TH and Allan-Herndon-Dudley syndrome: In addition, since MCT8 is a thyroid-hormone (TH) transporter, AHDS patients exhibit endocrine alterations in their TH parameters, with decreased plasma concentration of the prohormone 3,5,3′,5′-triiodo-L-thyronine/thyroxine (T4) and increased concentration of the active form 3,5,3′-triiodo-L-thyronine (T3) [3], [4].