Very active and specific thyroid hormone transporter. Stimulates cellular uptake of thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (rT3) and diidothyronine. Does not transport Leu, Phe, Trp or Tyr. Homodimer. Expressed at highest levels in liver, lower levels in brain, kidney and heart (at protein level). Belongs to the major facilitator superfamily. Monocarboxylate porter (TC 2.A.1.13) family.