Effects of thyroxine treatment on histology and behavior using the methimazole model of congenital hypothyroidism in the rat.

•There is a therapeutic window for T4 replacement therapy in congenital hypothyroidism.•The sensitivity of behavioral assessments is necessary in this area of research.•T4 treatment by P14 is necessary to protect dendritic branching.•T4 treatment by P7 is necessary to protect NGF expression.•T4 replacement should be provided as early as possible for the treatment of CH.