The prevalence of overt autoimmune thyroid disease (AITD) is no less than around 23% in the adult population and increases with age. Subclinical thyroid autoimmunity is around 10 times higher, with a female preponderance. Although the major phenotypes (Graves disease, Hashimotos thyroiditis with or without goitre, and chronic asymptomatic thyroiditis) are well characterized our knowledge of the aetiology of these disorders is incomplete. Studying twins constitutes an ideal starting point in the evaluation of a possible genetic background for a given disorder, and the evaluation of possible aetiological environmental factors is facilitated by the study of disease discordant twins. Despite this, very little research has been conducted using twins as a resource. Studies in twins suggest that there is a considerable genetic susceptibility in the aetiology of AITD, based on higher concordance rates (around 3050%) in monozygotic (MZ) than in dizygotic (DZ) twins (around 5%). The main environmental factors seem to be related to iodine intake and cigarette smoking. Other suggested players (such as: low birth weight and various infections) are less likely.
The hurdle to overcome in the next years is to define, using e.g. molecular biological techniques, the genetic background for these phenotypes (so far with limited success), to disclose the relative contribution of environmental factors, alone or in concert, and to dig deeper into the understanding of genegene, environmentenvironment, and geneenvironment interactions. The possibilities are of Galactic dimensions and the hitherto used methodology quite insufficient. The ultimate goal is a better understanding of the aetiology of AITD, enabling a more focused intervention and ultimately eradication of some phenotypes.