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 ( ). 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 . Increases the access of these substrates to the intracellular sites where they are metabolized by the deiodinases . 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 (, ). Transports T4 and E1S in a pH-insensitive manner . Facilitates the transport of thyroid hormones across the blood-brain barrier and into glia and neuronal cells in the brain .
Subcellular locations: Cell membrane
Expressed in both luminal and abluminal membranes of brain capillary endothelial cells. Localized to the apical membrane and basal surfaces of choroid plexus.
Highly expressed in brain and in Leydig cells in testis (, ). Localized in nests of Leydig cells (at protein level) . Expressed in choroid plexus (at protein level) . Not strongly enriched in cerebral microvessels .