Figure 1

Illustration of the TH transporters expressed at the blood-brain barrier (BBB). The TH transporters MCT8, OATP1C1, LAT1, and LAT2 are expressed at the BBB, whereby LAT2 shows lower expression levels than the LAT1. The prohormone T₄ enters the astrocyte and is converted to the active form T₃ by an outer ring deiodination. According to current concepts, T₃ enters the neurons by the TH transporter MCT8.