Serum thyroid hormone levels are closely related to the normal functioning of the cochlea. However, the relationship between initial auditory brainstem response (ABR) results and levels of thyroid hormone remained unclear until we adopted ABR as a prognostic factor in Idiopathic sudden sensorineural hearing loss (ISSNHL) patients. This investigation aimed to elucidate the association between ABR and outcomes in patients with ISSNHL. Thirty-three patients presenting with unilateral ISSNHL underwent blood sampling and ABR tests on the day of admission. The mean latencies of the ABR results were compared among the groups which were classified by ISSNHL outcome, based on Siegel's criteria. The association between the ABR results and the thyroid hormone serum levels (TSH, T3, and free T4) were assessed. The overall successful recovery rate was 60.6% ( = 20). The mean latencies of all the ABR parameters were not significantly different between the different treatment outcome groups (Mann-Whitney test). Wave latency, III-V interval and I-V interval were negatively associated with T3 serum levels. The results indicate that clinical caution should be exercised when conducting ABR tests without assessing thyroid hormone levels.