Iodine deficiency, thyroid function and hearing deficit: a review

Acquired hypothyroidism, iodine status and hearing impairment in adults: A pilot study

OBJECTIVES

Hearing impairment can have major impacts on behavior, educational attainment, social status, and quality of life. In congenital hypothyroidism, the incidence of hearing impairment reaches 35-50%, while in acquired hypothyroidism there is a reported incidence of 25%. Despite this, knowledge of the pathogenesis, incidence and severity of hearing impairment remains greatly lacking. The aim of our study was to evaluate hearing in patients with acquired hypothyroidism.

METHODS

30 patients with untreated and newly diagnosed peripheral hypothyroidism (H) and a control group of 30 healthy probands (C) were enrolled in the study. Biochemical markers were measured, including median iodine urine concentrations (IUC) µg/L. The hearing examination included a subjective complaint assessment, otomicroscopy, tympanometry, transitory otoacoustic emission (TOAE), tone audiometry, and brainstem auditory evoked potential (BERA) examinations. The Mann-Whitney U test, Fisher's Exact test and multivariate regression were used for statistical analysis.

RESULTS

The H and C groups had significantly different thyroid hormone levels (medians with 95% CI) TSH mU/L 13.3 (8.1, 19.3) vs. 1.97 (1.21, 2.25) p =  0 and fT4 pmol/L 10.4 (9.51, 11.1) vs. 15 (13.8, 16.7) p =  0. The groups did not significantly differ in age 39 (34, 43) vs. 41 (36,44) p =  0.767 and IUC 142 (113, 159) vs. 123 (101, 157) p =  0.814. None of the hearing examinations showed differences between the H and C groups: otomicroscopy (p =  1), tympanometry (p =  1), TOAE (p =  1), audiometry (p =  0.179), and BERA (p =  0.505).

CONCLUSIONS

We did not observe any hearing impairment in adults with acquired hypothyroidism, and there were no associations found between hearing impairment and the severity of hypothyroidism or iodine status. However, some forms of hearing impairment, mostly mild, were very common in both studied groups.

Safety assessment of Tafamidis: a real-world pharmacovigilance study of FDA adverse event reporting system (FAERS) events

OBJECTIVE

Tafamidis-associated adverse events (AEs) were investigated retrospectively by data mining the US Food and Drug Administration Adverse Event Reporting System (FAERS) to inform clinical safety.

METHODS

Data were gathered from the FAERS database, which spans the second quarter of 2019 to the fourth quarter of 2023. A total number of 8532 reports of Tafamidis-related adverse events were detected after evaluating 8,432,351 data. Disproportionality analyses were used to quantify the signal and assess the significance of Tafamidis-associated AEs using four algorithms, including the reporting odds ratio (ROR), the proportional reporting ratio (PRR), the multi-item gamma Poisson shrinker (MGPS) and the Bayesian confidence propagation neural network (BCPNN).

RESULTS

Among the 8532 reports of AEs with Tafamidis as the primary suspected drug, Tafamidis-induced AEs were identified as occurring in 27 system organ classes (SOC). A total of 207 Tafamidis-induced AEs were detected which simultaneously complied with the four algorithms. Our analysis also identified new adverse reactions including Hypoacusis, Deafness, and Essential hypertension. The median onset of adverse reactions associated with Tafamidis was 180 days (interquartile range [IQR] 51-419 days).

CONCLUSION

Tafamidis is a drug that has shown favorable safety and tolerability results in clinical trials. However, a number of adverse reactions associated with Tafamidis have been identified through analysis of the FAERS database. In clinical applications, it is recommended to closely monitor patients' hearing while using Tafamidis. In addition, it is hoped that further experimental and clinical studies will be conducted in the future to understand the mechanism of occurrence between Tafamidis and adverse reactions such as primary hypertension, hyperlipidemia, and height reduction.

