To ascertain whether triiodothyronine (T3) sulfotransferase coeluted with the known phenol sulfotransferases (PSTs) during purification, human liver thermostable PST, thermolabile PST, and T3 sulfotransferase were assayed with p-nitrophenol, dopamine, and T3, respectively. Thermostable PST eluted from an ion-exchange column in two sequential peaks of activity (Peaks I and II), followed by a peak of thermolabile PST activity. There were three peaks of T3 sulfotransferase with thermostable PST: two within thermostable PST Peak I, and one peak of T3 sulfotransferase activity within thermostable PST Peak II. There was a minor peak of T3 sulfotransferase with thermolabile PST. Further purification of thermostable Peak I showed coelution of T3 sulfotransferase with thermostable PST during gel filtration and affinity chromatography. SDS-PAGE revealed a major protein band at 31 kDa. Dehydroepiandrosterone sulfotransferase comprised only 4% of the final activity. This report demonstrates coelution of T3 sulfotransferase with thermostable PST, shows a potential additional isozyme of T3 sulfotransferase, and points out the apparent minimal role of dehydroepiandrosterone sulfotransferase in T3 sulfation. The findings support the hypothesis that thermostable PST is the predominant human liver T3 sulfotransferase activity.