Pendred syndrome is an autosomal recessive familial disorder, defined as a sensorineural deafness coinciding with a goiter related to an iodine organification defect. It constitutes 4 to 10 % of all causes of familial congenital hearing loss. It is now well established that this condition was caused by mutations in the SLC26A4 gene which codes for pendrin, a protein involved in the transport of anions (I-, Cl, HCO3-), particularly in apical iodine efflux in thyroid cells and chloride (Cl-|HCO3-) at the cochlear level. A current issue is to determine factors that distinguish Pendred syndrome from other varieties of isolated deafness also including enlargement of the vestibular aqueduct, but without evidence of thyroid expression. A prospective investigation of these 2 situations, conducted in Lille, leads us to redefine diagnostic criteria and assessment methods, and to discuss genetic or environmental factors contributing to the phenotypes.