Hypo-hyperdontia
is an extremely rare mixed numerical dental abnormality with the presence of
supernumerary teeth and  absence of the teeth concomitantly
in the same individual.

Although
its etiology is unknown, hypo-hyperdontia may appear as a result of genetic or
possible environmental factors. This report presents the first case for
posterior hypo-hyperdontia with distodens and absent premolar. Furthermore, it
is the second case report in the literature for hypo-hyperdontia with a thyroid
disorder