The long-term effects of severe hypothyroidism on craniofacial growth and dental development are illustrated in this case. It is apparent that given a favorable diet, the primary dentition can persist for a long period (early childhood to at least the age of 19) without the development of dental caries. It is also clear that the dental structures can still respond to the effects of L-thyroxine at a relatively late age, with the exfoliation of primary dentition and eruption of the secondary dentition. Impacted mandibular second molars appear to be rare. The lack of proper growth of the mandible and failure of normal resorption of the internal aspect of the ramus associated with deposition of bone on the external aspect with the development of normal-size teeth, resulted in a lack of space for the eruption of mandibular second molars. The impaction of the mandibular second molars in this patient seems to be caused by a dissociation of ramus growth and dental development, resulting in insufficient space for proper eruption of these teeth.