Mediates the Na(+)-independent high affinity transport of organic anions such as the thyroid hormones L-thyroxine (T4), L- thyroxine sulfate (T4S), and 3,3',5'-triiodo-L-thyronine (reverse T3, rT3) at the plasma membrane (PubMed:12351693, PubMed:18566113, PubMed:19129463, PubMed:40680733). Regulates T4 levels in different brain regions by transporting T4, and also by serving as an export pump for T4S, which is a source of T4 after hydrolysis by local sulfatases (PubMed:18566113). Increases the access of these substrates to the intracellular sites where they are metabolized by the deiodinases (PubMed:18566113). Other potential substrates, such as triiodothyronine (T3), 17-beta-glucuronosyl estradiol (17beta-estradiol 17-O-(beta-D- glucuronate)), estrone-3-sulfate (E1S) and sulfobromophthalein (BSP) are transported with much lower efficiency (PubMed:12351693, PubMed:19129463, PubMed:40680733). Transports T4 and E1S in a pH- insensitive manner (PubMed:19129463). Facilitates the transport of thyroid hormones across the blood-brain barrier and into glia and neuronal cells in the brain (PubMed:30296914). {ECO:0000269|PubMed:12351693, ECO:0000269|PubMed:18566113, ECO:0000269|PubMed:19129463, ECO:0000269|PubMed:30296914, ECO:0000269|PubMed:40680733}. This is the function of SLCO1C1 (solute carrier organic anion transporter family member 1C1, Ensembl gene identifier ENSG00000139155).