Résumé : 2-Iodohexadecanal (IHDA) has been identified as a major thyroid iodolipid which can be formed upon addition of iodine to the vinyl ether group of plasmalogens (Pereira et al., 1990). In order to test whether IHDA plays a role in the thyroid autoregulation by iodide, we have investigated its effects on the production of H2O2 by cultured dog thyroid cells. IHDA inhibited the formation of H2O2 in dog thyroid cells stimulated by carbamylcholine (CCHOL). In the presence of BSA, which potentiated its action, the effect of IHDA was maximal after 2 h and had an IC50 around 5 microM. The effect of IHDA was not decreased by methimazole, which abolished the inhibition by iodide. IHDA also inhibited the stimulatory effect of bradykinin, but had only a marginal effect on the production of H2O2 induced by ionomycin or phorbol 12-myristate 13-acetate (PMA). The accumulation of inositol phosphates in CCHOL-stimulated thyroid cells was decreased by IHDA. As evaluated by measurements of 51Cr release and [3H]thymidine incorporation into DNA, IHDA had no adverse effect on thyroid cell viability. Several analogs of IHDA, of which the synthesis is described, have been tested for their inhibitory activity. This allowed the identification of two major structural features required for the biological activity: the carbonyl group at C1 and an halogen atom at C2, with iodine conferring a greater activity than bromine, while chlorine and fluorine were inactive. In conclusion, IHDA inhibits the production of H2O2 in CCHOL-stimulated dog thyroid cells by decreasing the phospholipase C cascade activity. This effect involves both the aldehyde function and the iodine atom. These results suggest that IHDA might be the mediator of some of the regulatory actions of iodide on the thyroid gland.