SLC16A2 and Allan-Herndon-Dudley syndrome: AHDS is caused by inactivating mutations in the monocarboxylate transporter 8 (MCT8), a highly specific thyroid hormone (TH) transporter protein that facilitates the passage of the prohormone 3,3′,5,5′-tetraiodothyronine (T4) and the TH receptor active form 3,3′,5-triiodothyronine (T3) across the plasma membrane [1].