The major findings of this study are that, in lactation and adolescent stage of development rats, developmental ID and hypothyroidism (15 ppm PTU) significantly reduced the mean number of surviving cells in hippocampus and decreased ERK1/2 and CREB expression in hippocampal CA1 and CA3; even after the thyroid hormones back to normal, surviving cells, ERK1/2 and CREB were still lower than the controls. This evidence concerns the gene TG and hypothyroidism.