Nutrition Research Reviews

Research Article

Iodine deficiency, thyroid function and hearing deficit: a review

Alida Melse-Boonstraa1 c1 and Ian Mackenziea2

a1 Division of Human Nutrition, Wageningen University, Wageningen, The Netherlands

a2 World Health Organization Collaborating Centre for Prevention of Deafness, Child and Reproductive Health, Liverpool School of Tropical Medicine, Liverpool, UK

Abstract

Iodine deficiency affects an estimated 241 million school-aged children in the world. Little is known about iodine deficiency in relation to auditory function, except for the fact that deaf–mutism is one of the features of cretinism. In the present review, we documented the scientific knowledge on the role of iodine and hypothyroidism in the auditory system. We found that ear development and hearing function depend on thyroid hormones. Multiple pathways are involved in this, including both inner ear morphology as well as neurological processes. Conductive as well as sensorineural hearing loss is found in studies with animals that were rendered hypothyroidic. In humans, auditory impairment is reported frequently in relation to hypothyroidism, ranging from mild disturbances to severe handicap. Auditory impairment has been related more explicitly to congenital hypothyroidism than to acquired hypothyroidism. The critical period for thyroid function-related hearing maturation is the first and second trimesters of pregnancy. Although only a limited number of studies have directly investigated the relationship between iodine deficiency and auditory function, most studies point toward an association. However, evidence from good randomised controlled trials is lacking. Inclusion of auditory outcomes in iodine supplementation studies is therefore to be recommended, especially for trials in pregnancy. Hearing deficit is an invisible abnormality, but has major consequences for educational and social skills if not detected. In view of this, auditory impairment should be mapped in iodine-deficient areas in order to realistically estimate the magnitude of the problem.

Key Words:

  • Iodine deficiency;
  • Thyroid function;
  • Hearing deficit;
  • School-aged children

Correspondence

c1 Corresponding author: Alida Melse-Boonstra, fax +31 317 483342, email alida.melse@wur.nl

Footnotes

  Abbreviations: ABR, auditory brainstem response; Tg, thyroglobulin; TR, thyroid hormone receptor; TSH, thyroid-stimulating hormone; T3, triiodothyronine; T4, thyroxine; UI, urinary iodine concentration