Pendred syndrome is an autosomal recessive disorder characterized by sensorineural deafness, goiter and a partial defect in iodide organification. Goiter development and hypothyroidism vary and appear to depend on nutritional iodide intake. Pendred syndrome is caused by biallelic mutations in the SLC26A4 gene, which encodes pendrin, a multifunctional anion exchanger. Pendrin is mainly expressed in the thyroid, the inner ear, and the kidney. In the thyroid, pendrin localizes to the apical membrane of thyrocytes, where it may be involved in mediating iodide efflux. Loss-of-function mutations in the SLC26A4 gene are associated with a partial iodide organification defect, presumably because of a reduced iodide efflux into the follicular lumen. In the kidney, pendrin functions as a chloride/bicarbonate exchanger. In the inner ear, pendrin is important in the maintenance of a normal anion transport and the endocochlear potential. Elucidation of the function of pendrin has provided unexpected novel insights into the pathophysiology of thyroid hormone biosynthesis, chloride retention in the kidney, and composition of the endolymph.