Characterization of a human thyroid microtissue model for testing thyroid disrupting chemicals

Perturbation of thyroid hormone (T4) synthesis is known to cause numerous developmental, metabolic, and cognitive disorders in humans. Due to species differences in sensitivity to chemical exposures, there is a need for human-based in vitro approaches that recapitulate thyroid cellular architecture and T4 production when screening. To address these limitations, primary human thyrocytes, isolated from healthy adult donor tissues and cryopreserved at passage one (p'1) were characterized for cellular composition, 3D follicular architecture, and thyroglobulin (TG)/T4 expression and inhibition by prototype thyroid disrupting chemicals (TDC). Flow analysis of the post-thaw cell suspension showed >80% EpCAM-positive cells with 10%-50% CD90-positive cells. When seeded onto 96-well Matrigel®-coated plates and treated with bovine thyroid stimulating hormone (TSH), thyrocytes formed 3D microtissues during the initial 4-5 days of culture. The microtissues exhibited a stable morphology and size over a 14-day culture period. TG and T4 production were highest in microtissues when the proportion of CD90-positive cells, seeding density and thyroid stimulating hormone concentrations were between 10%-30%, 6K-12K cells per well, and 0.03-1 mIU/mL, respectively. At maximal TG and T4 production levels, average microtissue diameters ranged between 50 and 200 µm. The T4 IC50 values for two prototype TPO inhibitors, 6-propyl-2-thiouracil and methimazole, were ∼0.7 µM and ∼0.5 µM, respectively, in microtissue cultures treated between days 9 and 14. Overall, p'1 cryopreserved primary human thyrocytes in 3D microtissue culture represent a promising new model system to prioritize potential TDC acting directly on the thyroid as part of a weight-of-evidence hazard characterization.

A review of the auditory-gut-brain axis

Hearing loss places a substantial burden on medical resources across the world and impacts quality of life for those affected. Further, it can occur peripherally and/or centrally. With many possible causes of hearing loss, there is scope for investigating the underlying mechanisms involved. Various signaling pathways connecting gut microbes and the brain (the gut-brain axis) have been identified and well established in a variety of diseases and disorders. However, the role of these pathways in providing links to other parts of the body has not been explored in much depth. Therefore, the aim of this review is to explore potential underlying mechanisms that connect the auditory system to the gut-brain axis. Using select keywords in PubMed, and additional hand-searching in google scholar, relevant studies were identified. In this review we summarize the key players in the auditory-gut-brain axis under four subheadings: anatomical, extracellular, immune and dietary. Firstly, we identify important anatomical structures in the auditory-gut-brain axis, particularly highlighting a direct connection provided by the vagus nerve. Leading on from this we discuss several extracellular signaling pathways which might connect the ear, gut and brain. A link is established between inflammatory responses in the ear and gut microbiome-altering interventions, highlighting a contribution of the immune system. Finally, we discuss the contribution of diet to the auditory-gut-brain axis. Based on the reviewed literature, we propose numerous possible key players connecting the auditory system to the gut-brain axis. In the future, a more thorough investigation of these key players in animal models and human research may provide insight and assist in developing effective interventions for treating hearing loss.

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.

Iodine status during child development and hearing ability: a systematic review

Iodine, through the thyroid hormones, is required for the development of the auditory cortex and cochlea (the sensory organ for hearing). Deafness is a well-documented feature of endemic cretinism resulting from severe iodine deficiency. However, the range of effects of suboptimal iodine intake during auditory development on the hearing ability of children is less clear. We therefore aimed to systematically review the evidence for the association between iodine exposure (i.e. intake/status/supplementation) during development (i.e. pregnancy and/or childhood) and hearing outcomes in children. We searched PubMed and Embase and identified 330 studies, of which thirteen were included in this review. Only three of the thirteen studies were of low risk of bias or of good quality, this therefore limited our ability to draw firm conclusions. Nine of the studies (69 %) were in children (one RCT, two non-RCT interventions and six cross-sectional studies) and four (31 %) were in pregnant women (one RCT, one cohort study and two case reports). The RCT of iodine supplementation in mildly iodine-deficient pregnant women found no effect on offspring hearing thresholds. However, hearing was a secondary outcome of the trial and not all women were from an iodine-deficient area. Iodine supplementation of severely iodine-deficient children (in both non-RCT interventions) resulted in improved hearing thresholds. Five of six cross-sectional studies (83 %) found that higher iodine status in children was associated with better hearing. The current evidence base for the association between iodine status and hearing outcomes is limited and further good-quality research on this topic is needed.

