The enzyme(s) responsible for the sulfate conjugation of L-T3 in man has not been characterized. T3 sulfotransferase (T3-ST) activity was characterized in normal human liver tissue obtained during clinically indicated surgical resection. Subcellular distribution studies showed that the T3-ST activity was localized to the cytoplasmic fraction. This finding raised the possibility that T3-ST activity might be similar to the 2 previously identified forms of cytoplasmic phenol sulfotransferase (PST) found in human tissue. A thermostable (TS) form of PST catalyzes the sulfate conjugation of micromolar concentrations of p-nitrophenol, and a thermolabile (TL) form catalyzes the sulfate conjugation of dopamine and other monoamines. Thermal stability and enzyme inhibitor experiments showed that T3-ST activity in pooled liver homogenates was very similar to the TS form of PST. The apparent similarity of T3-ST to TS PST was studied further by measuring T3-ST, TS PST, and TL PST activities in 20 individual liver samples. T3-ST activities correlated significantly with TS PST activities (r = 0.939; P less than 0.001) measured with p-nitrophenol, but not with TL PST activities (r = -0.118; P greater than 0.6) measured with dopamine. However, sulfation of T3 by the TL form of the enzyme might have been masked by the 18-fold higher specific activity of TS than TL PST in human liver homogenates. When the two forms of PST were separated by ion exchange chromatography, T3 was found to be a substrate for both the TS and TL forms of PST. "True" Km values for T3 were similar for TS and TL PST (81 and 127 microM, respectively).