3,5,3'-Triiodothyronine (T3) 4-sulfate is a major metabolite of T3 in humans (LoPresti and Nicoloff 1994), and seven SULT enzymes, SULT1A1 (Li et al. 2001), 1A3 (Kester, Kaptein et al. 1999), 1B1 (Wang et al. 1998), 1C1 (Li et al. 2000), 1E1 (Li and Anderson 1999; Kester, van Dijk et al. 1999), 2A1 (Li and Anderson 1999), and 4A1 (Sakakibara et al. 2002) can catalyze this reaction in vitro or in tissue culture model systems. All of these enzymes except SULT4A1 also catalyze the sulfonation of 3,3'-diiodothyronine (T2).