Thyroid-Related Hormone Levels in Clinical Patients With Moderately Severe-to-Profound Sudden Sensorineural Hearing Loss: A Prospective Study

Objectives: Sudden sensorineural hearing loss (SSNHL) is a common otological emergency, causing a measure of hearing loss and affecting the quality of life. This study aims to investigate the association of thyroid-related hormone levels with moderately severe-to-profound SSNHL. Methods: The study included 70 patients with moderately severe-to-profound SSNHL and 100 age- and sex-matched healthy controls. Peripheral venous blood samples were taken from the participants, and their thyroid-related hormone levels were measured at admission and 1 week after treatment. Results: In moderately severe-to-profound SSNaHL patients, the concentrations of total triiodothyronine (TT3), total thyroxine (TT4), free triiodothyronine (FT3), and thyroid-stimulating hormone (TSH) (all P < 0.05) were significantly lower than in the control group. The TT3, TT4, FT3, and TSH levels were significantly higher in the effective group than in the ineffective group (all P < 0.05). Linear correlation analysis revealed that TSH level (R = 0.707, P < 0.05) elevation after treatment successfully predicted a favorable outcome of hearing recovery. Logistic regression analyses suggested low FT3 and TSH levels to be independent occurrence predictors, while the increase of TSH level may be an independent favorable outcome predictor. Conclusions: The results suggest that low FT3 and TSH levels are risk factors for moderately severe-to-profound SSNHL. By discovering the positive association between TSH elevation and hearing recovery, along with the potential novel predictors of FT3 and TSH, our study may contribute valuable insights to the research and treatment of moderately severe-to-profound SSNHL.


INTRODUCTION
Sudden sensorineural hearing loss (SSNHL) is defined as a rapid onset of hearing impairment with more than 30 dB decrease in at least three continuous frequencies within 72 h (1). Although the morbidity rate of SSNHL in China has been increasing in recent years, large-scale epidemiological data are still lacking. Referring to data from the United States, the incidence of SSNHL ranges from 5 to 27 per 100,000 population, with about 66,000 new cases per year (2)(3)(4). SSNHL usually occurs unilaterally, and is sometimes accompanied by tinnitus, vertigo, ear fullness, and nausea. It is a particularly devastating disease mainly due to a lack of understanding about its causes, exacerbated by delays and limited options for treatment (5). Hearing loss frequency and degree, age, presence of vertigo, and the initial time of therapy are all factors that may influence the prognosis of SSNHL (6). Among these factors, the degree of hearing loss plays the most important role (7). Sheehy et al. (8) reported that primary hearing loss with a decrease in intensity of 45 dB or lower presented satisfactory recovery prognosis. In contrast, Enache et al. (9) reported that hearing loss with a decrease in frequency of more than 50-60 dB, even with adequate treatment, leads to a recovery of no more than 20-30 dB. Steroids that are effective for most cases have no effect when the initial hearing loss is 90 dB or greater (10). Generally, high hearing loss severity translates to a worse prognosis. According to the latest World Health Organization hearing classification, hearing loss of more than 50 dB is classified as moderate to severe deafness, where patients experience difficulty in hearing conversational speech (11). Thus, our research focused on moderately severe-to-profound SSNHL at all frequencies.
The guidelines of China (2015) (6) emphasize that moderately severe-to-profound SSNHL at all frequencies may be caused by vasospasms or endothelial dysfunction, and inner ear embolism or thrombosis. Thyroid disorders are risk factors for cardiovascular and cerebrovascular disease, including acute ischemic stroke, atherosclerosis, and myocardial infarction (12)(13)(14). Hypothyroidism has a significant causal association with a worse profile of atherosclerotic risk factors and may play a role in atherothrombotic myocardial infarction (15). Given the similar pathogenesis of cardiovascular disease, cerebrovascular disease, and moderately severe-to-profound SSNHL, we hypothesize that comparing the thyroid function tests conducted upon admission and after treatment may assist with the diagnosis and prognosis of moderately severe-to-profound SSNHL. These cover measurements of total triiodothyronine (TT3), total thyroxine (TT4), free triiodothyronine (FT3), free thyroxine (FT4), and thyroid stimulating hormone (TSH). Thyroid hormones play vital roles in cochlear development and in the maintenance of adulthood hearing (16,17). Ng et al. (18) and Forrest et al. (19) demonstrated that thyroid hormones and their receptors are required for the development of hearing. Richter et al. (20) and Ng et al. (21) supported the same concept reporting that T3 is essential for normal cochlear function and morphology in mice, where a lack of T3 could lead to important alterations in cochlear morphology and loss of cochlear function. T3 regulates not only the development of auditory function but also the maturation of auditory sensitivity, which is evident through reports from Li et al. (22) who reported hypothyroidism to be associated with sensorineural hearing loss.
To investigate the potential roles that thyroid-related hormone may play in the development of moderately severe-toprofound SSNHL, we designed this prospective study to compare the thyroid hormone levels before and after treatment for the first time. A regression analysis was done to determine the occurrence and prognosis, and linear regression was performed to compare thyroid-related hormone vs. the severity of hearing loss, and the elevation of thyroid-related hormone vs. hearing recovery. Receiver operating characteristic (ROC) curve analysis was used to assess the predictive value of thyroid-related hormone for SSNHL.

