Thyroid dysfunction is rare in tuberous sclerosis, although papillary adenomas (hamartomas) of the thyroid gland have been reported in a few autopsy cases. We describe a child with tuberous sclerosis and primary congenital hypothyroidism secondary to a dysgenetic thyroid gland. To our knowledge, this association has not been reported previously. Although the association of these two disorders in one patient may be merely coincidence, we speculate that the dysgenetic thyroid gland in this patient may represent a ''hamartia'' as a consequence of the tuberous sclerosis gene. (C) 1993 Wiley-Liss, Inc.