The purpose of this paper is to examine the mental development of children with congenital hypothyroidism. There are few developmental disorders which show such striking somatic response to treatment; the literature abounds with reports and illustrations of the miraculous changes which have taken place in retarded, misshapen children after administration of an adequate amount of thyroid. Details of physical progress are usually described accurately; references to mental development are commonly vague. Even the most enthusiastic reports convey some uneasiness about the ultimate mental capacity of these children.
Only a few authors1 have attempted to evaluate as objectively and quantitatively as possible the relationship of mental development to physical progress. The reports of these few contain data which point to a certain lack of dependence of mental development on the adequacy of treatment and the degree of restoration of the physical state. However, the great majority of reports—only a