Audiological evaluation (128-20,000Hz) in women with autoimmune thyroiditis: The role of antibodies vs. l-thyroxine deficiency

BACKGROUND AND AIM

Although sensorineural hearing loss may have different aetiologies, we focused on autoimmune hearing loss since it may be reversible with corticosteroid therapy; this entity is sometimes associated with systemic autoimmune diseases. Hashimoto's thyroiditis or chronic autoimmune thyroiditis shows antibodies and may be harmful to hearing thresholds regardless of hypothyroidism effect. To date this effect has not been sufficiently studied and never with extended high frequencies. The aim of this work is to study by age groups whether hearing thresholds in the human auditory range (128-20,000Hz) are affected in Hashimoto's disease.

MATERIALS AND METHODS

Two groups of 128 patients affected by Hashimoto's thyroiditis were included. First group: patients with pathological antithyroid antibodies who do not need L-thyroxine treatment. Second group: patients controlled with L-thyroxine substitutive treatment. Audiometric threshold study comparing between the groups of patients and a group of 209 controls was performed. All patients underwent complete otorhinolaryngological examination, antithyroid antibodies, TSH, T3 and T4 blood levels, tympanometry, conventional pure-tone audiometry, and extended-high-frequency audiometry.

RESULTS

All patients were women. Both groups showed worst audiometric thresholds than the control group; both study groups showed worse hearing than controls, this difference was statistically significant in all frequencies. In the 8-20kHz frequency range, this difference was more than 10dB, and in the 9-16kHz and 20kHz range this difference was more than 20dB. When separated by age groups, in younger subjects (20-29 years) these differences were found in all frequencies, except for conversational frequencies (500-4,000Hz); between 30 and 49 years the difference is statistically significant in all frequencies; and from 50 to 69 years differences are found, especially in the conversational frequencies.

CONCLUSIONS

This first work studying the human auditory range in the chronic autoimmune thyroiditis or Hashimoto's thyroiditis confirms that hearing loss related to the autoimmune disorder predominates at extended-high-frequencies initially. But ends up involving all frequencies in pure-tone conventional audiometry, then it may be detected in routine clinical tests. These results support the role of extended-high-frequencies audiometry to diagnose subclinical hearing loss in patients affected by Hashimoto's thyroiditis.

No Association Was Found Between Mild Iodine Deficiency During Pregnancy and Pregnancy Outcomes: a Follow-up Study Based on a Birth Registry

BACKGROUND

Severe iodine deficiency during gestation is associated with adverse pregnancy outcomes; however, the impact of mild-to-moderate iodine deficiency, though prevalent in pregnancy, remains unclear.

METHODS

We extracted follow-up data for 7435 pregnant women from a national iodine deficiency disorders monitoring program from 2016 to 2018 and a mother-child cohort study in 2017 based on a birth registry in Shanghai. Birth outcomes were collected from the registry. Spot urine and household salt samples were collected for iodine testing. Single-factor analysis and logistic regression were used to evaluate the association between maternal iodine status and pregnancy outcomes.

RESULTS

The median urine iodine level in pregnant women was 137.5 μg/L (interquartile range 82.4-211.5), suggesting mild deficiency according to WHO standards. The incidence of pregnancy termination, preterm birth, congenital malformations, low birth weight, and cesarean section was 3.2%, 4.3%, 1.4%, 2.7%, and 45.2% in the mildly iodine-deficient group and 3.4%, 4.5%, 1.4%, 2.7%, and 44.5% in the normal group, respectively. After adjusting for maternal age and education, trimesters, and preterm birth rate in the general population, the odds ratios for any outcome did not differ significantly between the two groups.

CONCLUSION

The present study suggests that mild maternal iodine deficiency is not associated with adverse pregnancy outcomes.

A disease-associated mutation in thyroid hormone receptor α1 causes hearing loss and sensory hair cell patterning defects in mice

