Iodine is required for the production of thyroid hormones which are essential for hearing function and ear development during pregnancy. While severe iodine deficiency in pregnancy is known to cause deafness (as in cretinism), less is known about the effect of mild-to-moderate iodine deficiency on hearing function. In the UK, pregnant women are classified as mildly-to-moderately iodine deficient and we have previously shown in the ALSPAC cohort that this is significantly associated with lower IQ and reading scores at 8-9 years.