Iodide transport by thyrocytes involves two transporters, namely the Na(+)/I (-) symporter located at the basolateral pole and possibly pendrin in the apical membranes of the cell. Recently, we identified a human gene and its protein product, designated hAIT, as a putative new transporter involved in iodide transfer across the apical membrane of thyrocytes. In the present report, we analyzed both hAIT gene and protein expressions in a large series of benign and malignant human thyroid tissues. Using immunohistochemistry, hAIT staining was detected in normal thyroid tissue in about 10% of follicles; in positive follicles, 10-40% of thyrocytes, mostly the tall cells, were stained. In thyroid tissues obtained from patients with Graves' disease and toxic adenomas, hAIT mRNA and protein levels were similar to those found in normal tissue. In hypofunctioning adenomas, hAIT mRNA levels were slightly decreased, and apical iodide transporter (AIT) immunostaining was similar to that observed in normal thyroid tissue. AIT staining was stronger in Hürthle cell adenomas and in microfollicular adenomas. In thyroid carcinomas, the mean and median hAIT mRNA levels were significantly decreased. Expression of AIT protein was undetectable in most papillary carcinomas and was weak but detectable in most follicular carcinomas; it was negative in anaplastic carcinomas.