Resistance to thyroid hormone due to mutations in THRA, which encodes the thyroid hormone receptor α (TRα1), shows variable clinical presentation. Mutations affecting TRβ1 and TRβ2 cause deafness in mice and have been associated with deafness in humans. To test whether TRα1 also affects hearing function, we used mice heterozygous for a frameshift mutation in Thra that is similar to human THRA mutations (Thra^S1/+ mice) and reduces tissue sensitivity to thyroid hormone. Compared to wild-type littermates, Thra^S1/+ mice showed moderate high-frequency sensorineural hearing loss as juveniles and increased age-related hearing loss. Ultrastructural examination revealed aberrant orientation of ~20% of sensory outer hair cells (OHCs), as well as increased numbers of mitochondria with fragmented morphology and autophagic vacuoles in both OHCs and auditory nerve fibers. Molecular dissection of the OHC lateral wall components revealed that the potassium ion channel Kcnq4 was aberrantly targeted to the cytoplasm of mutant OHCs. In addition, mutant cochleae showed increased oxidative stress, autophagy, and mitophagy associated with greater age-related cochlear cell damage, demonstrating that TRα1 is required for proper development of OHCs and for maintenance of OHC function. These findings suggest that patients with THRA mutations may present underdiagnosed, mild hearing loss and may be more susceptible to age-related hearing loss.

Relationship between Nutrient Intake and Hearing Loss According to the Income Level of Working-Aged Adults: A Korean National Health and Nutrition Survey

The relationship between hearing impairment and nutrition has been extensively investigated; however, few studies have focused on this topic in working-age adults by income level. Herein, we aimed to determine the differences in hearing impairment among working-age adults by income level and identify the nutritional factors that affect hearing loss in various socioeconomic groups. Seven-hundred-and-twenty participants had hearing impairment, while 10,130 had normal hearing. After adjustment for propensity score matching, income and smoking status were identified as significant variables. By assessing the relationship between hearing impairment and nutrient intake by income level using multiple regression analyses, significant nutrients differed for each income category. Carbohydrate and vitamin C levels were significant in the low-income group; protein, fat, and vitamin B1 levels were significant in the middle-income group; and carbohydrates were significant in the high-income group. Income was significantly associated with hearing impairment in working-age adults. The proportion of individuals with hearing impairment increased as income decreased. The association between hearing impairment and nutritional intake also differed by income level. Our findings may enable the establishment of health policies for preventing hearing impairment in working-age adults by income level.

Hypothyroidism and related comorbidities on the risks of developing tinnitus

This is a retrospective longitudinal study that uses data from the National Health Insurance Research Database (NHIRD) of Taiwan of which hypothyroid patients who received a diagnosis between 2000 and 2010 were selected and followed up until 2011. The primary outcome of this study was the occurrence of tinnitus (ICD-9-CM code 388.3). The relevant comorbidities were selected as potential confounders according to the literature, which included vertigo (ICD-9-CM code 386), insomnia (ICD-9-CM code 780), anxiety (ICD-9-CM code 300.00), and hearing loss (ICD-9-CM code 388–389). The overall incidence of tinnitus was significantly higher in the hypothyroidism cohort than in the non-hypothyroidism cohort (9.49 vs. 6.03 per 1000 person-years), with an adjusted HR of 1.35 (95% CI 1.18–1.54) after adjusting potential confounders. The incidences of tinnitus, as stratified by gender, age, comorbidity, and follow-up time, were all significantly higher in the hypothyroidism cohort than those in the non-hypothyroidism cohort. The incidence of tinnitus significantly increased with age (aHR = 1.01, 95% CI 1.01–1.02). In conclusion, we report the relationship between hypothyroidism and the increased risk for tinnitus. We also found that hypothyroidism patients are at increased risk of developing tinnitus when associated with comorbidities including vertigo, hearing loss, and insomnia.

Low Thyroid-stimulating Hormone Levels Are Associated With Annoying Tinnitus in Adult Women: Korea National Health and Nutrition Examination Surveys

OBJECTIVE

This study aimed to investigate the relationship between thyroid function and tinnitus.

STUDY DESIGN

A cross-sectional study.

SETTING

The Korean National Health and Nutrition Examination Survey from 2013 was used.

PATIENTS AND INTERVENTIONS

A total of 1,165 participants ≥ 40 years old who were surveyed for the presence of tinnitus and underwent thyroid function tests were included. The presence of discomfort from tinnitus was defined as annoying tinnitus. The control group included participants with "no tinnitus" or "no discomfort from tinnitus." The participants were divided into the annoying tinnitus group and the control group.

MAIN OUTCOME MEASURES

The associations of free thyroxine and thyroid-stimulating hormone (TSH) with annoying tinnitus were analyzed using logistic regression with complex sampling methods. Subgroup analyses were performed according to sex.

