Identification of 3-methyl-2-thiohydantoin, a metabolite of carbimazole, in man.

1. Classical and high-pressure liquid chromatographic separations were devised for the separation and isolation of the metabolite 3-methyl-2-thiohydantoin from the urine of patients receiving carbimazole orally. 2. 3-methyl-2-thiohydantoin was identified by comparing its absorption and mass spectral properties with authentic material. 3. 3-methyl-2-thiohydantoin was also detected in the plasma of patients receiving methimazole intravenously.