The gene mutated in Pendred syndrome (PDS), the PDS gene, is expressed in the inner ear, kidney, and thyroid. It encodes a membrane protein named pendrin that is endowed with the function of anion transporter or exchanger. It has been postulated that in the thyroid pendrin could participate in the transport of iodide from the cell to the lumen of follicles. We generated antipeptide antibodies directed against the C- terminal sequence of human pendrin 1) to characterize the protein expressed in the human thyroid, and 2) to analyze its expression level in relation to the functional activity of thyroid tissue. In denaturing conditions, a single molecular species of 110-115 kDa was identified in human thyroid membrane fractions. After treatment of thyroid membranes with N-glycosidase F, pendrin had an apparent molecular mass of 85 kDa. Analyzed by ultracentrifugation on sucrose gradient in nondenaturing conditions, pendrin sedimented as a main 120- to 140-kDa component. Pendrin was assayed by semiquantitative Western blot in thyroid membrane fractions from 25 hyper- or hypofunctioning tumors and paired normal tissue samples. Pendrin was increased 2-fold in toxic adenomas, was not significantly altered in follicular adenoma, and was decreased, on the average, by 35% in papillary carcinomas compared with levels in paired normal tissue. The variations in the pendrin tissue content and PDS transcript levels, assayed by RT-PCR on duplicate samples of the same tumors, were similar. In conclusion, we show that pendrin expressed by the human thyroid gland is a mainly monomeric glycoprotein and that the level of expression of pendrin, although somewhat related, only moderately varied with the functional status of the thyroid tissue.