Association of Iodine Deficiency With Hearing Impairment in US Adolescents Aged 12 to 19 Years

Abstract

This analysis of data from NHANES 2007-2010 evaluates associations between urinary iodine concentrations and hearing impairment among US adolescents.

Hearing impairment is poorly recognized and underreported. Thyroid hormones are necessary for maturation of the cochlea and the central auditory areas.¹ Congenital and acquired hypothyroidism in humans are associated with hearing impairment²; in animal models, higher auditory thresholds have been reported in animals with induced hypothyroidism.² Because iodine is an essential component of thyroid hormones, iodine deficiency may impair hearing by contributing to thyroid dysfunction. Few studies have investigated the relationship between iodine deficiency and hearing impairment. Speech-frequency hearing loss (SFHL) is more severe in preschool-aged children with urinary iodine concentrations (UICs) less than 100 μg/L.³ Reports of improved auditory function after iodine supplementation may indicate a possible association.²

Methods

The National Health and Nutrition Examination Survey (NHANES) is a cross-sectional, nationally representative survey of the noninstitutionalized civilian population of the United States. We analyzed data from 1198 adolescent (aged 12-19 years) participants from NHANES 2007-2010 with audiometric and urinary iodine measurements and demographic and related variables. The Centers for Disease Control and Prevention /Agency for Toxic Substances and Disease Registry (CDC/ATSDR) determined that our study met the criteria for research not involving identifiable human subjects per the Federal Policy for the Protection of Human Subjects (45 CFR 46) and therefore did not require institutional review board approval.

Speech-frequency hearing loss is defined as pure-tone audiometric average at 0.5, 1, 2, and 4 kHz greater than 15 dB HL in the better ear, and high-frequency hearing loss (HFHL) is defined at 3, 4, and 6 kHz (HFHL) greater than 15 dB HL in the better ear.⁴ Although iodine can be measured in the blood, urine, and saliva, UIC is a well-accepted and cost-efficient indicator of iodine status and reflects the current dietary intake of iodine.⁵ Because there are no cutoffs to define iodine deficiency at the individual level, we categorized UIC based on the World Health Organization classification for population-level iodine status but did not use the World Health Organization labels. Our 3 UIC categories were as follows: (1) less than 100 μg/L, (2) 100 to 199 μg/L, and (3) greater than 200 μg/L.⁵ Further analyses categorized the lowest UICs into 2 levels: 50 to 99 μg/L and less than 50 μg/L.

Statistical analyses were performed from October 30 to November 24, 2017. The a priori covariates included in the models were age, sex, race/ethnicity, poverty income ratio, body weight status based on body mass index z score percentiles, serum cotinine concentration, self-reported ear infection, urinary creatinine concentration, and thyrotropin concentration. Analyses were performed using SAS, version 9.3 (SAS Institute Inc) and SUDAAN, version 11.0 (RTI International). The UIC subsample weights of NHANES cycles were combined according to NHANES guidelines. Multivariable logistic regressions were performed to calculate adjusted odds ratios (ORs) and 95% CIs.

Results

Table 1 presents the weighted characteristics of the study population. The geometric mean (SE) age of the 1198 participants was 15 (0.9) years, and 679 participants (weighted percentage [SE], 52.1% [1.84%]) were male. The weighted prevalence of SFHL was 7.7% and of HFHL was 11.2%. Urinary iodine concentrations of 100 to 199 μg/L were reported in 39.6 (29.9% [SE, 1.81%]) of participants, 353 (31.2% [SE, 1.61%]) had a UIC less than 100 μg/L, and 503 (38.8% [SE, 1.81%]) had a UIC of 200 μg/L or greater (Table 1). In multivariable analyses (Table 2), adolescents with a UIC less than 100 μg/L had greater odds of having SFHL (OR, 2.10; 95% CI, 1.04-4.26). No association was found between SFHL and participants with a UIC greater than 200 μg/L. Further analyses found an association between a UIC less than 50 μg/L and SFHL (OR, 5.52; 95% CI, 1-15.68). Urinary iodine concentration was not associated with HFHL (Table 2).

Table 1. Weighted Characteristics of 1198 Adolescents Aged 12 to 19 Years, NHANES 2007-2010.

