Loss of function mutations in thyroid hormone transporter MCT8 lead to severe X-linked psychomotor retardation and elevated serum T3 levels. Most patients, for example those with mutations V235M, S448X, insI189, or delF230, cannot stand, walk, or speak. Patients with mutations L434W, L568P, and S194F, however, walk independently and/or develop some dysarthric speech. To study the relationship between mutation and phenotype, we transfected JEG3 and COS1 cells with wild-type or mutant MCT8. Expression and function of the transporter were studied by analyzing T₃ and T₄ uptake, T₃ metabolism (by cotransfected type 3 deiodinase), Western blotting, affinity-labeling with N-bromoacetyl-T₃, immunocytochemistry, and quantitative RT-PCR. Wild-type MCT8 increased T₃ uptake and metabolism 5-fold compared to empty vector controls. Mutants V235M, S448X, insI189, and delF230 did not significantly increase transport. However, S194F, L568P, and L434W showed 20, 23, and 37% of wild-type activity. RT-PCR did not show significant differences in mRNA expression between wild-type and mutant MCT8. Immunocytochemistry detected the non-functional mutants V235M, insI189 and delF230 mostly in the cytoplasm, whereas mutants with residual function were expressed at the plasma membrane. Mutants S194F and L434W showed high protein expression, but low affinity for N-bromoacetyl-T₃; L568P was detected in low amounts, but showed relatively high affinity. Mutations in MCT8 cause loss of function through reduced protein expression, impaired trafficking to the plasma membrane or reduced substrate affinity. Mutants L434W, L568P, and S194F showed significant residual transport capacity, which may underlie the more advanced psychomotor development observed in patients with these mutations.