Study Population
A total of 70 consecutive patients with moderately severe-toprofound SSNHL diagnosed at our hospital between July 2018 and December 2020 were prospectively enrolled. All participants provided written informed consent for their inclusion in the database and the use of their data for research purposes. The study protocol was approved and implemented according to the ethical standards of the Institutional Ethics Committee of the Shanghai Jiao Tong University Affiliated Sixth People's Hospital [2018-KY-036(K)]. The progress was conducted in accordance with the spirit of the Helsinki Declaration. Patients included in the study visited the hospital for the first time within 7 days after the onset of moderately severe-to-profound SSNHL. All participants underwent standard laboratory tests and audiological diagnostic procedures. Included patients were those with hearing loss at all frequencies and mean pure tone audiometry (PTA) across 0.25-8 kHz ≥50 dB. Those excluded had acute inflammatory conditions, obstructive sleep apnea, connective tissue diseases, abnormal ear examination findings, a previous history of thyroid disorder, chronic otitis media, a history of acoustic trauma or otologic surgery, conductive hearing loss, or had used ototoxic medications. Patients with malignant disease, psychiatric conditions, dementia, hepatitis B or C, or other major comorbidities (heart failure; stroke; and severe hepatic, pulmonary, or renal dysfunction) were also excluded. A group of 100 sex-and age-matched controls without any disease at regular health check-ups was used for comparison. The exclusion criteria were the same as those for the SSNHL group.

Data Collection
The baseline characteristics included age, sex, height, weight, body mass index (BMI), and blood pressure on admission (systolic blood pressure and diastolic blood pressure). The clinical characteristics included affected side, accompanying symptoms (including tinnitus, vertigo, ear fullness), history of hypertension and diabetes, time to treatment, and hearing level on admission. All hearing assessments were performed in standard shielding rooms and PTA was performed for both air and bone conduction at 0.125, 0.25, 0.5, 1, 2, 4, and 8 kHz before and after 7 days' course of systemic treatment. The hearing loss of each individual was calculated by averaging the PTA value of damaged frequencies after onset. The extent of hearing recovery is calculated using PTA after onset minus PTA after treatment. All patients underwent temporal bone computed tomography or inner ear magnetic resonance imaging, ensuring no ear structural abnormality and tumors were found. Upon admission, thyroid function tests of TT3, TT4, FT3, FT4, and TSH levels were performed using blood samples obtained from the antecubital veins of all patients between 6 and 7 a.m. after an overnight fast. The thyroid function test was performed again post-treatment and its difference (post-minus pre) from the first test was used to evaluate the changes in thyroid-related hormone.

Treatment Procedure
Once SSNHL was diagnosed, patients were hospitalized for 1 week. All the patients underwent comprehensive treatment, including treatment with steroids and batroxobin following the 2015 China guideline for the diagnosis and treatment of sudden deafness (6). The comprehensive treatment consisted of intravenous injection of Prednisone (1 mg/kg/day) for 3-5 days, followed by a reduced dosage for the remaining days according to the hearing improvement, and intravenous batroxobin (10U batroxobin for the first time and then reduced to 5U batroxobin, once every other day, 1-3 times in total according to the level of fibrinogen). Patients that experienced hearing recovery were divided into the effective group (PTA of impaired frequencies which improved more than or equal to 15 dB, or back to normal/unaffected ear) and the ineffective group (PTA of impaired frequencies which improved <15 dB).

Statistical Analyses
Statistical analyses were performed using SPSS for Windows version 22.0 (IBM Corp., Armonk, NY). Data on quantitative variables are presented as mean ± standard deviation and qualitative variables as numbers (percentage). The Chi-squared test was used for categorical variables. The independent samples t-test was used to compare continuous variables. Linear correlation was performed to assess the association between thyroid-related hormone hearing loss and hearing recovery. ROC curve analysis was used to assess the relationship between thyroid-related hormone and the occurrence of moderately severe-to-profound SSNHL. Binary logistic regression models were used to estimate the odds ratios (OR) and 95% confidence intervals (CI) for the correlation between thyroid-related hormone and the occurrence and outcome of moderately severeto-profound SSNHL. The collinearity of all continuous variables was examined before performing the logistic regression using the variance inflation factor. P < 0.05 was considered significant for all tests. The figures were generated using GraphPad Prism 7.0 for Windows (GraphPad Software Inc., CA).

