Investigation of the effect of hypothyroidism duration on hearing by experimental animal model

AIM: It is well known that congenital hypothyroidism may have effect on hearing function in children but there is not enough information regarding with the relationship between acquiered hypothyroidism and hearing function. In addition there is not enough data concerning with the effect of hypothyroidism duration on hearing loss. The purpose of this study is to investigate the effect of duration of hypothyroidism on hearing func tion by using animal model. MATERIALS and METHODS: Four groups (15, 30, 45 days hypothyroidism groups and control) were included in this study. There were 8 animals in each group. 10 mg/kg/day PTU were given to the experiment groups intaperitonealy. OAE and ABR were done in control and experiment groups at the beginning and end of the 15, 30, 45 days. Intracardiac blood samples were taken to evaluate T4 and TSH levels before decapitation. After that temporal bone of rats were dissected and cochleas of rats were collected. Changes in outer hair cell, stria vascularis and spiral ganglion were scored by using histopathological evaluation. RESULTS: According to click ABR results, threshold increased and latency delayed in hypothyroid groups (H15, H30, H45) at the end of the experiment (p<0,05). When all groups were compared according to threshold and latency results, although there was no significant difference in terms of threshold between groups, there was significant difference in terms of latency (p<0,05). According to OAE results while SNR's decreased in 2002 Hz and higher frequencies in H15 group, they decreased in 2380-5652 Hz in H30 group (p<0,05). Histopathological evaluation results showed that outer hair cells, stria vascularis and spiral ganglion scores in hypothroid groups were different from control group (p<0,05). CONCLUSION: In this experimental animal model, it was detected that hypothyroidism effected threshold and latency in ABR and SNR in OAE. In addition, according to histopathological evaluation results outer hair cells, stria vascularis and spiral ganglions were also effected due to hypothyroidism. However, while extension of duration of hypothyroidism did not change the threshold, it extended latency in ABR. According to our study results, hypothyroidism may disrupt hearing function by effecting inner ear both electrophiyslogically and at the tissue level.