The detrimental effects of iodine deficiency on brain development and the protective effects of thyroid hormone replacement therapy in the critical period were closely examined. Animals in the experiment were divided into four categories: Normal control (NL), Low iodine diet group (LI), Normal iodine diet group (NI) and the thyroid hormone supplement group (LI+T). Results showed that the body weight, motility and the indices of learning ability including total time (TT), the number of conditioned reflex (CL), conditioned and unconditioned reflex latency and the correction rate of conditioned reflex in the LI+T were all improved to such a condition that there was no significant difference between the LI+T and N, NI. (P greater than 0.05), whereas there did have significant difference between LI+T and LI (p less than 0.01). The experiment confirmed the protective effects offered by the thyroid hormone from damaging by severe iodine deficiency.