Effects of acute thyroxin depletion on hearing in humans

The Effect of Hormone Replacement Treatment on Hearing Function in Hypothyroid Patients

Congenital hypothyroidism causes physiologic, morphologic and developmental abnormalities of the auditory system. However, the effect of acquired hypothyroidism and hormone replacement treatment (HRT) on hearing function is still controversial. This study aimed to investigate hearing impairment and the effect of HRT on hearing function in patients with acquired hypothyroidism.

MATERIALS AND METHODS

Fifty hypothyroid patients were included in this study. Levothyroxine (0.05-0.2 mg/dl) was used for HRT and its dosage was gradually increased until the patients became euthyroid. Otoscopy and microscope was used to evaluate tympanic membrane and hearing thresholds and pure tone avarages (PTA) were estimated by using pure tone audiometry before and after treatment.

RESULTS

Patients with lower baseline free T4 (FT4) had significantly higher air conduction PTA (p < 0.05). Negative correlation between the severity of hypothyroidism and hearing gain were found (p < 0.05). Hearing improvements were at 250 and 8000 Hz after HRT.

CONCLUSION

Due to the correlation between baseline FT4 and hearing impairment in a negative direction, disease severity may have an effect on hearing impairment. In addition, patients with lower FT4 and higher thyroid-stimulating hormone levels had lower PTA improvement after HRT. HRT may not significantly improve hearing disorders in severe hypothyroidism.

Prognostic Factors in Sudden Sensorineural Hearing Loss: Our Experience and a Review of the Literature

Objectives:

We investigated prognostic factors in sudden sensorineural hearing loss (SSNHL).

Methods:

Our study group consisted of 133 patients with SSNHL who were treated at our department between 1980 and 2000. Eighty-one of them (group B) were treated between 1980 and 1996; they received vasodilators and small doses of steroids. The others (52 patients; group A) were treated between 1997 and 2000; they received vasodilators, steroids at high doses, and hyperbaric oxygen. A multivariate stepwise linear regression was used to identify the prognostic factors that were related to hearing improvement as measured by objective change of gain in the overall average (0.5, 1, 2, 4, 6, 8 kHz), the pure tone average (0.5, 1, 2 kHz), the high tone average (4, 6, 8 kHz), and the pure middle tone average (0.5, 1, 2, 4 kHz). The following factors were included in the analysis: group (method of treatment), age, gender, seasonal occurrence of disease, presence of tinnitus and vestibular symptoms, time delay before first visit, type of initial audiogram, and type of caloric reaction. In group A, an additional analysis was conducted to include the results of certain laboratory tests: blood morphology parameters, erythrocyte sedimentation rate, glucose level, coagulogram, lipidogram, thyroid-stimulating hormone, autoantibodies (antimitochondrial antibodies, smooth muscle antibodies, and anti-brush border antibodies), and immunoglobulins G, A, and M. Values for p of less than .05 were considered significant.

Results:

Our analysis suggests the presence of the following prognostic factors for SSNHL: method of SSNHL treatment (better results in group A); time delay before the start of treatment (better results when treatment started within 10 days of the first symptoms of SSNHL); and type of caloric reactions (worse results in patients with canal paresis). In group A, the factors for poor prognosis for absolute hearing improvement were as follows: delayed treatment, labyrinth responsiveness disorders, and decreased level of thyroid-stimulating hormone. In group A, better hearing improvement was observed in those patients in whom SSNHL was diagnosed in the spring.

Conclusions:

A short time delay before starting treatment (within 10 days), treatment with high doses of steroids and hyperbaric oxygen, preserving complete caloric function of the labyrinths, normal function of the thyroid, and seasonal occurrence of the disease in the spring were positive prognostic factors for hearing recovery in SSNHL.