RESULTS

The low TSH level group had 2.35-fold greater odds of annoying tinnitus than the control group (95% confidence interval  = 1.10-5.12, p = 0.027). Even in patients with a normal free thyroxine level, a low TSH level was related to 2.78-fold higher odds of annoying tinnitus (95% confidence interval  = 1.21-6.38, p = 0.016). In subgroup analyses, this association was apparent in the female subgroup. The male subgroup did not show a relationship between low TSH levels and annoying tinnitus.

CONCLUSIONS

Subclinical hyperthyroidism was related to an increased risk of annoying tinnitus. This relationship was apparent in the female subgroup.

Association of Maternal Iodine Status With Child IQ: A Meta-Analysis of Individual Participant Data

CONTEXT

Although the consequences of severe iodine deficiency are beyond doubt, the effects of mild to moderate iodine deficiency in pregnancy on child neurodevelopment are less well established.

OBJECTIVE

To study the association between maternal iodine status during pregnancy and child IQ and identify vulnerable time windows of exposure to suboptimal iodine availability.

DESIGN

Meta-analysis of individual participant data from three prospective population-based birth cohorts: Generation R (Netherlands), INMA (Spain), and ALSPAC (United Kingdom); pregnant women were enrolled between 2002 and 2006, 2003 and 2008, and 1990 and 1992, respectively.

SETTING

General community.

PARTICIPANTS

6180 mother-child pairs with measures of urinary iodine and creatinine concentrations in pregnancy and child IQ. Exclusion criteria were multiple pregnancies, fertility treatment, medication affecting the thyroid, and preexisting thyroid disease.

MAIN OUTCOME MEASURE

Child nonverbal and verbal IQ assessed at 1.5 to 8 years of age.

RESULTS

There was a positive curvilinear association of urinary iodine/creatinine ratio (UI/Creat) with mean verbal IQ only. UI/Creat <150 µg/g was not associated with lower nonverbal IQ (-0.6 point; 95% CI: -1.7 to 0.4 points; P = 0.246) or lower verbal IQ (-0.6 point; 95% CI: -1.3 to 0.1 points; P = 0.082). Stratified analyses showed that the association of UI/Creat with verbal IQ was only present up to 14 weeks of gestation.

CONCLUSIONS

Fetal brain development is vulnerable to mild to moderate iodine deficiency, particularly in the first trimester. Our results show that potential randomized controlled trials investigating the effect of iodine supplementation in women with mild to moderate iodine deficiency on child neurodevelopment should begin supplementation not later than the first trimester.

Association of Nutritional Factors with Hearing Loss

Hearing loss (HL) is a major public health problem. Nutritional factors can affect a variety of diseases, such as HL, in humans. Thus far, several studies have evaluated the association between nutrition and hearing. These studies found that the incidence of HL was increased with the lack of single micro-nutrients such as vitamins A, B, C, D and E, and zinc, magnesium, selenium, iron and iodine. Higher carbohydrate, fat, and cholesterol intake, or lower protein intake, by individuals corresponded to poorer hearing status. However, higher consumption of polyunsaturated fatty acids corresponded to better hearing status of studied subjects. In addition to malnutrition, obesity was reported as a risk factor for HL. In studies of the relationship between middle ear infection and nutrition in children, it was reported that lack of vitamins A, C and E, and zinc and iron, resulted in poorer healing status due to vulnerability to infection. These studies indicate that various nutritional factors can affect hearing. Therefore, considering that multifactorial nutritional causes are responsible, in part, for HL, provision of proper guidelines for maintaining a proper nutritional status is expected to prevent some of the causes and burden of HL.

Interplay between Nutrition and Hearing Loss: State of Art

Hearing loss has been recently ranked as the fifth leading cause of years lived with disability, ahead of many other chronic diseases such as diabetes, dementia, or chronic obstructive pulmonary disease. Moreover, according to the World Health Organization, moderate-to-profound hearing loss affects about 466 million people worldwide. Its incidence varies in each population segment, affecting approximately 10% of children and increasing to 30% of the population over 65 years. However, hearing loss receives still very limited research funding and public awareness. This sensory impairment is caused by genetic and environmental factors, and among the latter, the nutritional status has acquired relevance due its association to hearing loss detected in recent epidemiological studies. Several experimental models have proved that the onset and progression of hearing loss are closely linked to the availability of nutrients and their metabolism. Here, we have reviewed studies focused on nutrient effects on auditory function. These studies support the potential of nutritional therapy for the protection against hearing loss progression, which is especially relevant to the aging process and related quality of life.

