Thyroid hormone synthesis requires H(2)O(2) produced by dual oxidases (Duoxes) and thyroperoxidase (TPO). Defects in this system lead to congenital hypothyroidism. H(2)O(2) damage to the thyrocytes may be a cause of cancer. The objective of the study was to investigate whether Duox and TPO, the H(2)O(2) producer and consumer, might constitute a complex in the plasma membrane of human thyroid cells, thus maximizing efficiency and minimizing leakage and damage. The interaction between Duox and TPO was studied by coimmunoprecipitation and Western blotting of plasma membranes from incubated follicles prepared from freshly resected human thyroid tissue from patients undergoing thyroidectomy, and COS-7 cells transiently transfected with the entire Duoxes or truncated [amino (NH2) or carboxyl (COOH) terminal]. The following results were reached: 1) Duox and TPO from membranes are coprecipitated, 2) this association is up-regulated through the Gq-phospholipase C-Ca(2+)-protein kinase C pathway and down-regulated through the Gs-cAMP-protein kinase A pathway, 3) H(2)O(2) increases the association of Duox1 and Duox2 to TPO in cells and in membranes, and 4) truncated NH(2)- or COOH-terminal Duox1 and Duox2 proteins show different binding abilities with TPO. Coimmunoprecipitations show that Duox and TPO locate closely in the plasma membranes of human thyrocytes, and this association can be modulated by H(2)O(2), optimizing working efficiency and minimizing H(2)O(2) spillage. This association could represent one part of a postulated pluriprotein complex involved in iodination. This suggests that defects in this association could impair thyroid hormone synthesis and lead to thyroid insufficiency and cell damage.