Age-Related Hearing Loss and Degeneration of Cochlear Hair Cells in Mice Lacking Thyroid Hormone Receptor β1

A key function of the thyroid hormone receptor β (Thrb) gene is in the development of auditory function. However, the roles of the 2 receptor isoforms, TRβ1 and TRβ2, expressed by the Thrb gene are unclear, and it is unknown whether these isoforms promote the maintenance as well as development of hearing. We investigated the function of TRβ1 in mice with a Thrb(b1) reporter allele that expresses β-galactosidase instead of TRβ1. In the immature cochlea, β-galactosidase was detected in the greater epithelial ridge, sensory hair cells, spiral ligament, and spiral ganglion and in adulthood, at low levels in the hair cells, support cells and root cells of the outer sulcus. Although deletion of all TRβ isoforms causes severe, early-onset deafness, deletion of TRβ1 or TRβ2 individually caused no obvious hearing loss in juvenile mice. However, over subsequent months, TRβ1 deficiency resulted in progressive loss of hearing and loss of hair cells. TRβ1-deficient mice had minimal changes in serum thyroid hormone and thyrotropin levels, indicating that hormonal imbalances were unlikely to cause hearing loss. The results suggest mutually shared roles for TRβ1 and TRβ2 in cochlear development and an unexpected requirement for TRβ1 in the maintenance of hearing in adulthood.

Cochlear dysfunction in patients with acute hypothyroidism

The effect of acute hypothyroidism on the cochlear function was studied prospectively, in a group of 52 patients with thyroid carcinoma who underwent total thyroidectomy. All patients were examined before surgery and 6-8 weeks postoperatively. During this period there was no replacement with levothyroxine and the magnitude of thyroxin depletion was monitored by serum thyroid-stimulating hormone levels. Pure-tone audiometry, tympanometry and transiently evoked otoacoustic emissions were performed. A group of healthy volunteers of similar age and sex were used as controls. Tympanograms were normal, either on initial or on repeat testing. Audiometry showed elevation of all postoperative hearing thresholds, whereas the thresholds varied significantly across frequency. Transiently evoked otoacoustic emission testing showed response signal-to-noise ratios lower in the postoperative session (hypothyroid state) than in the preoperative session on all measured frequencies. Emission levels varied significantly across frequency, with maximum response observed at 2 kHz. Comparison of significant pure-tone and otoacoustic emission shifts for individual ears showed more ears affected in otoacoustic emission testing, indicating subclinical involvement. Comparing hearing thresholds and otoacoustic emission levels between patients and controls showed significant differences on postoperative testing. It may be thus concluded that acute hypothyroidism causes elevation of hearing thresholds in humans and to a greater degree subclinical damage of the cochlear function.

Hexachlorobenzene, a dioxin-like compound, disrupts auditory function in rat

Hexachlorobenzene (HCB) is a dioxin-like compound widely distributed in the environment. In this study, we investigated the effects of HCB on the cochlea. Conscious free-moving rats were given HCB per os daily for 4 weeks at doses of 0.16, 4 or 16 mg/kg in olive oil, whereas the control group received olive oil only. The effects of HCB were evaluated at various time intervals, by measuring auditory nerve acoustic thresholds and plasma thyroid hormone concentration by radioimmunoassay. Histological evaluation involved surface preparation and scanning electron microscopy observations of cochlear hair cells. At a dose of 0.16 mg/kg, HCB induced no loss of acoustic sensitivity, whereas at 4 mg/kg, it induced cochlear sensitivity deficits at the mid-frequencies (2-16 kHz) with complete recovery once treatment was stopped. At a dose of 16 mg/kg, permanent threshold shifts were observed at all frequencies tested (from 1 to 32 kHz). Morphological studies showed no cochlear hair cell loss or alteration of stereocilia. HCB treatment reduced circulating thyroxine concentrations. Thyroidectomy had no effect on cochlear sensitivity in control animals. Thus, HCB is a potent oto-toxicant, and its ototoxicity may be independent of its thyroidal effects.

Click-evoked otoacoustic emissions recorded from untreated congenital hypothyroid newborns

Thyroid hormone plays an important role in hearing development. Both a genetic or non-genetic hypothyroidism is often associated with congenital hearing loss. The exact incidence of hearing impairment in untreated congenital hypothyroid (CH) patients is unknown. This paper will present the results of measuring of the transient-evoked otoacoustic emissions (TEOAE) in a population of 29 newborns, who tested positive on a screening test for hypothyroidism (CH group) and in 68 well babies (control group) randomly chosen from all the newborns, classified as PASS, included in the Hearing Screening Program of the San Raffaele Hospital in Milan. TEOAE were recorded in all newborns within 1 month after birth and before beginning L-thyroxine treatment with conventional commercial instrumentation. Both temporal and time-frequency analyses of the emitted responses were conducted by means of a wavelet transform. The comparison of the characteristics of the temporal and frequency content of the responses of the two groups (CH and control) showed no statistically significant difference. No correlation was found between outer hair cell dysfunction and hypothyroidism.