Influence of tetraiodothyronine on hearing maturation in rats

Thyroid hormones are essential to the maturation of the central nervous system and sensory organs. Hypothyroidism induces various kinds of neural retardation, including hearing deficits. Despite careful attention to replacement therapy in human neonatal hypothyroidism, psychological disorders and cerebellar malfunction may occur in later life and may be related to overdoses. Brain stem response audiometry (BSRA) is used in this study as an indicator of acoustic pathway maturation in newborn rats. Normal litters are compared to tetraiodothyronine (T4)-overtreated rats from birth to day 12 of life. Onset of hearing and acoustic maturation are accelerated in treated animals as compared to controls (p<0.0001). Differences between groups progressively vanish after cessation of T4 administration. However, as long as 4 months later, small differences are still present, leading one to suspect a permanent change in the functioning of the acoustic pathways.