Evidence that urinary excretion of thiouracil in adult bovine submitted to a cruciferous diet can give erroneous indications of the possible illegal use of thyrostats in meat production.

Thyrostats have been banned for use as veterinary drugs in Europe since 1981 because of their carcinogenic and teratogenic properties. Until now, the identification of thiouracil in animal biological matrices has been interpreted as the consequence of an illegal administration. The present paper studies the influence of a cruciferous-based feed on the occurrence of thiouracil as a residue in urine. Urine samples collected from two heifers fed on cabbage or rapeseed cakes were analysed for the presence of thiouracil by 3-iodobenzylbromide derivatization and liquid chromatography-electrospray ionization tandem mass spectroscopy (LC-ESI(-)-MS/MS) analysis. Urine collected after cabbage or rapeseed feeding showed thiouracil concentrations in the range 3-7 and 2-9 microg l-1, respectively, demonstrating a relationship between a diet based on cruciferous vegetables and the occurrence of thiouracil in urine. Thiouracil was excreted in urine in the hours following cruciferous intake. Complete elimination (<0.8 microg l-1) of the compound occurred within 5 days. The precursors in cruciferous vegetables responsible for the thiouracil excretion in urine were proved not to be thiouracil itself.