SLCO4A1 (solute carrier organic anion transporter family member 4A1) is a transmembrane transporter protein that functions as an organic anion antiporter with broad substrate specificity. The protein recognizes diverse substrates including thyroid hormones (T3, T4, rT3), conjugated steroids (estrone 3-sulfate, estradiol 17-beta glucuronide), bile acids (taurocholate), and prostanoids (prostaglandin E2), operating in a tissue-specific manner. SLCO4A1 may facilitate uptake of metabolites from circulation into organs such as kidney, liver, and placenta, and potentially drives selective transport of thyroid hormones and estrogens coupled to an outward glutamate gradient across placental membranes. In disease contexts, SLCO4A1 expression correlates with poor prognosis in colon adenocarcinoma patients 1 and shows negative associations with infiltrating immune cells including B cells, CD8+ T cells, and dendritic cells 1. Genetic polymorphisms in SLCO4A1 have been associated with eye involvement in Behçet's syndrome 2. The antisense RNA SLCO4A1-AS1 is frequently overexpressed in various cancers and functions as an oncogenic regulator through multiple pathways 345, suggesting the SLCO4A1 locus has broader regulatory roles beyond its primary transport function.