• hTFR1-targeting HCAbs developed from RenNano® mice have nM range affinity to human TFR1, and have functional capabilities, including internalization capacity. • hTFR1-targeting HCAbs are able to penetrate the BBB and deliver drug payload to the brain parenchyma efficiently. • Together, these data highlight the tremendous potential for HCAbs and its variable domain sdAbs for transporting cargo across the BBB. Due to their smaller size and simpler structure, sdAbs could ultimately provide therapeutic benefit for neurodegenerative diseases, and offer promising potential for tumor penetration.