Thyroid hormone induces earlier onset of auditory function in neonatal rats

The effect of thyroid hormone injection on the development of auditory function in neonatal rats was evaluated using auditory nerve-brainstem evoked responses (ABR). The hormone induced earlier onset of auditory function. In order to differentiate between conductive and sensorineural factors, both air-conducted (AC) and bone-conducted (BC) ABR responses were recorded. Neonatal rats were injected with thyroxine (T4), or with saline (control animals), from day of birth (post-natal day-PND-0), daily, until PND 9. AC- and BC-ABRs were recorded from PND 6 up to PND 20. It was found that both AC- and BC-ABR thresholds were lower in the T4-injected rats up to PND 15, after which no difference was found between the two groups. This indicated earlier maturity of both conductive (external and middle ears) and sensorineural (inner ear) factors and is probably due to the earlier appearance in the blood of higher T4 levels, following injection, than that occurring naturally during the neonatal period in these animals.