Objective To elucidate the role of thyroid hormones during the embryo and juvenile periods in mammal, and to observe the effects of thyroid hormones on the cerebrum development. Method During mid pregnancy of the female sheep with twins, one fetus of the twins was thyroidectomized. The female sheep with twins were kept pregnancy to birth after the operation. The pyramidal cells in the cerebral visual cortex of the twins were with Golgi staining for observation. Result The number of spinous process, the growing cones and basal dendrites of the thyroidectomized sheep were different from the normal group. Conclusion The deficiency of thyroid hormones can influence the development of pyramidal cells in the cerebrum.