Thyroid hormone (TH) is a multifaceted hormone, promoting neurodevelopment, metabolism and mental wellbeing. The discovery of the TH transporter MCT8, whose mutations were linked to a rare psychomotor disability syndrome, raised questions on how its functional absence contributes to the physiopathology of these patients. In Leuven, Belgium, we used the chicken embryo to knock down MCT8 and reduce TH action in specific neural stem cell (NSC) populations that give rise to key cell types such as the Purkinje cells and GABAergic cells, and observed cell cycle perturbations, hampered migration and impaired differentiation.