Auditory brainstem responses in thyroid diseases before and after therapy

The aim of this study was to evaluate through the auditory brainstem responses (ABRs) the electrical events generated along the auditory pathway in 56 adult patients affected with hyper- and hypothyroidism. Twenty-four normal-hearing hyperthyroid patients affected with Graves' disease and 32 normal-hearing hypothyroid patients (9 with subclinical and 23 with overt hypothyroidism) were subjected to standard (clicks at 21 pps) and sensitized ABR with masking wide-band (masking noise). In addition, thyroid scintiscan and ultrasonography, free T3 and T4, total T3 and T4, TSH, antimicrosomal and antithyroglobulin antibodies, audiogram and impedance tests were performed in all the patients. This study was repeated after 6-12 months of treatment in conditions of euthyroidism. The results showed changes of ABRs both in the standard procedure as well as in the sensitized test in 6 hyperthyroid (25%) and 8 hypothyroid patients (25%). All the patients with abnormal ABRs had overt hypothyroidism (8/23; 34.7%). The ABRs became normal in 5 out of 6 Graves' patients after 6-12 months of methimazole treatment. ABRs remained abnormal in all the hypothyroid patients despite their having been on L-thyroxine treatment for 6-12 months and were euthyroid for at least 5 months before the study was repeated. These findings suggest that ABR abnormalities are indicative only of a nonspecific injury in the bulbo-ponto-mesencephalic centers. Alterations of ABRs in thyroid diseases are not specific in relation to hyper- or hypothyroidism. Lastly, there is a relationship between ABR abnormalities and the degree of hypothyroidism, even if ABR alterations are not always reversible after long-term therapy.