Characteristic	Finding (N = 1198)
Hearing loss type, No. (%) [SE]
Speech frequency	104 (7.7) [1.33]
High frequency	164 (11.2) [1.85]
Urinary iodine concentration, μg/L, No. (%) [SE]
<100	353 (31.2) [1.62]
<50	111 (9.2) [1.54]
50-99	242 (22.0) [1.40]
100-199	396 (29.9) [1.81]
≥200	503 (38.8) [1.81]
Male, No. (%) [SE]	679 (52.1) [1.84]
Body weight status based on BMI z score percentile, No. (%) [SE]a
Underweight/normal	776 (66.3) [1.69]
Overweight	208 (15.7) [1.36]
Obese	268 (18.0) [1.41]
Race/ethnicity, No. (%) [SE]
Non-Hispanic white	413 (60.2) [2.93]
Non-Hispanic black	301 (14.3) [1.45]
Mexican American	309 (12.8) [1.95]
Other Hispanic	162 (5.8) [1.42]
Other	67 (6.9) [1.39]
Annual household income, No. (%) [SE]b
≤Poverty level	405 (23.2) [1.88]
>Poverty level	847 (76.8) [1.88]
≥3 Ear infections, No. (%) [SE]	382 (37.4) [1.86]
Urinary iodine concentration, GM (SE), μg/L	154.3 (5.79)
Urinary creatinine concentration, GM (SE), mg/d	118.3 (3.30)
Serum cotinine level, GM (SE), ng/mL	0.10 (0.02)
Thyrotropin concentration, GM (SE), μUI/mL	1.4 (0.03)
Age, GM (SE), y	15.3 (0.9)

Abbreviations: BMI, body mass index; GM, geometric mean; NHANES, National Health and Nutrition Examination Survey.

^aUnderweight/normal indicates a BMI below the 85th percentile; overweight, at the 85th to less than the 95th percentile; and obese, at the 95th percentile or higher.

^bPoverty level indicates the calculated ratio of household income to the poverty threshold after accounting for inflation and family size.

Table 2. Multivariable Logistic Regression of Hearing Impairment and Iodine Intake Levels for US Adolescents Aged 12 to 19 Years, NHANES 2007-2010^a.

Category Analysisb	Risk of Hearing Impairment, OR (95% CI)
	Speech Frequency		High Frequency
	Model 1	Model 2	Model 1	Model 2
Urinary iodine concentration, μg/L
<100	2.07 (1.01-4.22)	2.10 (1.04-4.26)	1.10 (0.62-1.97)	1.09 (0.61-1.97)
100-199	1 [Reference]	1 [Reference]	1 [Reference]	1 [Reference]
≥200	1.24 (0.52-2.97)	1.23 (0.52-02.93)	1.04 (0.61-1.77)	1.03 (0.61-1.74)
Urinary iodine concentration, μg/L
<50	5.52 (1.94-15.70)	5.52 (1.94-15.68)	2.21 (0.92-5.33)	2.14 (0.91-5.05)
50-99	1.57 (0.69-3.58)	1.60 (0.71-3.62)	0.91 (0.47-1.75)	0.90 (0.46-1.17)
100-199	1 [Reference]	1 [Reference]	1 [Reference]	1 [Reference]
>200	1.19 (0.49-2.86)	1.18 (0.49-2.82)	1.00 (0.58-1.72)	1.00 (0.59-1.69)

Abbreviations: NHANES, National Health and Nutrition Examination Survey; OR, odds ratio.

^aModel 1 was adjusted for age, sex, race/ethnicity, poverty income ratio, body weight status (normal/underweight, overweight, obese), serum cotinine concentration, self-reported ear infection, and urinary creatinine concentration. Model 2 was adjusted for all of the preceding plus thyrotropin concentration.

^bThe 2 breakdowns of urinary iodine concentration are explained in the Methods section.

Discussion

To our knowledge, this is the first analysis of the association between UIC and hearing impairment in a national survey. Urinary iodine concentration less than 100 μg/L was a predictive risk factor for having SFHL among adolescents and more specifically among those with UICs less than 50 μg/L. Youth with hearing impairment experience academic difficulties, have behavioral problems, and demonstrate lower performance in oral language compared with peers with normal hearing.⁶ In light of our finding and the public health implication of lower iodine intake in adolescents, further studies are needed.