The functional role of thyroid hormone (TH) in the cortex and hippocampus of mouse during neuronal development was investigated in this study. TH insufficiency showed a decrease in the expression of parvalbumin (PV) in the cortex and hippocampus of pups at postnatal day (PD) 14, while treatment with thyroxine from PD 0 to PD 14 ameliorated the PV loss. On the other hand, treatment with antithyroid agents in adulthood did not result in a decrease in the expression of PV in these areas. These results indicate the existence of a critical period of TH action during the early postnatal period. A decrease in MeCP2-positive neuronal nuclei was also observed in the cortical layers II-IV of the cerebral cortex. The brains were then stained with CUX1, a marker for cortical layers II-IV. In comparison with normal mice, CUX1 signals were decreased in the somatosensory cortex of the hypothyroid mice, and the total thickness of cortical layers II-IV of the mice was lower than that of normal mice. These results suggest that TH insufficiency during the perinatal period strongly and broadly affects neuronal development.