Dose-dependent effect of fluoride on clinical and subclinical indices of fluorosis in school going children and its mitigation by supply of safe drinking water for 5 years: an Indian study

Fluorosis is a public health problem in India; to know its prevalence and severity along with its mitigation measures is very important. The present study has been undertaken with the aim to assess the F dose-dependent clinical and subclinical symptoms of fluorosis and reversal of the disease by providing safe drinking water. For this purpose, a cross-sectional study was undertaken in 1934 schoolgoing children, Nalgonda district. Study villages were categorized into control (category I, F = 0.87 mg/L), affected (category II, F = 2.53 mg/L, and category III, F = 3.77 mg/L), and intervention categories (category IV, F = < 1.0 mg/L). School children were enrolled for dental grading by modified Dean Index criteria. Anthropometric measurements (height and weight) were used to assess nutritional status of the children. The biochemical parameters like serum T3, T4, TSH, PTH, ALP, 25-OH vitamin D, and 1,25-(OH)₂ vitamin D were analyzed. The results showed a positive correlation between the drinking water and urinary fluoride (UF) in different categories. However, there was a significant decrease in the UF levels in the intervention category IV compared to affected group (category III). Fluoride altered the clinical (dental fluorosis and stunting) and subclinical indices (urine and blood) of fluorosis in a dose-dependent manner. In conclusion, the biochemical indices were altered in a dose-dependent manner and intervention with safe drinking water for 5 years in intervention group-mitigated clinical and subclinical symptoms of fluorosis.

Endocrine Glands and Hearing: Auditory Manifestations of Various Endocrine and Metabolic Conditions

The aetiology of hearing loss in humans is multifactorial. Besides genetic, environmental and infectious causes, several endocrine and metabolic abnormalities are associated with varying degrees of hearing impairment. The pattern of hearing loss may be conductive, sensori-neural or mixed. The neurophysiology of hearing as well as the anatomical structure of the auditory system may be influenced by changes in the hormonal and metabolic milieu. Optimal management of these conditions requires the integrated efforts of the otolaryngologist and the endocrinologist. The presence of hearing loss especially in the young age group should prompt the clinician to explore the possibility of an associated endocrine or metabolic disorder for timely referral and early initiation of treatment.

A narrative review of obesity and hearing loss

The comorbidities related to obesity are already extensive, but as the prevalence of obesity increases globally, so do the number of its associated conditions. The relationship between hearing impairment and obesity is a relatively recent research interest, but is significant as both conditions have the ability to substantially reduce an individual's quality of life both physically and psychologically. Obesity has a significant effect on vascular function, and this may have an impact on highly vascular organs such as the auditory system. This review aims to provide an overview of the existing literature surrounding the association between hearing loss and obesity, in order to emphasise these two highly prevalent conditions, and to identify areas of further investigation. Our literature search identified a total of 298 articles with 11 articles of relevance to the review. The existing literature in this area is sparse, with interest ranging from obesity and its links to age-related hearing impairment (ARHI) and sudden sensorineural hearing loss (SSNHL), to animal models and genetic syndromes that incorporate both disorders. A key hypothesis for the underlying mechanism for the relationship between obesity and hearing loss is that of vasoconstriction in the inner ear, whereby strain on the capillary walls due to excess adipose tissue causes damage to the delicate inner ear system. The identified articles in this review have not established a causal relationship between obesity and hearing impairment. Further research is required to examine the emerging association between obesity and hearing impairment, and identify its potential underlying mechanisms.

Developmental exposure to PCBs alters the activation of the auditory cortex in response to GABAA antagonism

