Hearing Profile in Thyroid Dysfunction

Introduction

Multiple factors can affect hearing losslikegenetic or environmental. Also several endocrine and metabolic abnormalities may lead todifferent degrees of hearing loss.The neurophysiology of hearing and anatomical structure of the auditory pathwayis affected by changes in the hormonal and metabolic systems.¹

Before the very onset of hearing thyroid hormones act through thyroid hormone β receptors to influence hearing by initiating myelinogenesis of the vestibulocochlear nerve (VIII cranial nerve).²In addition tothe expression ofprestin(cochlear protein), that regulates functions of outer hair cells, has been proved to reduce in thyroid hormonedeficiency.³

Hypothyroidism is considered a cause ofthe diminution of hearing. About twenty-five percent of patients with acquired hypothyroidism complain hearing loss, with partial reversibility after thyroid hormone replacementtreatment.⁴

Patients and Methods

Subjects:
A control group of 20 healthyvolunteers (Group A) with normal thyroid function test werehearing-assessed in the department of Audiology and Vestibular medicine at Al-Azhar University Hospitals,the group included10 males and 10 females with a mean age of32.5 years and hearing threshold better than 25 dBHTLconcerning frequency range 250-8000 Hz.

The hyperthyroid group (GroupB) comprised 20 patients with acquiredhyperthyroidismof mean age 35.4; 13 were males and 7 were females. While the hypothyroid group (Group C) comprised 20 patients with acquiredhypothyroidism of mean age 37.95; 10 were males and 10 were females; both groups were diagnosed in the Department of Endocrinology and Metabolism at Al-Azhar University Hospitals. Any other causes of hearing loss were excluded as so any neurological disorders or systemic diseases that may interrupt with electrophysiological measures.

Based on findings of a previous paper Thornton and Jarvis,⁵ regarding pure tone thresholds of the average 2-4 kHz, the difference between hypothyroid (24.3±11) and control (11.7±5) was significant (p<0.005); so based on that with the power of 80.0% and confidence level of 95% the minimum sample size calculated to be fifteen in each group and with the non-responder rate we took twenty.

Equipments:
Two-channel Audiometer Inter-acoustics model AC40 with headphones TDH 39 and bone vibrator B71, Acoustic Immittancemeter Inter-acoustics model AZ26 with 220 Hz probe tone and Evoked potential system (ABR) Smart Intelligent.

Procedures:
All subjects were submitted to carefully taken full history, thyroid function test, otological examination, and hearingassessment includingimmittancemetry with ipsilateral acoustic reflexes, puretoneaudiometry(PTA), speech audiometry,and auditory brain stem response (ABR) at a low repetition rate 21.1 clicks/sec and high repetition rate 71.1 clicks/sec.

Statistical.Analysis:
Data obtained through history taking, clinical assessment, and laboratory tests were analyzed using Microsoft Excel software. The data were then imported into Statistical Package for the Social Sciences (SPSS version 20.0) software for analysis. Based on the form of data; qualitative data expressed as number and percentage while quantitative data for the continuous group is represented by mean ± SD. The following measures were used to assess significant differences inthe qualitative variable by the Chi-square test (X²). Differences between quantitative independent multiple groups by ANOVA.

Results

Table (1) shows age and gender distribution among studied groups. Mean age was 32.5±8.55, 35.4±9.17, and 37.95±8.1 years old among control, hyperthyroid and hypothyroid groups respectivelywith no significant difference among studied groups p>0.05.

Pure tone audiogramthresholdsamong three studied groups are shown inTable (2) showing that there is no significant difference between the three groups as regard frequency range 250 Hz – 2000 Hz. While the hypothyroid group wassignificantly higher than the other two groups regarding4000 Hz and 8000Hz on both right and left ears.

Table (3) showsthe distribution of low and high-frequencyaudiogramthresholds among studied groups showing thatthe hypothyroid group was significantly higher than the other two groups regarding high-frequency thresholds.

Distribution of right and left word discrimination scores among studied groups are shown inTable (4)showing no significant difference betweenthe three studied groups.

Acoustic reflexes ofthe studied groups are distributed and tabulated in Table (5) showing thatthe hypothyroid group was significantly higher than the other two groups regarding 1000 Hz and 2000 Hzonthe right ear and 500 Hz, 1000 Hz, and 2000 Hzon the left ear.

Low and High repetition rate ABR among studied groups was compared in Table (6) showing that ABR results as regarding wave V latencies were significantly higher amongthe hypothyroid group on both right and left ears.

Table (1): age and gender distribution among studied groups.

Figure (1):Mean age distribution among the three studied groups showing no significant difference.

Table (2):right and left pure tone audiogram thresholds distribution among the three studied groups.

Figure (2a):right ear pure tone thresholds of the three studied groups showing that Group C was significantly higher than the other two groups regarding 4kHz and 8kHz.

Figure (2b):left ear pure tone thresholds of the three studied groups showing that Group C was significantly higher than the other two groups regarding 4kHz and 8kHz.

Table (3):Low and high-frequency pure tone thresholdsof both right and left ears among studied groups.

Figure (3):Low and high-frequency pure tone thresholds on both earsamongthe three studied groups showing that Group C was significantly higher than the other two groups regarding high frequency.

Table (4):right and left distribution ofword discrimination score amongstudied groups.

Figure (4): distribution ofright and left word discrimination score among the three groups showing no significant difference.

Table (5): right and left ipsilateral reflexes thresholds distribution among the studied groups.

Figure (5a):Right ear ipsilateral reflexes distribution among studied groupsshowing that Group C was significantly higher than the other two groups regarding 1kHz and 2kHz.

