The transfer of sulfate to thyroid hormones and their analogs by a monkey hepatocarcinoma cell extract was compared to this reaction as catalyzed by two homogeneous aryl sulfotransferases from rat liver. The thyroid hormones 3,5,3'-triiodo-L-thyronine and L-T4 as well as analogs of these hormones, were used as substrates. While these compounds vary widely in their capacity as sulfate acceptors, the catalytic abilities of the three enzyme preparations generally reflect a broad pattern of overlapping specificity. The finding that T4 is a very poor substrate, that 3'-iodothyronine and 3,3'-diiodothyronine are very good substrates, and that T3 is intermediate in its acceptor activity probably explains the relative sulfation of these compounds in vivo and by living hepatocytes. (Endocrinology 108: 454, 1981)