated in multiple reaction monitoring mode. Urine samples were spiked with TS solution at levels corresponding to 5,10,15,and 20 μg/L. The accuracy (internal standard corrected)ranged from 92% to 107%,with a repeatability precision (relative standard deviation, RSD)less than 15% for all five analytes. The RSDs within-laboratory reproducibility was less than 26%. The decision limits (CCα)and detection capabilities (CCβ)were obtained from a calibration curve and were in the ranges of 3. 1-6. 1 μg/L and 4. 0-7. 4 μg/L,respectively. The CCα and CCβ values were below the recommen-ded concentration,which was set at 10 μg/L. The results show that the described method is suitable for the direct detection of TSs in bovine urine. This method can also be used to determine TSs in porcine urine.