Transport of thyroid hormone across the cell membrane is required for its action and metabolism. Recently, a T-type amino acid transporter was cloned which transports aromatic amino acids but not iodothyronines. This transporter belongs to the monocarboxylate transporter (MCT) family and is most homologous with MCT8 (SLC16A2). Therefore, we cloned rat MCT8 and tested it for thyroid hormone transport in Xenopus laevis oocytes. Oocytes were injected with rat MCT8 cRNA, and after 3 days immunofluorescence microscopy demonstrated expression of the protein at the plasma membrane. MCT8 cRNA induced an ϳ10-fold increase in uptake of 10 nM 125 I-labeled thyroxine (T 4 ) Thyroid hormone is important for the development of different tissues and the metabolic function of these tissues throughout life. A well known example is the critical role of thyroid hormone in brain development (1). Transport of thyroid hormone across the cell membrane is required for its metabolism and action. Thyroxine (T 4 ) 1 is the predominant (pro)hormone secreted by the thyroid follicular cells, but 3,3Ј,5-triiodothyronine (T 3 ) is the receptor-active form of thyroid hormone.For its conversion to T 3 , T 4 must enter cells that express the enzymes catalyzing this reaction. The three iodothyronine deiodinases (D1-D3) involved in the activation and inactivation of thyroid hormone are homologous selenoproteins with different catalytic profiles, tissue distributions, and physiological functions (2-4).