Developmental exposure of rats to polychlorinated biphenyls (PCBs) causes impairments in hearing and in the functioning of peripheral and central auditory structures. Additionally, recent work from our laboratory has demonstrated an increase in audiogenic seizures. The current study aimed to further characterize the effects of PCBs on auditory brain structures by investigating whether developmental exposure altered the magnitude of activation in the auditory cortex (AC) in response to electrical stimulation of thalamocortical afferents. Long-Evans female rats were fed cookies containing either 0 or 6mg/kg of an environmental PCB mixture daily from 4 weeks prior to breeding until postnatal day 21. Brain slices containing projections from the thalamus to the AC were collected from adult female offspring and were bathed in artificial cerebrospinal fluid (aCSF) alone, aCSF containing a gamma-aminobutyric acid (GABA) receptor antagonist (200nM SR95531), and aCSF containing an and N-methyl-d-aspartate (NMDA) receptor antagonist (50μM AP5). During each of these drug conditions, electrical stimulations ranging from 25 to 600μA were delivered to the thalamocortical afferents. Activation of the AC was measured using flavoprotein autofluorescence imaging. Although there were no differences seen between treatment groups in the aCSF condition, there were significant increases in the ratio of aCSF/SR95531 activation in slices from PCB-exposed animals compared to control animals. This effect was seen in both the upper and lower layers of the AC. No differences in activation were noted between treatment groups when slices were exposed to AP5. These data suggest that developmental PCB exposure leads to increased sensitivity to antagonism of GABA_A receptors in the AC without a change in NMDA-mediated intrinsic excitability.

Low Urinary Iodine Concentrations Associated with Dyslipidemia in US Adults

Iodine is an essential component of the thyroid hormone which plays crucial roles in healthy thyroid function and lipid metabolism. However, the association between iodine status and dyslipidemia has not been well established at a population level. We aimed to test the hypothesis that the odds of dyslipidemia including elevated total cholesterol, triglycerides, low-density lipoprotein (LDL) cholesterol and apolipoprotein B, and lowered high-density lipoprotein (HDL) cholesterol and HDL/LDL ratio are associated with urinary iodine concentration (UIC) in a population perspective. Data of 2495 US adults (≥20 years) in the National Health and Nutrition Examination Survey 2007–2012 were used in this study. Two subgroups (i.e., UIC below vs. above the 10th percentile) were compared of dyslipidemia as defined based on NCEP ATP III guidelines. The differences between the groups were tested statistically by chi-square test, simple linear regressions, and multiple logistic regressions. Serum lipid concentrations differed significantly between two iodine status groups when sociodemographic and lifestyle covariates were controlled (all, p < 0.05). Those with the lowest decile of UIC were more likely to be at risk for elevated total cholesterol (>200 mg/dL) (adjusted odds ratio (AOR) = 1.51, 95% confidence interval (CI): 1.03–2.23) and elevated LDL cholesterol (>130 mg/dL) (AOR = 1.58, 95% CI: 1.11–2.23) and lowered HDL/LDL ratio (<0.4) (AOR = 1.66, 95% CI: 1.18–2.33), compared to those with UIC above the 10th percentile. In US adults, low UIC was associated with increased odds for dyslipidemia. Findings of the present cross-sectional study with spot urine samples highlight the significant association between UIC and serum lipids at population level, but do not substantiate a causal relationship. Further investigations are warranted to elucidate the causal relationship among iodine intakes, iodine status, and serum lipid profiles.

Iodine status and thyroid function among Spanish schoolchildren aged 6-7 years: the Tirokid study

I deficiency is still a worldwide public health problem, with children being especially vulnerable. No nationwide study had been conducted to assess the I status of Spanish children, and thus an observational, multicentre and cross-sectional study was conducted in Spain to assess the I status and thyroid function in schoolchildren aged 6-7 years. The median urinary I (UI) and thyroid-stimulating hormone (TSH) levels in whole blood were used to assess the I status and thyroid function, respectively. A FFQ was used to determine the consumption of I-rich foods. A total of 1981 schoolchildren (52 % male) were included. The median UI was 173 μg/l, and 17·9 % of children showed UI<100 μg/l. The median UI was higher in males (180·8 v. 153·6 μg/l; P<0·001). Iodised salt (IS) intake at home was 69·8 %. IS consumption and intakes of ≥2 glasses of milk or 1 cup of yogurt/d were associated with significantly higher median UI. Median TSH was 0·90 mU/l and was higher in females (0·98 v. 0·83; P<0·001). In total, 0·5 % of children had known hypothyroidism (derived from the questionnaire) and 7·6 % had TSH levels above reference values. Median TSH was higher in schoolchildren with family history of hypothyroidism. I intake was adequate in Spanish schoolchildren. However, no correlation was found between TSH and median UI in any geographical area. The prevalence of TSH above reference values was high and its association with thyroid autoimmunity should be determined. Further assessment of thyroid autoimmunity in Spanish schoolchildren is desirable.

