A method based on ultra-high performance liquid chromatography was developed and validated to detect six thyreostatic compounds: tapazole, thiouracil, methylthiouracil, dimethylthiouracil, propylthiouracil, and phenylthiouracil in faeces of bovine. Thyreostats were extracted from the matrix with a mixture of methanol and buffer (pH = 8). Next step was derivatization of analytes with 3-iodobenzylbromide. The liquid chromatographic separation of derivatives was obtained on a SB-C18 column (50 × 2.1 mm; 1.8 μm, Agilent) with gradient elution using a mobile phase consisting of acetonitrile/0.1% acetic acid within 7.5 min. The analysis was performed on a Shimadzu NEXERA X2 ultra-high performance liquid chromatograph with triple quadrupole MS 8050 instrument operating in positive electrospray ionization mode. Depending on the target compound, two or three diagnostic signals (selected reaction monitoring transitions) were monitored. The procedure was validated according to the Commission Decision 2002/657/EC. Recovery and repeatability met the performance criteria specified by this document for banned compounds. The recovery ranged from 97.5 to 110.5%, and repeatability did not exceed 14.1%. Decision limits and detection capabilities were below 10 μg/kg. The highest decision limits and detection capabilities concentrations were observed for phenylthiouracil of 3.48 and 6.96 μg/kg, respectively.