Deficiency of the Monocarboxylate Transporter 8 (MCT8) severely impairs thyroid hormone (TH) transport into the brain, disrupting brain development as well as peripheral TH homeostasis. Studies assessing MCT8 expression patterns and tissue-specific pathologies induced by local TH-deficiency are often inconclusive due to unreliable antibody staining and the lack of functional tools to specifically target MCT8-expressing cells....