Narrative review: iodine-thyroidal and extrathyroidal actions

Background and Objective: Discussion of iodine in endocrine practice is usually reserved to its role in thyroid hormonogenesis. The involvement of iodine in other organs such as breast, placenta, parotid and skin and in the environment is less well understood. The aim of this review is to present data on roles for iodine in thyroidal and extra thyroidal functions and in the wider environment. Methods: Sources of iodine, dietary supplementation and its assessment will be examined. The role of iodine deficiency and excess and its relevance to thyroid function in pregnancy and the neonate and in thyroid cancer will be described. Extrathyroidal actions of iodine in breast, parotid and skin as well as the possible association of diseases of the breast with iodine intake and thyroid disorders explored. Finally, the important role in influencing weather patterns in seaside areas of gaseous iodine arising from seaweeds and potentially affecting iodine intake in populations residing in coastal communities is discussed. Key Content and Findings: In addition to considering the well documented areas of iodine deficiency, excess and iodine supplementation, this review will look at iodine involvement in other organs such as the breast, placenta parotid and skin. The importance in iodine assessment of defining the urinary iodine (UI) cut off point to avoid inappropriate iodine supplementation will be emphasised. In particular, the role of iodine in breast physiology, pathology and possible therapy and in placental function will be described. Finally, the important role in influencing weather patterns of iodine released by seaweeds in coastal areas and potentially affecting iodine intake in coastal communities is discussed. Conclusions: Hopefully this review, by providing a more complete picture of thyroidal and extrathyroidal interactions will increase the appetite for looking at iodine involvement in other tissues and its possible relationship to thyroidal and extrathyroidal disorders.