Monocarboxylate transporter 8 (MCT8) is a thyroid hormone (TH)-specific 
transporter. Mutations in the MCT8 gene are associated with Allan-Herndon-Dudley 
Syndrome (AHDS), consisting of severe psychomotor retardation and disturbed TH 
parameters. To study the functional consequences of different MCT8 mutations in 
detail, we combined functional analysis in different cell types with live-cell 
imaging of the cellular distribution of seven mutations that we identified in 
patients with AHDS. We used two cell models to study the mutations in vitro: 1) 
transiently transfected COS1 and JEG3 cells, and 2) stably transfected Flp-in 
293 cells expressing a MCT8-cyan fluorescent protein construct. All seven 
mutants were expressed at the protein level and showed a defect in T3 and T4 
transport in uptake and metabolism studies. Three mutants (G282C, P537L, and 
G558D) had residual uptake activity in Flp-in 293 and COS1 cells, but not in 
JEG3 cells. Four mutants (G221R, P321L, D453V, P537L) were expressed at the 
plasma membrane. The mobility in the plasma membrane of P537L was similar to WT, 
but the mobility of P321L was altered. The other mutants studied (insV236, 
G282C, G558D) were predominantly localized in the endoplasmic reticulum. In 
essence, loss of function by MCT8 mutations can be divided in two groups: 
mutations that result in partial or complete loss of transport activity (G221R, 
P321L, D453V, P537L) and mutations that mainly disturb protein expression and 
trafficking (insV236, G282C, G558D). The cell type-dependent results suggest 
that MCT8 mutations in AHDS patients may have tissue-specific effects on TH 
transport probably caused by tissue-specific expression of yet unknown 
MCT8-interacting proteins.