Specific thyroid hormone transmembrane transporter, that mediates both uptake and efflux of thyroid hormones across the cell membrane independently of pH or a Na+ gradient. Major substrates are the iodothyronines T3 and T4 and to a lesser extent rT3 and 3,3-diiodothyronine (3,3'-T2).Acts as an important mediator of thyroid hormone transport, especially T3, through the blood-brain barrier.