Figure (5b):left ear ipsilateral reflexes distribution among studied groups showing that Group C was significantly higher than the other two groups regarding 500Hz, 1kHz, and 2kHz.

Table (6): low and high repetition rateABR wave V latencies in msec. of both right and left ears among studied groups.Group C was significantly higher than the other two groups regarding low and high repetition rates.

Hypothyroidism is defined as a decrease in serum concentration of thyroid hormones that leads to elevated serum TSH concentration and hyperthyroidism is described by low serum level of TSH and increased thyroid hormones serum concentration.The normal range of FT3, FT4 and TSH serum levels is0.2-0.5 ng/dL, 0.7-2.5 ng/dL and 0.4-4.2 mU/L respectively.⁶

In the current study, the assessment of patients with acquired hypothyroidism and hyperthyroidism was studied in detailwhen compared to the control group to define the degree of hearing loss. Results showed that there was no significant difference between the three groups regardingfrequency range 250 Hz – 2000 Hzin both ears, but when high frequencies were recorded; subjects of group C were significantly higher than the other two groups at 4000 HZ and 8000 HZ. This is an agreement with Thornton and Jarvis,⁵compared between hypothyroid and hyperthyroid patients; found that the average threshold was above twenty-five dB in hypothyroid patients resultswhile no significant difference between control and hyperthyroid patients results. They supposed that the significant alteration may be due to variation in the hormonal level which leads to changes in the metabolism and physiological criteriawithin cells in the retro-cochlear auditory pathway.

Also Karaliand Guclu,⁷Studied twenty-five hypothyroid patients and reported that comparison of the airway threshold values obtained by audiometry revealed that the hypothyroid group was found higher at high frequencies when compared to the control group. However there was no significant difference at 500 Hz - 4000 Hz, they found a statistically significant difference at 8000 Hz.In a study conducted on 30 hypothyroid patients, Santos et al.⁸ detected a moderate sensorineural hearing loss in twenty-two of the sixty ears and reported that there were bilateral hearing losses.

On the other hand, Berker et al.⁹examinedtwenty-twocasesof Grave’s disease anddocumented sensorineural hearing loss particularly at high frequencies in most cases.

Acoustic reflexes in the 3 groups; control, hyperthyroid and hypothyroid patients were measured and results showeda highly significant difference (p<0.05) in ipsilateral acoustic reflexes among group C when compared to group A and group B specifically at 1000 HZ and 2000 Hz on the right ear and at 500 Hz, 1000 Hz and 2000 Hz on the left ear. Results match up with Bruschiniet al.¹⁰ who stated elevated acoustic reflexes in patients with hypothyroidism when compared with the normal group. On the other hand, also stated elevated acoustic reflexes in hyperthyroid patients and mentioned that acoustic reflexes of both groups were elevated when compared withthe control group.

There was nosignificant variation between the 3 studied groups whenword discrimination scorewasmeasured, that might be due to the fact ofdiscrimination score is more strongly correlated with the average of frequencies 500 Hz, 1000 Hz, 20000 Hz,and 4000 Hz.¹¹Regarding this average of frequencies; discrimination does not depend only on 4000 Hz which was earlier proved to be affected by low thyroid hormones in group C of hypothyroid patients.

It is known that electrophysiological changes are seen in thyroid diseases. Calcium absorption decreases in hypothyroidism. Calcium is effective on synaptic transmission in the nervous system, so that hearing loss due to brain stem pathologies may occur in hypothyroidism.⁵

In the current study regardingthe ABR test witha low repetition rate at a rate of 21.1 clicks/sec or high repetition rate at a rate of 71.1 clicks/sec when results of hyperthyroid patients are compared to the control group, no statistically significant difference was found about wave V absolute latency. Although a statistically significant prolongation in absolute wave V latency could be detected in patients with hypothyroidism.

Thosefindings were following Santos et al.⁸ who believedthere may be some sluggish peripheral conduction in hypothyroidism. Also,Figueiredo et al.¹²who found a marked increase in absolute wave III and V latencies and interpeak I – III or I – V latenciesin hypothyroid patients.

In addition to Karaliand Guclu,⁷found that thedifference between wave III and V latencies or interpeak I - III latencywas statistically significant (p <0.05).Ismail et al.¹³founda statistically significant delay in absolute wave III and V latencies of ABR in recently diagnosed patients.

The same as Chanderasekhar et al.¹⁴ who also have reported an increase in absolute wave latencies in hypothyroid patientsmay be due to reduced tissue metabolism which may contribute to impaired recruitment of neuronal pools at brainstem wave generators and slow peripheral conduction. Results revealed there was no dramatic difference (p>0.05) between the control group and hyperthyroidism. Kumar et al.¹⁵ detected significant difference for waves I and III absolute latencies of both ears and wave V in right.

Khedr et al.¹⁶ also confirmed elongation of all absolute and inter-peak wave latencies affecting both central and peripheral conduction time suggesting that dysfunction in the nervous system is diffuse. Lowtemperature, decreased myelin development and cerebral metabolism alteration may be the potentialexploration for the prolongation of the conduction time.

Thornton and Jarvis,⁵ found no key difference (p>0.05) in hyperthyroidism for absolute or inter-peak latencies of waves by evaluatingthe audiometry and ABR results of twelve hyperthyroid patients as compared to control.

On the other hand, Anjana et al.¹⁷have found a significant decrease in latency to wave V.with no critical difference in inter-peak latencies for hypothyroid patients, as well as Vanasse et al.¹⁸ who compared hypothyroid patients to the control group butdid not detect a big difference in ABR outcomes. The same as Ozata et al.¹⁹who found no abnormalities in ABR of patients with hypothyroidism.

Conclusion