Thyreostatic drugs (TS), illegally administrated to livestock for fattening purposes, are banned in the European Union since 1981 (Council Directive 81/602/EC). This paper reviews the trends in the analytical approaches for the determination of TS drugs in biological matrices. After a brief introduction on the different groups of compounds with a thyreostatic action, the most relevant legislation regarding the residue control of these compounds is presented. An overview of the analytical possibilities for the determination of TS in animal matrices, covering sample extraction, purification, separation techniques and detection methods is provided. Additionally, a brief outline of animal experiments is described that illustrates the excretion and distribution profiles of TS residues. Finally, the novel developments in TS analysis are highlighted. Also the possible semi-endogenous status of thiouracil is discussed.