Baseline and Clinical Characteristics of Participants
The baseline characteristics of participants are summarized in Table 1. About half of the patients (n = 38, 54.29%) were male and the mean age of the patients was 51.00 ± 15.89 years. There were no differences in age, sex distribution, height, weight, BMI, and blood pressure on admission between the two groups. From the results, the thyroid-related hormone levels of the moderately severe-to-profound SSNHL group were significantly lower than those of the control group (TT3, TT4, FT3, and TSH; all P < 0.05) as shown in Table 1. There is a known association between hypertension and diabetes with altered thyroid function (23,24). We further compared the thyroid parameters of moderately severe-to-profound SSNHL with or without hypertension and diabetes. It was found that there were no significant differences in moderately severeto-profound SSNHL subgroups (all P > 0.05) (as shown in Supplementary Tables 1, 2). This indicated that thyroid parameters between moderately severe-to-profound SSNHL group and control group may not be influenced by the clinical history of diabetes and hypertension.
Patients that experienced hearing recovery were divided into effective group (n = 30) and ineffective group (n = 40). As shown in Table 2, the baseline characteristics of the two groups had no significant differences (all P > 0.05). No significant differences were found between the two groups in terms of the affected side, accompanying symptoms such as tinnitus and ear fullness, and history of hypertension and diabetes (all P > 0.05). However, the number of patients with vertigo in the ineffective group was significantly higher than that in the effective group (P < 0.05). The time to treatment and the hearing loss level of the ineffective group were 4.95 ± 1.54 days and 82.11 ± 12.50 dBHL, respectively, and both were significantly higher than that in the effective group (3.57 ± 1.98 days and 75.43 ± 12.40 dBHL) (P < 0.05). There were no significant differences in thyroid-related hormone levels (all P > 0.05), however, changes in TT3, TT4, FT3, and TSH levels were significantly higher in the effective group than in the ineffective group (all P < 0.05).

Thyroid-Related Hormone and the Functional Outcome of Moderately Severe-to-Profound SSNHL
In the linear correlation analysis and scatterplots of the elevation of thyroid-related hormone levels vs. hearing recovery (Figure 3), an association was found between moderately severeto-profound SSNHL and TSH when hearing loss was treated as a continuous variable. The elevation of TSH level (R = 0.707, P < 0.05) after treatment predicted a favorable outcome of hearing recovery. In the univariate logistic regression analysis, the ORs FIGURE 3 | Plots of the change in thyroid-related hormone levels against the recovery of hearing. TT3, total triiodothyronine; TT4, total thyroxine; FT3, free triiodothyronine; FT4, free thyroxine; TSH, thyroid stimulating hormone; R, correlation coefficient; , the difference of thyroid-related hormone after treatment minus that before treatment. Data are presented as correlation coefficients and P-values. *The correlation was significant at the 0.05 level (P < 0.05). Hearing recovery was positively correlated with the elevation of TSH levels (R = 0.707, P < 0.05).
for the outcome of moderately severe-to-profound SSNHL with parameters are presented in Table 4. We found parameters that include vertigo, time to treatment, hearing level, and the change in TT3, FT3, and TSH levels to show strong associations with the treatment outcome. Given that cardiovascular risk factors such as hypertension and diabetes may be associated with SSNHL, we performed a multivariate logistic regression model adjusted using these two factors along with those that were significant in the univariate analysis. Using the variance inflation factor, all parameters were deemed non-collinear and could be used in further analysis. It was found that the change in TSH level remained the independent outcome predictor with an adjusted OR of 1.456 (95% CI, 1.069-1.983, P < 0.05).

