Monocarboxylate transporter 8 (MCT8, SLC16A2) is a thyroid hormone (TH) 
transmembrane transport protein mutated in Allan-Herndon-Dudley syndrome, a 
severe X-linked psychomotor retardation. The neurological and endocrine 
phenotypes of patients deficient in MCT8 function underscore the physiological 
significance of carrier-mediated TH transmembrane transport. MCT8 belongs to the 
major facilitator superfamily of 12 transmembrane-spanning proteins and mediates 
energy-independent bidirectional transport of iodothyronines across the plasma 
membrane. Structural information is lacking for all TH transmembrane 
transporters. To gain insight into structure-function relations in TH transport, 
we chose human MCT8 as a paradigm. We systematically performed conventional and 
liquid chromatography-tandem mass spectrometry-based uptake measurements into 
MCT8-transfected cells using a large number of compounds structurally related to 
iodothyronines. We found that human MCT8 is specific for L-iodothyronines and 
requires at least one iodine atom per aromatic ring. Neither thyronamines, 
decarboxylated metabolites of iodothyronines, nor triiodothyroacetic acid and 
tetraiodothyroacetic acid, TH derivatives lacking both chiral center and amino 
group, are substrates for MCT8. The polyphenolic flavonoids naringenin and 
F21388, potent competitors for TH binding at transthyretin, did not inhibit T(3) 
transport, suggesting that MCT8 can discriminate its ligand better than 
transthyretin. Bioinformatic studies and a first molecular homology model of 
MCT8 suggested amino acids potentially involved in substrate interaction. 
Indeed, alanine mutation of either Arg(445) (helix 8) or Asp(498) (helix 10) 
abrogated T(3) transport activity of MCT8, supporting their predicted role in 
substrate recognition. The MCT8 model allows us to rationalize potential 
interactions of amino acids including those mutated in patients with 
Allan-Herndon-Dudley syndrome.