Researchers conducted a study of 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.

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. The study was published online 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.  

Hearing Improved

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.

Researchers aimed to investigate the association between urinary iodine levels and two types of hearing loss speech-frequency and high-frequency. With the speech frequency hearing loss, a person "may have difficulty understanding conversational speech.

High-frequency Hearing

The more severe the loss, the greater the difficulty in comprehension, particularly in difficult listening situations such as in a noisy environment. A person who has a 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.  

Iodine Deficiency

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 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 a speech-frequency hearing loss, and 11.2% had a 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 urinary iodine of < 50 µg/L were significantly more likely to have a speech-frequency hearing loss, after adjusting for multiple variables.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.

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 and aims to harmonize the iodine supply across Europe.