Do antithyroid antibodies affect hearing outcomes in patients with pediatric euthyroid Hashimoto's thyroiditis?

OBJECTIVES

Hashimoto's thyroiditis (HT) is the most common autoimmune thyroid disease in children. HT is a multifaceted disease with a variable clinicopathological presentation, including hearing impairment. It is known that hearing function is negatively affected in patients with thyroid disorders. The literature includes a very limited number of studies on hearing function in euthyroid pediatric patients with HT. The aim of this study was to determine the relationship between cochlear function and HT, independent of thyroid function.

MATERIALS AND METHODS

The study included 48 children and adolescents (42 females and 6 males) aged 9-18 years that were diagnosed as HT, and 30 gender- and age-matched healthy controls. Hearing was assessed via otoscopy, tympanometry, pure-tone audiometry, and measurement of distortion product otoacoustic emissions.

RESULTS

There weren't any significant differences in pure tone thresholds between the 2 groups based on pure-tone audiometry, except in the right ear at 6kHz and 8kHz. Distortion product otoacoustic emissions signal to noise ratios were significantly lower in the HT group than in the control group at 4 different frequencies (6kHz [left ear], 8kHz [left ear], 1.5kHz [right ear], and 6kHz [right ear]) (P<0.05). The signal to noise ratios at all frequencies were <6dB in 3% of left ears and 2.5% of right ears in the control group, versus 12.5% of left ears and 9.6% of right ears in the HT group. Distortion product amplitudes were significantly lower in the HT group than in the control group for both left and right ears at 1kHz, 1.5kHz, 3kHz, and 8kHz, and at 2kHz for left ears only (P<0.05).

CONCLUSIONS

The present findings show that cochlear function was lower in the HT group than in the control group. Accordingly, we think that hearing in patients with HT should be monitored periodically, even if their hearing thresholds are within normal limits. Thyroid autoimmunity appears to play an important role in a decrease in cochlear activity in pediatric HT patients.

Serum Iodine Is Increased in Subjects Having Budd-Chiari Syndrome

This study was performed to investigate the status of serum iodine concentration among the Budd-Chiari syndrome (BCS) patients and its effect on thyroid hormone. The study group serum specimens were collected from 233 BCS patients and 60 healthy people. Serum iodine was analyzed with the Sandell-Kolthoff method, and the ELISA method was used to detect thyroid function: TSH, T3, T4, FT3, and FT4. The serum iodine level of patients with BCS was 316.7 ± 256.8 μg/L, greatly higher than 76.3 ± 25.7 μg/L of serum iodine for control group (p < 0.001), but with no significant difference among different types of BCS. There were no statistically significant differences in thyroid hormone levels (TSH, T3, T4, FT3, and FT4) between people with BCS and control group, although the TSH level of BCS group is slightly higher than that of normal control group. This study demonstrates that iodine may be related to the pathogenesis of BCS and needs to be paid more attention.

Maternal Hypothyroxinemia-Induced Neurodevelopmental Impairments in the Progeny

Maternal hypothyroxinemia can induce neurodevelopmental impairments in the developing fetus. We here review recent studies on the epidemiology and molecular mechanisms associated with this important public health issue. In 2011, the American Thyroid Association defined maternal hypothyroxinemia as low serum free thyroxine (FT4) levels (<5th or <10th percentile) existing in conjunction with normal serum free triiodothyronine (FT3) or thyroid stimulating hormone (TSH) levels during pregnancy. Compared to clinical or subclinical hypothyroidism, hypothyroxinemia is more commonly found in pregnant women. Hypothyroxinemia usually ensues in response to several factors, such as mild iodine deficiency, environmental endocrine disrupters, or certain thyroid diseases. Unequivocal evidence demonstrates that maternal hypothyroxinemia leads to negative effects on fetal brain development, increasing the risks for cognitive deficits and poor psychomotor development in resulting progeny. In support of this, rodent models provide direct evidence of neurodevelopmental damage induced by maternal hypothyroxinemia, including dendritic and axonal growth limitation, neural abnormal location, and synaptic function alteration. The neurodevelopmental impairments induced by hypothyroxinemia suggest an independent role of T4. Increasing evidence indicates that adequate thyroxine is required for the mothers in order to protect against the abnormal brain development in their progeny.