DISCUSSION
SSNHL is a common otological emergency with an increasing incidence rate worldwide. It can cause long-term hearing loss, irreversible hearing loss, and tinnitus (25, 26), severely affecting patients' quality of life. Recently, many studies have found that hematological indices, such as routine blood parameters, serum lipid level, and coagulation function are convenient indicators for prognosis in patients with SSNHL (26)(27)(28). A large, representative population cohort studied by Kim et al. (29) showed that SSNHL patients are more likely to have goiter and hypothyroidism than normal people. In addition, a large case-control study in Taiwan reported that pre-existing hypothyroidism and hyperthyroidism are associated with SSNHL susceptibility (30). This study is the first to show an association between thyroid-related hormone before and after treatment in patients with moderately severe-to-profound SSNHL.
Lower Thyroid-Related Hormone Levels Reflect Higher Risk of Moderately Severe-to-Profound SSNHL In this study, although still within normal ranges, we found that the levels of thyroid-related hormones were significantly lower in patients with moderately severe-to-profound SSNHL than those of the control group ( Table 1). Analyzing the relationship between thyroid-related hormones and SSNHL through ROC curve suggested that the AUC for FT3 diagnosis of SSNHL is 0.837, with a cut-off point at 4.00 pmol/L (sensitivity, 86.00%; specificity, 65.71%). In terms of TSH, diagnosis for SSNHL is 0.903, with a cut-off point at 2.37 mIU/L (sensitivity, 80.00%; specificity, 88.57%). This shows that early detection of FT3 and TSH are valuable for the diagnosis of SSNHL (Figure 2). Lower FT3 and TSH levels in the early stages of moderately severe-toprofound SSNHL were independent predictors of the occurrence of moderately severe-to-profound SSNHL ( Table 3).

Ischemia and Lower Neuroprotection
Contribute to the Higher Risk of Moderately Severe-to-Profound SSNHL The mechanisms behind our findings may be understood from several established research. Hypothyroidism is related to the severity of atherosclerosis risk factors (hypertension, hyperlipemia, hyperhomocysteinemia) and may be a risk factor of atherothrombotic myocardial infarction (15). Low triiodothyronine (T3) level also plays a role in vascular diseases, including complications after brain tumor surgery, respiratory failure, and acute cardiovascular events (31)(32)(33). Given that the blood supply of the inner ear is mainly dependent on end arterioles, the function of the inner ear is greatly affected by ischemia, and ischemia may lead to hearing loss at all frequencies when thrombosis occurs in the peripheral arterioles (34). In regard to the association of poor outcomes of moderately severe-to-profound SSNHL with low serum thyroid hormone levels in patients, one possible mechanism is secondary neuronal damage after moderately severe-to-profound SSNHL. Wang et al. (35) reported that a decrease in TSH levels and an increase in the basal metabolic rate may lead to a higher risk of poststroke fatigue; this could lead to the production of excess reactive oxygen species and free radicals, resulting in neurotoxicity (36). In addition, thyroid-related hormones and their derivatives have a great influence on the repair of injured neurons (37,38). Sadana et al. (39) reported that T3 can reduce infarct and edema in a focal ischemia model. Therefore, we speculate that patients with hypothyroidism may experience lower neuroprotection and worsened secondary damage, leading to a poor outcome.
Furthermore, a growing body of research found that thyroidrelated hormone levels are greatly associated with the secretion of many neurotrophic factors, including nerve growth factors (40,41). Therefore, we can infer that patients with hypothyroidism may experience inhibition of endogenous neuron repair systems, leading to poorer functional outcomes. The elevation of TSH levels after treatment predicts a favorable outcome of hearing recovery (Figure 3), evident from the greater increase in TT3, TT4, FT3, and TSH levels for the effective group than that in the ineffective group ( Table 2), while TSH level remained an independent outcome predictor ( Table 4). Tamura et al. (42) found that steroid can not only improve thyroid function but also lead to transient decrease in thyroid hormone and an increase in reverse thyroid hormone. Most patients presented an upward trend of thyroid-related hormone levels after glucocorticoid treatment. This may explain why some patients had thyroid hormone increase after glucocorticoid treatment and thus a better prognosis.
Our study has some limitations. First, being a preliminary study, the repeatability of the results needs to be verified in more prospective studies to determine their stability and effectiveness. Second, although the thyroid hormones were correlated with hypertension and diabetes in previous reports (23,24), this correlation was not significant in our study. This could be due to the small sample size, where the association between thyroid-related hormone levels and cardiovascular status could not be identified. Third, the study has not proven a causal relationship between moderately severe-to-profound SSNHL and thyroid-related hormone levels, thus a randomized controlled trial is needed for further confirmation.

CONCLUSIONS
In summary, our study indicated that thyroid-related hormones play an important role in the clinical characteristics and outcome of moderately severe-to-profound SSNHL. During in-hospital monitoring, thyroid function testing may be important as higher levels of thyroid-related hormones have been associated with better functional outcomes. Therefore, treatment to normalize and/or elevate thyroid-related hormone levels among patients with moderately severe-to-profound SSNHL could be beneficial to their recovery. Further prospective studies with longer-term follow-up are needed to confirm our findings.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

ETHICS STATEMENT
The studies involving human participants were reviewed and approved by Shanghai Jiaotong University Affiliated Sixth People' s Hospital Ethics Committee. The patients/participants provided their written informed consent to participate in this study.