Within the last decades multiple iodolipid-classes have been identified in thyroid tissue. For a long time they have been supposed to be involved in thyroid autoregulation, but for the time being no specific compounds could be isolated. A new approach was stimulated by the finding that thyroid cells were able to iodinate polyunsaturated fatty acids to form iodolactones and by the identification of alpha-iodohexadecanal (alpha-IHDA) as the major compound of an iodolipid fraction. alpha-IHDA exerts multiple inhibitory effects on adenylate cyclase, NADPH-oxidase and thyroid peroxidase. Therefore, it is speculated as a mediator of the Wolff-Chaikoff-effekt and to be involved in the autoregulation of specific thyroid functions mediated by the cyclic adenosine-3',5'-monophosphate (cAMP)-pathway. Meanwhile 6-iodo-5-hydroxy-8,11,14-eicosatrienoic acid delta-lactone (delta-iodolactone) has been identified in human thyroid tissue and it could be demonstrated that this iodoeicosanoid specifically inhibits signal transduction pathways induced by local growth factors such as epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). Therefore, delta-iodol-actones seem to act as mediators of iodine, especially in the autoregulation of cAMP-independent thyroid cell proliferation. We will summarize these important new findings and discuss the role of these iodolipids on thyroid cell growth regulation.