Recent studies have provided insights into the neurodevelopmental functions of thyroid hormone signaling. The nuclear thyroid hormone receptors (TRs) are ligand-activated transcription factors and a variety of TR isotypes, generated by two genes, mediate distinct processes. In addition, deiodinase enzymes that regulate levels of the main active form of thyroid hormone, T3, are likely to cooperate closely with TRs in specifying a localized and timely response to thyroid hormones in target tissues. Some of the most sensitive processes controlled by these pathways are in the auditory and visual sensory systems.