SPORADIC (nongoitrous) cretinism occurring in one of monozygotic twins strongly suggests the etiology of that condition to be not genetically conditioned but, rather, related to direct influences of environmental factors on the development of the thyroid anlage in the fetus. This is to be differentiated from goitrous sporadic cretinism, which is thought to be genetically conditioned.1 The near absence of familial coincidence of nongoitrous sporadic cretinism lends further support to that hypothesis. To date, three proved instances, and one probable, of discordant cretinism in monozygotic twins are reported.*
In contrast, where congenital disorders are considered genetically conditioned, it is common to observe manifest disease in each of identical twins; for example, in known instances where vitamin-D-resistant rickets has occurred in one monozygotic twin, it has also been present in the others.†
This communication records a fourth proved instance of discordance between identical twins, one of whom manifested congenital cretinism.