Low Iodine Intake Linked to Hearing Loss in US Teens

US teens with the lowest levels of urinary iodine were five-times more likely to have hearing loss associated with speech comprehension than their peers with higher iodine levels, researchers report.

This relationship has important public health implications because "youth with hearing impairment experience academic difficulties, have behavioral problems, and demonstrate lower performance in oral language compared with peers with normal hearing," Franco Scinicariello, MD, MPH, and Melanie C. Buser, MPH, from the Agency for Toxic Substances and Disease Registry, Centers for Disease Control and Prevention, Atlanta, Georgia, report.

More work is needed to study the association, they conclude in their research letter published online June 7 in JAMA Otolaryngology–Head & Neck Surgery.

Thyroid Hormones Essential for Maturation of the Cochlea

This is the first analysis of the link between iodine levels and hearing loss in teens in a national survey, according to the researchers.

Previous work has shown that thyroid hormones are necessary for maturation of the cochlea, and congenital and acquired hypothyroidism is associated with hearing impairment.

Iodine is essential to produce thyroid hormones, and studies have shown that preschool children with urinary iodine concentrations below 100 μg/L had speech-frequency hearing loss. Other work has shown improved hearing after iodine supplementation.

Scinicariello and Buser aimed to investigate the association between urinary iodine levels and two types of hearing loss — speech-frequency and high-frequency — in a nationally representative sample of 1198 teens who were 12- to 19-years-old and had participated in the 2007–2010 National Health and Nutrition Examination Survey (NHANES).

With speech frequency hearing loss, a person "may have difficulty understanding conversational speech," Scinicariello explained to Medscape Medical News in an email.

The more severe the loss, the greater the difficulty in comprehension, "particularly in difficult listening situations such as in a noisy environment (for example, a restaurant) or on the telephone."

A person who has high-frequency hearing loss, on the other hand, may have difficulty discriminating between "tin," "fin," and "thin," for example, and may not hear leaves rustling or birds singing.

Because there are no cutoffs to define iodine deficiency at the individual level (only at the population level), researchers divided the teens into three groups, based on World Health Organization population levels of urinary iodine: < 100 μg/L, 100 to 199 μg/L (reference group), and > 200 μg/L.

They then subdivided the lowest category into two categories of urinary iodine: 50 to 99 μg/L and < 50 μg/L.

The participants were a mean age of 15 years and 52% were male.

Overall, the teenagers had a mean urinary iodine of 154 µg/L, and levels < 100 µg/L in 31% of the teens, 100 to 199 µg/L in 30%, and > 200 µg/L in 39%.

A total of 7.7% of the teens had speech-frequency hearing loss and 11.2% had high-frequency hearing loss.

Further Studies Needed on Implications of Low Iodine Diet

Compared with the reference group (urinary iodine 100 to 199 µg/L), teens with a urinary iodine of < 50 µg/L were significantly more likely to have speech-frequency hearing loss, after adjusting for multiple variables.

Risk of Speech-Frequency Hearing Loss, Low vs Reference Urinary Iodine Levels*

Urinary Iodine, µg/L	Adjusted OR (95% CI)**
< 100	2.10 (1.04 – 4.26)
< 50	5.52 (1.94 – 15.68)
50 – 99	1.60 (0.71 – 3.62)

*Reference group: urinary iodine 100–199 µg/L.

**Adjusted for age, sex, race/ethnicity, poverty, body weight (normal/underweight, overweight, obese), serum cotinine, self-reported ear infection, urinary creatinine, and thyrotropin.

There were no significant associations between speech-frequency hearing loss and high levels of urinary iodine or between high-frequency hearing loss and any levels of urinary iodine.

"In light of our finding and the public health implication of lower iodine intake in adolescents, further studies are needed," Scinicariello and Buser conclude.

The major dietary sources of iodine are iodized salt, seafood, dairy, and grains, Scinicariello said.

In many countries, including the United States, iodized salt is available, but in some developed nations it is not, such as the UK, where there have been calls to iodize salt.

The 3-year, European Union–funded EUthyroid project, which began in 2015, includes 24 member countries plus six additional countries (Iceland, Israel, Macedonia, Norway, Switzerland, and Turkey), and aims to harmonize the iodine supply across Europe.

The authors have reported no relevant financial relationships.

JAMA Otolaryngol Head Neck Surg. Published online June 7, 2018. Research Letter

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