UDP-glucuronosyltransferase (UGT) that catalyzes phase II biotransformation reactions in which lipophilic substrates are conjugated with glucuronic acid to increase the metabolite's water solubility, thereby facilitating excretion into either the urine or bile (PubMed:18177842, PubMed:24641623). Essential for the elimination and detoxification of drugs, xenobiotics and endogenous compounds (PubMed:18177842). Involved in the glucuronidation of calcidiol, which is the major circulating form of vitamin D3 essential for the regulation of calcium and phosphate homeostasis (PubMed:24641623). Also glucuronidates the biologically active form of vitamin D3, calcitriol, probably leading to its biliary transport and intestinal reabsorption (PubMed:18177842). Lacks UDP-glucuronosyltransferase (UGT) activity but acts as a negative regulator of isoform 1. glucuronate acceptor + UDP-alpha-D-glucuronate = acceptor beta-D-glucuronoside + H(+) + UDP calcidiol + UDP-alpha-D-glucuronate = calcidiol 25-O-(beta-D-glucuronide) + H(+) + UDP calcidiol + UDP-alpha-D-glucuronate = calcidiol 3-O-(beta-D-glucuronide) + H(+) + UDP calcitriol + UDP-alpha-D-glucuronate = calcitriol 25-O-(beta-D-glucuronide) + H(+) + UDP Some kinetic parameters were assessed using commercial enzymes, which may represent a mix of both active and inactive protein forms, and therefore modify the kinetic values. Homodimer (PubMed:17179145). Homooligomer (Probable). Interacts with UGT1A1, UGT1A3, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 to form heterodimers (PubMed:17179145). Isoform 1 interacts with isoform 2/i2 suggesting that oligomerization is involved in negative regulation of transferase activity by isoform 2. Isoform 1 also interacts with respective i2 isoforms of UGT1A1, UGT1A3, UGT1A6, UGT1A7, UGT1A8, UGT1A9 and UGT1A10 (PubMed:20610558). Expressed in liver (PubMed:1339448, PubMed:18004212). Expressed in kidney, colon and small intestine (PubMed:18004212). Not expressed in esophagus (PubMed:18004212). Not expressed in skin (PubMed:1339448). Expressed in liver, kidney, colon, esophagus and small intestine. Induced by phenobarbital. UGT1A4 isoform is part of the UGT1A complex locus which displays alternative use of promoters, first exons and terminal exons. The locus is defined by 13 first exons, which are alternatively spliced to 3 other common exons and 2 alternative terminal exons 5. From the 27 possible mRNA isoforms, 9 produce functionally active polypeptides (UGT1A1, 1A3, 1A4, 1A5, 1A6, 1A7, 1A8, 1A9 and 1A10) called isoforms 1 (i1). Use of an alternative exon 5 (5b) as terminal exon is leading to 9 additional alternatively spliced products termed isoforms i2 and which lack transferase activity. Belongs to the UDP-glycosyltransferase family.